Chapter Iv., the Probability of the Theory That the Trigeminal Group of

nerves of the vertebrate have been derived from the prosomatic group of nerves of the invertebrate can be put to the test by the answers to the following morphological and anatomical questions:--

1. Do we find in the vertebrate two segmentations in this region corresponding to the two segmentations in the branchial region, i.e. a somatic or dorsal series of segments, and a splanchnic or ventral series of segments? The latter would not be branchial, but rather of the nature of free tactile appendages; so that it is useless to look for or talk about gill-slits, although such appendages, being serially homologous with the branchial mesosomatic appendages, would readily give rise to the conception of branchial segments.

2. Is there morphological evidence that the trigeminal nerve is not the nerve belonging to a single segment, or even to two segments, but is really a concentration of at least six, probably seven, segmental nerves?

3. Is there morphological evidence that the oculomotor and trochlear nerves, which on all sides are regarded as belonging to the trigeminal segments, are not single nerves corresponding each {258}to a single segment, but are the somatic motor roots belonging to the same segments as those to which the trigeminal supplies the splanchnic roots?

4. Do the mesoderm segments, which give origin to the eye-muscles, and therefore do the head-cavities of this region, correspond with the trigeminal segments? Considering the concentration of parts in this region and the difficulty already presented by the want of numerical agreement between the prosomatic appendages and the prosomatic coelomic cavities in Limulus, it may very probably be difficult to determine the actual number of the mesoderm segments.

5. Is there anatomical evidence that the ganglion of origin of the motor part of the trigeminal nerve is not a single ganglion, but a representative of many, probably seven?

6. Is there anatomical evidence that the ganglia of origin of the oculomotor and trochlear nerves represent many ganglia?

7. Is there any evidence that the organs originally supplied by the motor part of the trigeminal nerve are directly comparable with prosomatic appendages?

It is agreed on all sides that in this region of the head there is distinct evidence of double segmentation, the dorsal mesoderm segments giving origin to the eye-muscles, and the ventral segments to the musculature innervated by the trigeminal nerve. Originally, according to the scheme of van Wijhe, two segments only were recognized, the dorsal parts of which were innervated by the IIIrd and IVth nerves respectively. Since his paper, the tendency has been to increase the number of segments in this region, as is seen in the following sketch, taken from Rabl, of the history of cranial segmentation.

HISTORY OF CRANIAL SEGMENTATION.

The first attempt to deal with this question was made by Goethe and Oken. They considered that the cranial skeleton was composed of a series of vertebræ, but as early as 1842 Vogt pointed out that only the occipital segments could be reduced to vertebræ. In 1869, Huxley showed that vertebræ were insufficient to explain the cranial segmentation, and that the nerves must be specially considered. The olfactory and optic nerves he regarded as parts of the brain, not true segmental nerves; the rest of the cranial nerves {259}were segmental, with special reference to branchial arches and clefts, the facial, glossopharyngeal, and separate vagus branches supplying the walls of the various branchial pouches. In a similar manner, the supra- and infra-maxillary branches of the trigeminal were arranged on each side of the mouth, and the inner and outer twigs of the first (ophthalmic) branch of the trigeminal on each side of the orbito-nasal cleft, the trabecular and the supra-maxillary arches being those on each side of this cleft. Thus Huxley considered that there was evidence of a series of pairs of ventral arches belonging to the skull, viz. the trabecular and maxillary in front of the mouth, the mandibular, hyoid, and branchial arches behind, and that the Vth, VIIth, IXth, and Xth nerves were segmental in relation to these arches and clefts. Gegenbaur, in 1871 and 1872, considered that the branchial arches represented the lower arches of cranial vertebræ, and therefore corresponded to lower arches in the spinal region, i.e. the skull was composed of as many vertebræ as there are branchial arches. These vertebræ were confined to the notochordal part of the skull, the prechordal part having arisen secondarily from the vertebral part, while the number of vertebræ are at least nine, possibly more. The nerves which could be homologized with spinal nerves were, he thought, divisible into two great groups--(1) the trigeminal group, which included the eye-muscle nerves, the facial, and its dorsal branch, the auditory; (2) the vagus group, which included the glossopharyngeal and vagus.

Such was the outcome of the purely comparative anatomical work of Huxley and Gegenbaur--work that has profoundly influenced all the views of segmentation up to the present day.

Now came the investigations of the embryologists, of whom I will take, in the first instance, Balfour, whose observations on the embryology of the Selachians led him to the conclusion that besides the evidence of segmentation to be found in the cranial nerves and in the branchial clefts, further evidence was afforded by the existence of head-cavities, the walls of which formed muscles just as they do in the spinal region. He came to the conclusion that the first head-cavity belonged to one or more pre-oral segments, of which the nerves were the oculomotor, trochlearis, and possibly abducens; while there were seven post-oral segments, each with its head-cavity and its visceral arch, of which the trigeminal, facial, glossopharyngeal, and the four parts of the vagus were the respective nerves.

{260}Marshall, in 1882, considered that the cranial segments were all originally respiratory, and that all the segmental nerves are arranged uniformly with respect to a series of gill-clefts which have become modified anteriorly and have been lost, to a certain extent, posteriorly. He included the olfactory nerves among the segmental nerves, and looked upon the olfactory pit, the orbito-nasal lacrymal duct, the mouth, and the spiracle as all modified gill-slits, so that he reckoned three pre-oral and oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and eight post-oral segments belonging respectively to the VIIth and VIth nerves, and to the IXth nerve, and six segments belonging to the Xth nerve. He pointed out that muscles supplied by the oculomotor nerve develop from the outer wall of the first head-cavity; not, however, the obliquus superior and rectus externus, the latter originating probably from the walls of the third cavity.

In the same year, 1882, came van Wijhe's well-known paper, in which he showed that the mesoderm of the head in the selachian divided into two sets of segments, dorsal and ventral; that the dorsal segments were continuous with the body-somites, and that the ventral segments formed the lateral plates of mesoblast between each of the visceral and branchial pouches. He concluded that the dorsal somites were originally nine in number, that each was supplied with a ventral nerve-root, in the same way as the somites in the trunk, and that to each one a visceral pouch corresponded, whose walls were supplied by the corresponding dorsal nerve-root; of these nine segments, the ventral nerve-roots of the first three segments were respectively the oculomotor, trochlearis, and abducens nerves. The next three segments possessed no definable ventral root or muscles, and the seventh, eighth, and ninth segments possessed as ventral roots the hypoglossal nerve, with its muscular supply. The corresponding dorsal nerve-roots were the trigeminal, facial, auditory, glossopharyngeal and vagus nerves, the difference between cranial and spinal dorsal roots being that the former contain motor fibres.

Ahlborn, in 1884, drew a sharp distinction between the segments of the mesoderm and those of the endoderm. The former segmentation he called mesomeric, the latter branchiomeric. He considered the two segmentations to be independent, and concluded that the branchiomeric was secondary to the mesomeric, and therefore not of {261}segmental value. As to the segments of the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ remain permanently, and correspond to the three last segments in the selachian head. Of the anterior mesoderm segments, he considered that there were originally six, and that there are six typical eye-muscles in all Craniota, which have been compressed into three segments, as in Selachia.

Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal nerve belongs to three proto-vertebræ posterior to the vagus region, which were true spinal segments. He therefore modified Gegenbaur's conceptions to this extent: that portion of the skull designated by Gegenbaur as vertebral must be divided into two parts--a hind or occipital region, which is clearly composed of modified vertebræ and is the region of the hypoglossal nerves, and a front region, extending from the oculomotor to the accessorius nerves, which is characterized segmentally by the formation of branchial arches, but in which there is no evidence that proto-vertebræ were ever formed. He therefore divides the head-skeleton into three parts--

1. Gegenbaur's evertebral part--the region of the olfactory and optic nerves--which cannot be referred to any metameric segmentation.

2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be segmented from the consideration of the nerves and branchial arches, but not referable to proto-vertebræ--the region of the trigeminal and vagus nerves.

3. The vertebral spinal part--the region of the hypoglossal nerves.

He further showed that the ganglia of the specially branchial nerves, the facial, glossopharyngeal, and vagus, are at one stage in connection with the epidermis, so that these parts of the epidermis represent sense-organs which do not develop; these organs probably belonged to the lateral line system. As the connection takes place at the dorsal edge of the gill-slits, they may also be called rudimentary branchial sense-organs.

Since this paper of Froriep's, it has been generally recognized, and Gegenbaur has accepted Froriep's view, that the three hindmost metameres, which distinctly show the characteristics of vertebræ, belong to the spinal and not to the cranial region, so that the metameric segmentation of the cranial region proper has become {262}more and more associated with the branchial segmentation. Froriep's discovery of the rudimentary branchial sense-organs as a factor in the segmentation question has led Beard to the conclusion that the olfactory and auditory organs represent in a permanent form two of these rudimentary branchial sense-organs. He therefore includes both the olfactory and auditory nerves in his list of cranial segmental nerves, and makes eleven cranial branchial segments in front of the spinal segments represented by the hypoglossal.

A still larger number of cranial segments is supposed to exist, according to the researches of Dohrn and Killian, in the embryos of Torpedo ocellata. The former, holding to the view that vertebrates arose from annelids, considered that the head was formed of a series of metameres, to each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a segmental nerve and vessel belonged. He found in the front head-region of a Torpedo embryo, corresponding to van Wijhe's first four somites, no less than twelve to fifteen mesoderm segments, and concluded, therefore, that the eye-muscle nerves, especially the oculomotor, represented many segmental nerves, and were not the nerves of single segments; so, also, that the inferior maxillary part of the trigeminal and the hyoid nerve of the facial are probably not single nerves, but a fusion of several. Killian comes to much the same conclusion as Dohrn, for he finds seventeen to eighteen separate mesoderm segments in the head, of which twelve belong to the trigeminal and facial region.

Since Rabl's paper, a number of papers have appeared, especially from America, dealing with yet another criterion of the original segmentation of the head, viz. a series of divisions of the central nervous system itself, which are seen at a very early stage of development, and are called neuromeres; the divisions in the cranial region being known as encephalomeres, and those of the spinal region as myomeres. Locy's paper has especially brought these divisions into prominence as a factor in the question of segmentation. They are essentially segments of the epiblast and not of the mesoblast; they are conspicuous in very early stages, and appear to be in relation with the cranial nerves, according to Locy. He recognizes in Squalus acanthias, in front of the spino-occipital region, fourteen pairs of such encephalomeres and a median unsegmented termination, which may represent one more pair fused in the middle line, making at least fifteen. He distributes these fifteen segments as follows: {263}fore-brain three and unsegmented termination, mid-brain two, and hind-brain nine.

Again, Kupffer, in his recent papers on the embryology of Ammocoetes, asserts that especial information as to the number of primitive segments is afforded by the appearance in the early stages of a series of epibranchial ganglia in connection with the cranial nerves, which remain permanently in the case of the vagus nerves, but disappear in the case of pro-otic nerves. He considers that the evidence points to the number of segments in the mid- and hind-brain region as being primitively fifteen, viz. six segments belonging to the trigeminal and abducens group, three segments belonging respectively to the facial, auditory, and glossopharyngeal, and six to the vagus.

From this sketch we see that the modern tendency is to make six segments at least out of the region of the trigeminal nerves rather than two. In this region, as already mentioned, the evidence of segmentation is based more clearly on the somatic than on the splanchnic segments. We ought, therefore, in the first place, to consider the teaching of the eye-muscles and their nerves and the coelomic cavities in connection with them, and see whether the hypothesis that such muscles represent the original dorso-ventral somatic muscles of the palæostracan ancestor is in harmony with and explains the facts of modern research.

EYE-MUSCLES AND THEIR NERVES.

The only universally recognized somatic nerves belonging to these segments which exist in the adult are the nerves to the eye-muscles, of which, according to van Wijhe, the oculomotor is the nerve of the 1st segment, the trochlearis of the 2nd, and the abducens of the 3rd; while the nerves and muscles belonging to the 4th and 5th segments, i.e. the 2nd facial and glossopharyngeal segments respectively, show only the merest rudiments, and do not exist in the adult. One significant fact appears in this statement of van Wijhe, and is accepted by all those who follow him, viz. that the oculomotor nerve has equal segmental value with the trochlearis and the abducens, although it supplies a number of muscles, each of which, on the face of it, has the same anatomical value as the superior oblique or external rectus. Dohrn alone, as far as I know, as already pointed out, insists upon the multiple character of the oculomotor nerve.

{264}As far as the anatomist is concerned, the evidence is becoming clearer and clearer that the nucleus of the IIIrd nerve is a composite ganglion composed of a number of nuclei, each similar to that of the trochlearis, so that if the trochlearis nucleus is a segmental motor nucleus, then the oculomotor nucleus is a combined nucleus belonging to at least four segmental nerves, each of which has the same value as that of the trochlearis.

The investigations of a number of anatomists, among whom may be mentioned Gudden, Obersteiner, Edinger, Kölliker, Gehuchten, all lead directly to the conclusion that this oculomotor nucleus is composed of a number of separate nuclei, of which the most anterior as also the Edinger-Westphal nucleus contains small cells, while the others contain large cells. Thus Edinger divides the origin of the oculomotor nerve into a small-celled anterior part and a larger posterior part, of which the cells are larger and distinctly arranged in three groups--(1) dorsal, (2) ventral, and (3) median. Between the anterior and posterior groups lies the Edinger-Westphal nucleus, which is small-celled; naturally, the large-celled group is that which gives origin to the motor nerves of the eye-muscles, the small-celled being possibly concerned with the motor nerves of the pupillary and ciliary muscles. I may mention that Kölliker considers that the anterior lateral nucleus has nothing to do with the oculomotor nerve, but is a group of cells in which the fibres of the posterior longitudinal bundle and of the deep part of the posterior commissure terminate.

These conclusions of Edinger are the outcome of work done in his laboratory by Perlia, who says that in new-born animals the nucleus of origin of the oculomotor nerve is made up of a number of groups quite distinct from each other, each group being of the same character as that of the trochlearis. He finds the same arrangement in various mammals and birds. Further, he finds that some of the fibres arise from the nucleus of the opposite side, thus crossing, as in the trochlearis; these crossing fibres belong to the most posterior of the dorsal group of nuclei, i.e. to the nerve to the inferior oblique muscle.

The evidence, therefore, points to the conclusion that the oculomotor nucleus is a multiple nucleus, each part of which gives origin to one of the nerves of one of the eye-muscles.

Edinger says that such an array of clinical observations exists, {265}and of facts derived from post-mortem dissections, that one may venture to designate the portion of the nucleus from which the innervation of each individual ocular muscle comes. He gives Starr's table, the latest of these numerous attempts, begun by Pick. According to Starr, the nuclei of the nerves to the individual muscles are arranged from before backward, thus--

m. sphincter iridis. m. ciliaris. m. levator palpebræ. m. rectus internus. m. rectus superior. m. rectus inferior. m. obliquus inferior.

Further, the evidence of the well-known physiological experiments of Hensen and Völckers that the terminal branches of the oculomotor nerve arise from a series of segments of the nucleus, arranged more or less one behind the other in a longitudinal row, leads them to the conclusion that the nuclei of origin are arranged as follows, proceeding from head to tail:--

Nearest brain. 1. m. ciliaris. 2. m. sphincter iridis. 3. m. rectus internus. 4. m. rectus superior. 5. m. levator palpebræ. 6. m. rectus inferior. Most posterior. 7. m. obliquus inferior.

It is instructive to compare this arrangement of Hensen and Völckers with the arrangement of the origin of these muscles from the premandibular cavity as given by Miss Platt.

Thus she states that the most posterior part of the premandibular cavity is cut off so as to form a separate cavity, resembling, except in position, the anterior cavity; this separate, most posterior part gives origin to the inferior oblique muscle. She then goes on to describe how the dorsal wall of the remainder of the premandibular cavity becomes thickened, to form posteriorly the rudiment of the inferior rectus and anteriorly the rudiments of the superior and internal recti, a slight depression in the wall of the cavity separating these rudiments. The internal rectus is the more median of the two anterior muscles. In other words, her evidence points not only to a fusion of somites to form the premandibular cavity, but also to the arrangement of these somites as follows, from head to tail: (1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior {266}oblique--an order precisely the same as that of Hensen and Völckers, and of Starr.

I conclude, from the agreement between the anatomical, physiological, and morphological evidence, that the IIIrd and IVth nerves contain the motor somatic nerves belonging to the same segments as the motor trigeminal, in other words, to the prosomatic segments, so that the eye-muscles, innervated by III. and IV., represent segmental muscles belonging to the prosoma. Further, I conclude that originally there were seven prosomatic segments, the first of which is represented by the anterior cavity described by Miss Platt, and does not form any permanent muscles; that the next four belong to the premandibular cavity, and the muscles formed are the superior rectus, internal rectus, inferior rectus, and inferior oblique; and that the last two belong to the mandibular cavity, the muscles formed being Miss Platt's mandibular muscle and the superior oblique. It is, to say the least of it, a striking coincidence that such an arrangement of the coelomic cavities as here given should be so closely mimicked by the arrangement in the prosomatic region of Limulus as already mentioned; it suggests inevitably that the head-cavities of the vertebrate are nothing more than the prosomatic and mesosomatic segmental coelomic cavities, as found in animals such as Limulus. In the table on p. 253, I have inserted the segments in the vertebrate for comparison with those of Limulus.

Before we can come to any conclusion as to the original position of these eye-muscles, it is necessary to consider the VIth nerve and the external rectus muscle. This nerve and this muscle belong to van Wijhe's 4th segment. The muscle is, therefore, the somatic segmental muscle belonging to the same segment as the facial and is, in fact, a segmental muscle belonging not to the prosoma, but to the mesosoma. Neal comes to the conclusion that the existing abducens is the only root which remains permanent among a whole series of corresponding ventral roots belonging to the opisthotic segments, and further points out that the external rectus was originally an opisthotic muscle which has taken up a pro-otic position, or, translating this statement into the language of Limulus, etc., it is a mesosomatic muscle which has taken up a prosomatic position.

There is, however, another muscle--the Retractor oculi--belonging to the same group which is innervated by the VIth nerve. Quite recently Edgeworth has shown that in birds and reptiles this muscle {267}belongs to the hyoid segment; so that in this respect also the hyoid segment proclaims its double nature.

With respect to the external rectus muscle, Miss Platt has shown that the mandibular muscle is formed close alongside the external rectus, so that the two are in close relationship as long as the former exists.

Further, as already mentioned, the eye-muscles in Ammocoetes must be considered by themselves; they do not belong in structure or position to the longitudinal somatic muscles innervated by the spinal nerves; their structure is not the same as that of the tubular constrictor or branchial muscles, but resembles that structure somewhat; their position is dorso-ventral rather than longitudinal; they may be looked upon as a primitive type of somatic muscles segmentally arranged, the direction of which was dorso-ventral.

Anderson also has shown that the time of medullation of the nerves supplying these muscles is much earlier than that of the nerves belonging to the somatic trunk-muscles, their medullation taking place at the same time as that of the motor nerves supplying the striated visceral muscles; and Sherrington has observed that these muscles do not possess muscle-spindles, while all somatic trunk-muscles do. Both these observations are strong confirmation of the view that the eye-muscles must be classified in a different category to the ordinary somatic trunk muscle group.

What, then, is the interpretation of these various embryological and anatomical facts?

Remembering the tripartite division of each segmental nerve-group in Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-nerve, and (3) ventral somatic nerve, I venture to suggest that the three nerves--the oculomotorius, the trochlearis, and the abducens--represent the ventral somatic nerves of the prosoma, and partly also of the mesosoma; that they are nerves, therefore, which may have originally contained sensory fibres, and which still contain the sensory fibres of the eye-muscles themselves, as stated by Sherrington. According to this suggestion, the eye-muscles are the sole survivors of the segmental dorso-ventral somatic muscles, so characteristic of the group from which I imagine the vertebrates to have sprung. In the mesosomatic region the dorso-ventral muscles which were retained were those of the appendages and not of the mesosoma itself, because the presumed ancestor breathed after the fashion of the water-breathing Limulus, by means of the dorso-ventral muscles of its {268}branchial appendages, and not after the fashion of the air-breathing scorpion, by means of the dorso-ventral muscles of the mesosoma. The only mesosomatic dorso-ventral muscles which were retained were those of the foremost mesosomatic segments, i.e. those supplied by the VIth nerve, which were preserved owing to their having taken on a prosomatic position and become utilized to assist in the movements of the lateral eyes.

Let us turn now to the consideration of the corresponding musculature in Limulus and in the scorpion group. These muscles constitute the markedly segmental muscles to which I have given the name 'dorso-ventral somatic muscles.' They are most markedly segmental in the mesosomatic region, both in Limulus and in Scorpio, each mesosomatic segment possessing a single pair of these vertical mesosomatic muscles, as Benham calls them (cf. Fig. 58 (Dv.)). In the prosomatic region the corresponding muscles are not so clearly defined in Limulus; they are apparently attached to the plastron forming the group of plastro-tergal muscles. From Benham's description it is sufficiently evident that they formed originally a single pair to each prosomatic segment.

In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles are situated near the middle line on each side and form the following well-marked series of pairs of muscles, shown in Fig. 110, A, taken from her paper, and thus described by her:--

1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the dorso-ventral muscles. It is very small, and is attached to the carapace near the median line anteriorly to the central eyes.

2. The median dorso-preoral-entosclerite muscle (62) is a large muscle, between which and its fellow of the opposite side the eyes are situated. It is attached dorsally to the carapace and ventrally to the pre-oral entosclerite.

3. The anterior dorso-plastron muscle (63) is attached dorsally to the carapace in the middle line, being joined to its fellow of the opposite side. They separate, and are attached ventrally to the plastron. Through the arch thus formed the alimentary canal and the dorsal vessel pass.

4. The median dorso-plastron muscle (64) is attached dorsally to the posterior part of the carapace. It runs forward on the anterior surface of the posterior flap of the plastron to the body of the plastron, to which it is attached.

{269}[Illustration] A.

DORSO-VENTRAL MUSCLES ON CARAPACE OF SCORPION. (From MISS BECK.)

SIMILAR MUSCLES ON CARAPACE OF EURYPTERUS.

SIMILAR MUSCLES ON HEAD-SHIELD OF A CEPHALASPID.

l.e., lateral eyes; c.e., central eyes; Fro., narial opening.

62-65 refer to Miss Beck's catalogue of the scorpion muscles.

FIG. 110.

{270}To these may be added, owing to its attachment to the plastron,

5. The posterior dorso-plastron muscle (65). This is the first of the dorso-ventral muscles attached to the mesosomatic tergites, being attached to the tergite of the first segment of the mesosoma.

This muscle is of interest, in connection with the prosomatic dorso-ventral muscles, because it is attached to the plastron, and runs a course in close contact with the muscle (64), the two muscles being attached dorsally close together, on each side of the middle line, the one at the very posterior edge of the prosomatic carapace, and the other at the very anterior edge of the mesosomatic carapace.

Taking these muscles separately into consideration, it may be remarked with respect to (61) that the cheliceral segment in its paired dorso-ventral muscles, as in its tergo-coxal muscles, takes up a separate position isolated from the rest of the prosomatic segments.

Next comes (62) the median dorso-preoral-entosclerite muscle, which is strikingly different from all the other dorso-ventral muscles in its large size and the extent of its attachment to the dorsal carapace, according to Miss Beck's figures. The reason of its large size is clearly seen upon dissection of the muscles in Buthus, for I find that, strictly speaking, it is not a single muscle, but is composed of a series of muscle-bundles, separated from each other by connective tissue. There are certainly three separate muscles included in this large muscle, which are attached in a distinct series along the pre-oral entosclerite, and present the appearance given in Fig. 110, A, at their attachment to the prosomatic carapace. Of this muscle-group the most anterior and the most posterior bundle are distinctly separate muscles; I am not, however, clear whether the middle bundle represents one or two muscles.

This division of Miss Beck's muscle (62) into three or four muscles brings the prosomatic region of the scorpion into line with the mesosomatic, and enables us to feel sure that a single pair of dorso-ventral somatic muscles belongs to each prosomatic segment just as to each mesosomatic, and, conversely, that each such single pair of muscles possesses segmental value in this region as much as in the mesosomatic.

It is very striking to see how in all the Scorpionidæ, in which the two median eyes are the principal eyes, this muscle group (62) on the two sides closely surrounds these two eyes, so that with a fixed {271}pre-oral entosclerite, a slight movement of the eyes, laterally or anteriorly, owing to the flexibility of the carapace, might result as the consequence of their contraction. But this cannot be the main object of these muscles. The pre-oral entosclerite is firmly fixed to the camerostome, as is seen in Fig. 94, pr. ent., so that the main object of these muscles is, as Huxley has pointed out, the movement of this organ.

In order to avoid repetition of the long name given to this muscle group (62) by Miss Beck, because of their position, and for other reasons which will appear in the sequel, I will call this group of muscles the group of recti muscles. These recti muscles belong clearly to the segments posterior to the first prosomatic or cheliceral segment, and represent certainly three, probably four, of these segments, i.e. belong to the segments corresponding to the second, third, fourth, and fifth prosomatic locomotor appendages--the endognaths of the old Eurypterids.

The next pair of muscles is the pair of anterior dorso-plastron muscles (63). This muscle-pair evidently belongs to a segment posterior to the segments represented by the group already discussed, and belongs, therefore, in all probability to the same segment as the sixth pair of prosomatic appendages--the ectognaths of the old Eurypterids. This can be settled by considering either the nerve-supply or the embryological development. In the Eurypteridæ it seems most highly probable that the dorso-ventral muscles of each half of the segments belonging to the endognaths should be compressed together and separate from the dorso-ventral muscle belonging to the ectognathal segment, on account of the evident concentration and small size of the endognathal segments in contradistinction to the separateness and large size of the ectognathal segment.

The striking peculiarity of this muscle-pair, which distinguishes it from all other muscles in the scorpion, is the common attachment of the muscles of the two sides in the mid-dorsal line, so that the pair of muscles forms an arch through which the alimentary canal and dorsal blood-vessel pass.

The same dorso-ventral muscles are present in Phrynus, and in this animal the fibres of this pair of muscles (63) actually interlace before the attachment to the prosomatic carapace, so that the attachment of the muscle on each side overpasses the mid-dorsal line, and a true crossing occurs. In Fig. 108 the position of this pair of {272}muscles is shown just posteriorly to the brain-mass. This muscle I will call the oblique muscle.

Finally we come to the muscles (64) and (65), the median and posterior dorso-plastron muscles, which run close together. Both muscles are attached to the plastron, and, therefore, to that extent belong to the prosomatic region; they are attached dorsally close to the junction of the prosoma and mesosoma. This position of the first mesosomatic dorso-ventral muscle belonging to the opercular segment may be compared with the position of the first mesosomatic dorso-ventral muscle in Limulus which has become attached to the prosomatic carapace; in both cases we see an indication that the foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up a prosomatic position.

As to the pair of small muscles (64), I believe that they represent the dorso-ventral muscles of the seventh prosomatic segment (if the pair of muscles (63) belongs to the segment of the sixth locomotor prosomatic appendages), i.e. they belong to the chilarial segment or metastoma.

I desire to draw especial attention to the fact that the dorso-ventral muscle (64), which represents the seventh segment, always runs close alongside the dorso-ventral muscle (65), which represents the first mesosomatic or opercular segment.

The comparison, then, of these two sets of facts leads to the following conclusions:--

The foremost prosomatic or trigeminal segment stood separate and apart, being situated most anteriorly; the musculature of this segment does not develop, so that the only evidence of its presence is given by the anterior coelomic cavity. This corresponds, according to my scheme, with the first or anterior coelomic cavity of Limulus, and therefore represents, as far as the prosomatic appendages are concerned, the first prosomatic appendage-pair, or the cheliceræ; the appendage-muscles being the muscles of the cheliceræ, and the dorso-ventral somatic muscles the pair of dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of muscles, therefore, dwindle and disappear in the vertebrate.

Then came four segments fused together to form the premandibular segment, the characteristic of which is the apparent non-formation of any permanent musculature from the ventral mesoderm-segments, and the formation of the eye-muscles innervated by the {273}oculomotor nerve from the dorsal mesoderm segments. These four segments have been so fused together that van Wijhe looked upon them as a single segment, and the premandibular cavity as the cavity of a single segment. They represent, according to my scheme, the segments belonging to the endognaths, i.e. the second, third, fourth, fifth pairs of prosomatic appendages; the premandibular cavity, therefore, represents the second coelomic cavity in Limulus, which, according to Kishinouye, is the sole representative of the coelomic cavities of the second, third, fourth, fifth prosomatic segments. The muscles derived from the ventral mesoderm-segments represent the muscles of these appendages, which therefore dwindle and disappear in the vertebrate, with the possible exception of the muscles innervated by the descending root of the trigeminal. The muscles derived from the dorsal mesoderm-segments, i.e. the eye-muscles supplied by the oculomotor nerve, represent the dorso-ventral somatic muscles of these four segments, muscles which are represented in the scorpion by the recti group of muscles, i.e. the median dorso-preoral-entosclerite muscles (62).

Then came two segments, the mandibular, in which muscles are formed both from the ventral and from the dorsal mesoderm-segments. From the former arose the main mass of muscles innervated by the motor root of the trigeminal, from the latter the superior oblique muscle and the mandibular muscle of Miss Platt, of which the former alone survives in the adult condition. These two segments are looked upon as a single segment by van Wijhe, of which the mandibular cavity is the coelomic cavity. They represent, according to my scheme, the segments belonging to the sixth pair of prosomatic appendages or ectognaths, and the seventh pair, i.e. the chilaria or metastoma.

The first part, then, of the mandibular cavity represents the third coelomic cavity in Limulus and the muscles derived from the ventral mesoderm, in all probability the muscles of the tongue in the lamprey (cf. Chap. IX.), which represents the ectognaths or sixth pair of prosomatic appendages, while the muscles derived from the dorsal mesoderm, i.e. the superior oblique muscles, represent the dorso-ventral somatic muscles of this segment, muscles which are represented in the scorpion group by the pair of anterior dorso-plastron or oblique muscles (63).

The second part of the mandibular cavity represents the 4th {274}coelomic cavity in Limulus and the muscles derived from the ventral mesoderm, in all probability the muscles of the lower lip in the lamprey (cf. Chap. IX.), which represents the metastoma; while the muscles derived from the dorsal mesoderm, i.e. Miss Platt's pair of mandibular muscles, represent the dorso-ventral somatic muscles of this segment, muscles which are represented in the scorpion group by the pair of median dorso-plastron muscles (64).

In connection with this last pair of muscles we find that the external rectus in the vertebrate represents the first dorso-ventral mesosomatic muscle in the scorpion, i.e. the posterior dorso-plastron muscle (65), and, as already mentioned (p. 267), that it always lies closely alongside the mandibular muscle, just as in the scorpion group muscle (65) always lies alongside muscle (64).

In the invertebrate as well as in the vertebrate this muscle is a mesosomatic muscle which has taken up a prosomatic position.

The question naturally arises, what explanation can be given of the fact that these dorso-ventral muscles attached on each side of the mid-dorsal line to the prosomatic carapace became converted into the muscles moving the eyeballs of the two lateral eyes? An explanation which must take into account not only the isolated position of the abducens nerve, but also the extraordinary course of the trochlearis. The natural and straightforward answer to this question appears to me quite satisfactory, and I therefore venture to commend it to my readers.

I have argued the case out to myself as follows: The lateral eyes must have been originally situated externally to the group of muscles innervated by the oculomotor nerve, for a sheet of muscle representing the superior internal and inferior rectus muscles could only wrap round the internal surface of each lateral eye; i.e. the arrangement of the muscle-sheet, as in the scorpion, about two median eyes, is in the wrong position, for if those two eyes, which are the main eyes in the scorpion, were to move outwards to become two lateral eyes, then such a muscle-group would form a superior external and inferior rectus group. The evidence, however, of Eurypterus and similar forms is to the effect that the lateral eyes became big and the median eyes insignificant and degenerate. If, then, with the degeneration of the one and the increasing importance of the other, these lateral eyes came near the middle line, then the muscular group (62), which I have called the recti group, would naturally be pressed into their {275}service, and would form an internal and not an external group of eye-muscles.

In Fig. 110, A, taken from Miss Beck's paper, I have shown the relative position of the eyes and the segmental dorso-ventral prosomatic muscles on the carapace of the scorpion. In Fig. 110, B, I have drawn the prosomatic carapace of Eurypterus Scouleri, taken from Woodward's paper, with the eyes as represented there; in this I have inserted the segmental dorso-ventral muscles as met with in the scorpion, thereby demonstrating how, with the degeneration of the median eyes and the large size of the lateral eyes, the recti muscles of the scorpion would approach the position of an internal recti group to the lateral eyes, and so give origin to the group of muscles innervated by the oculomotor nerve. In the Eurypterus these large eyes are large single eyes, not separate ocelli, as in the scorpion.

All, then, that is required is that in the first formed fishes, which still possessed the dorso-ventral muscles of their Eurypterid ancestors, the lateral eyes should be the important organs of sight, large and near the mid-dorsal line. Such, indeed, is found to be the case. In amongst the masses of Eurypterids found in the upper Silurian deposits at Oesel, as described by Rohon, numbers of the most ancient forms of fish are found belonging to the genera Thyestes and Tremataspis. The nature of the dorsal head-shields of these fishes is shown in Fig. 14, which represents the dorsal head-shield of Thyestes verrucosus, and Fig. 111 that of Tremataspis Mickwitzi. They show how the two lateral eyes were situated close on each side of the mid-dorsal line in these Eurypterus-like fishes, in the very position where they must have been if the eye-muscles were derived from the dorso-ventral somatic muscles of a Eurypterid ancestor.

Fro., narial opening; l.e., lateral eyes; gl., glabellum plate over brain; Occ., occipital spine.]

In Lankester's words, one of the characteristics of the Osteostraci (Cephalaspis, Auchenaspis, etc.), as distinguished from the Heterostraci (Pteraspis), are the large orbits placed near the centre of the shield. The apparent exception of Thyestes mentioned by him is no {276}exception, for orbits of the same character have since been discovered, as is seen in Rohon's figure (Fig. 14). In Fig. 110, C, I give an outline of the frontal part of the head-shield of a Cephalaspid, in which I have drawn the eye-muscles as in the other two figures.

Although all the members of the Osteostraci possess large lateral eyes towards the centre of the head-shield, the other group of ancient fishes, the Heterostraci, are characterized by the presence of lateral eyes far apart, situated on the margin of the head-shield on each side (cf. Fig. 142, o, p. 350).

So, also, on the invertebrate side, the lateral eyes of Pterygotus and Slimonia are situated on the margin of the prosomatic carapace, while those of Eurypterus and Stylonurus are situated much nearer the middle line of the prosomatic carapace.

Next comes the question of the superior oblique muscle and the trochlearis nerve. Why does this nerve (n.IV. in Fig. 106, C and D) alone of all the nerves in the body take the peculiar position it always does take? The only suggestion that I know of which sounds reasonable and worth consideration is that put forward by Fürbringer, which is an elaboration of the original suggestion of Hoffmann. Hoffmann suggested in 1889 that the trochlearis nerve represented originally a nerve for a protecting organ of the pineal eye, which became secondarily a motor nerve for the lateral eye as the pineal eye degenerated. Fürbringer differs from Hoffmann in that he considers that the nerve was originally a motor nerve, and was not transformed from sensory to motor, yet thinks Hoffmann's suggestion is in the right direction.

He points out that the crossing of the trochlearis is not a crossing of fibres between two centres in the central nervous system, but may be explained by the shifting of the peripheral organ, i.e. the muscle, from one side to the other, and the nerve following this shift. Consequently, says Fürbringer, the course of the nerve indicates the original position of the muscle, and therefore he imagines that the ancestor of the superior oblique muscle was a muscle the fibres of which were attached in the mid-dorsal line, and interlaced with those of the other side, the two muscles thus forming an arch through which the nervous system with its central canal passed. Then, for the sake of getting a more efficient pull, the crossing muscle-fibres became more definitely attached to the opposite side of the middle line, and finally obtained a new attachment on the opposite side, with the {277}obliteration of the muscular arch; the nerve on each side, following the shifts of the muscle, naturally took up the position of the original muscular arch, and so formed the trochlear nerve, with its dorsal crossing. This explanation of Fürbringer's was associated by him with movements of the median pineal eyes, the length of their nerve, according to him, even yet indicating their previous mobility. This assumption is not, it seems to me, necessary. The length of the nerve is certainly no indication of mobility, for in Limulus and the scorpion group the nerve to each median eye is remarkably long, yet these eyes are immovably fixed in the carapace. All that is required is a pair of dorso-ventral muscles belonging to the segment immediately following the group of segments represented by the oculomotor nerves, the fibres of which should cross the mid-dorsal line at their attachment; for, seeing that the lateral eyes were originally so near this position, it follows that such muscles might form part of the muscular group belonging to the lateral eye without having previously moved the pineal eyes. In fact, Fürbringer's explanation requires as starting-point that the pair of muscles which ultimately become the superior oblique should have the exact position of the pair of dorso-ventral muscles in the scorpion, called by Miss Beck the anterior dorso-plastron muscles (63), which I have named the oblique muscles. Here, and here only, do we find an interlacement, across the mid-dorsal line, of the fibres of attachment of the muscles on the two sides, in consequence of which this pair of muscles is described by her as forming an arch encircling the alimentary canal and dorsal vessel. If, then, as I have previously argued, the primitive plastron formed a pair of trabeculæ, and the nervous system grew round the alimentary canal, such an arch would encircle the tubular central nervous system of the vertebrate.

Still more striking is this pair of muscles (63) in Phrynus (Fig. 108), where we see how the arch formed by them almost touches the posterior extremity of the supra-oesophageal brain-mass, crossing, therefore, over the beginning of the stomach region of the animal. The angle formed by the arch is much more obtuse than that formed in Scorpio, so that an actual crossing of the muscle-fibres has taken place at the point of attachment to the carapace. Also, only the part nearest the carapace is muscular, the rest forming a long tendinous prolongation of the plastron wall (the primordial cranium), as seen in the figure.

{278}This muscle-pair is, as it should be, the pair of dorso-ventral muscles belonging to the segment immediately following on the group of segments represented by the recti muscles, i.e. according to previous argument, the segment belonging to the sixth pair of locomotor appendages or ectognaths; a muscle, therefore, which would arise in the vertebrate from the mandibular, and not from the premandibular cavity. A similar muscle probably existed in Eurypterus (M.obl. in Fig. 106, B), and, as in the case of the formation of the oculomotor group, derived from the recti group of the scorpion, would form the commencement of the superior oblique muscle in Thyestes and Tremataspis.

l.e., lateral eyes; c.e., central eyes; C.N., central nervous system; Al., alimentary canal; c., aqueductus Sylvii.]

It is instructive to notice that the original position of attachment of this muscle is naturally posterior to that of the oculomotor group of muscles, and that Fürbringer, in his description of the eye-muscles of Petromyzon, asserts that this muscle in this primitive vertebrate {279}form is not attached as in other vertebrates, but is posterior to the other muscles, so that he calls it the posterior rather than the superior oblique. The nature of the change by which the muscle known in the scorpion as the anterior dorso-plastron muscle (63) was probably converted into the superior oblique muscle of the vertebrate, is represented in the drawings Fig. 112, in which also are indicated the dwindling of the median eyes, and the progressive superiority of the lateral eyes, as well as the transformation of the recti muscle-group of the scorpion into the muscles supplied by the oculomotor nerve of the vertebrate.

With respect to the external rectus muscle, it follows naturally that if the muscles (64) and (65) are to follow suit with the rest of the group and become attached to the lateral eyes, they must take up an external position. These two muscles, which always run together, as seen in Fig. 110, A, the one belonging to the prosoma and the other to the mesosoma, are represented by the mandibular muscle of Miss Platt and the external rectus, the former derived from the walls of the last pro-otic head-cavity, the latter from the foremost of the opisthotic head-cavities.

Such, then, is the simple explanation of the origin of the eye-muscles which follows from my theory, and we see that the successive alterations of the position of the orbit, and, therefore, of the globe of the eye with its muscles, as we pass from Thyestes to man, is the natural consequence of the growth of the frontal bone, i.e. of the brain.

THE TRIGEMINAL NERVES AND THE MUSCLES SUPPLIED BY THEM.

Turning now to the evidence as to the number of ventral segments, i.e. the motor and sensory supply to the prosomatic appendages afforded by the trigeminal nerve, we must, I think, come to the same conclusion as Dohrn, viz. that if there were originally seven dorsal or somatic segments in this region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles of the premandibular cavity, sup. rectus, inf. rectus, int. rectus, inf. oblique, supplied by IIIrd nerve; 6, 7, muscles of the mandibular cavity, sup. oblique, supplied by IVth nerve and muscle lost, there must have been also seven corresponding ventral or splanchnic segments supplied by the trigeminal. At present the evidence for such segments is nothing like so strong as for the corresponding somatic ones; there are, however, certain suggestive {280}facts which point distinctly in this direction in connection with both the motor and sensory parts of the trigeminal. The origin of the trigeminal motor fibres in the central nervous system is most striking. We may take it for granted that a nucleus of cells giving origin to one or more segmental motor nerves will possess a greater or less longitudinal extension in the central nervous system, according to the number of fused separate segmental centres it represents. Thus a nucleus such as that of the IVth nerve or of the facial is small and compact in comparison to the extensive conjoint nucleus of the vagus and cranial accessory.

Upon examination of the motor nucleus of the trigeminal, we find a compact or well-defined nucleus, the nucl. masticatorius, the nerves of which supply the masseter, temporal, and other muscles, so that the anatomical evidence at first sight appears to bear out van Wijhe's conclusion that the motor trigeminal supplies at most two segments. Further examination, however, shows that this is not all, for the extraordinary so-called descending root of the Vth must be taken into consideration in any question of the origin of the motor elements, just as the equally striking ascending root enters into the consideration of the meaning of the sensory elements of the Vth.

It is not necessary here to discuss the controversy as to whether this descending root is motor or sensory. It is universally considered at present to be motor, and is believed to supply, as Kölliker suggested, among other muscles, the m. tensor tympani and the m. tensor veli palati. It is thus described by Obersteiner--

"From the region of the mid-brain the motor root receives an important addition of thick fibres, which form the cerebral or descending root. The large, round vesicular cells from which the fibres of the descending root arise form no single compact group, but are partly single, partly arranged like little bunches of grapes, as far as the region of the anterior corpora quadrigemina. The further we go brainwards, the smaller is the number of fibres. In the region of the anterior corpora quadrigemina, the few cells of origin are found more and more median; so that the uppermost trigeminal fibres descend in curves almost from the mid-line, as is shown by the exceptional occurrence of one or more of the characteristic cells above the aqueduct. At the height of the posterior commissure one finds the last of these trigeminal cells."

{281}The anatomy of the Vth nerve reveals, then, three most striking facts:--

1. The motor nucleus of the Vth extends from the very commencement of the infra-infundibular region to nearly the commencement of the nucleus of the VIIth; in other words, the motor nucleus of the Vth extends through the whole prosomatic region, just as it must have done originally if its motor nerves supplied the muscles of the prosomatic appendages. Such an extended range of origin is indicative of the remains of an equally extended series of segmental centres or ganglia.

2. Of these centres the caudalmost have alone remained large and vigorous, constituting the nucleus masticatorius, which in the fish is divided into an anterior and posterior group, thus indicating a double rather than a single nucleus; while the foremost ones have dwindled away until they are represented only by the cells of the descending root, the muscles of these segments being still represented by possibly the tensor veli palati and the other muscles innervated from these cells.

3. The headmost of these cells takes up actually a position dorso-lateral to the central canal, so that the groups on each side nearly come together in the mid-dorsal line; a very unique and extraordinary position for a motor cell-group, but not improbable when we recall to mind Brauer's assertion as to the shifting of the foremost prosomatic ganglion-cells of the scorpion from the ventral to the dorsal side of the alimentary canal.

On the sensory side the evidence is also suggestive, the question here being not so much the distribution of the sensory nerves as the number of ganglia belonging to each of the cranial nerves.

With respect to this question, morphologists have come to the conclusion that there is a marked difference between spinal and cranial nerves, in that whereas the posterior root-ganglia of the spinal nerves arise from the central nervous system itself, i.e. from the neural crest, the ganglia of the cranial nerves arise partly from the neural crest, partly from the proliferation of cells on the surface of the animal; and because of the situation of these proliferating epidermal patches over the gill-clefts in the case of the vagus and glossopharyngeal nerves, they have been called by Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia into two sets, one connected with the neural ridges, called the neural ganglia, {282}and the other connected with the surface-cells, which he calls the lateral ganglia. This second set corresponds to Kupffer's epibranchial ganglia. Now it is clear that in the case of the vagus nerve, where, as is well shown in Ammocoetes, the nerve is not a single segmental nerve, but is in reality made up of a number of nerves going to separate branchial segments, the indication of such segments is not given by the main vagus ganglion or neural ganglion, but by the series of lateral ganglia. So also it is argued in the case of the trigeminal, that if in addition to the ganglion-cells arising from the neural crest separate ganglion-masses are found in the course of development, in connection with proliferating patches of the surface (plakodes, Kupffer calls them), then such isolated lateral ganglia are indications of separate segments, just as in the case of the vagus, even though the separate segments do not show themselves in the adult. So far the argument appears to me just, but the further conclusion that the presence of such plakodes shows the previous existence of branchial sense-organs, and, therefore, that such ganglia are epibranchial ganglia, indicating the position of a lost gill-slit, is not justified by the premises. If, as I suppose, the trigeminal nerve supplied a series of non-branchial appendages serially homologous with the branchial appendages supplied by the vagus, then it is highly probable that the trigeminal should behave with respect to its sensory ganglia similarly to the vagus nerve, without having anything to do with branchiæ.

Such plakodal ganglia, then, may give valuable indication of non-branchial segments as well as of branchial segments. The researches of Kupffer on the formation of the trigeminal ganglia in Ammocoetes are the chief attempt to find out from the side of the sensory ganglia the number of segments originally belonging to the trigeminal. The nature and result of these researches is described in my previous paper (Journal of Anatomy and Physiology, vol. xxxiv.), and it will suffice here to state that he himself concludes that the trigeminal originally supplied five at least, probably six, segments. As I have stated there, the evidence as given by him seems to me to indicate even as many as seven segments.

In the full-grown Ammocoetes, as is well known, there are two distinct ganglia belonging to the trigeminal, the one the ganglion of the ramus ophthalmicus, the other the main ganglion.

According to Kupffer the larval Ammocoetes possesses three sets of ganglia, not two, for between the foremost and hindmost ganglion {283}he describes a nerve (x., Fig. 113), with four epibranchial ganglia, which do not persist as separate ganglia, but either disappear or are absorbed into the two main ganglia (Fig. 113). This discovery of Kupffer's is very suggestive, for, as already stated, a transformation takes place when the Ammocoetes is 5 mm. long, so that the arrangement of the parts before that period is distinctly more indicative of the ancestral arrangement than any later one.

If we use the name plakodal ganglia to represent that part of these ganglia which was originally connected with the skin, then Kupffer's researches assert that in the larval Ammocoetes there were seven such plakodal ganglia, one in front belonging to the foremost trigeminal ganglion, two behind, parts of the hindmost ganglion, and four in between, which do not exist later as separate ganglia.

A-B, the line of epibranchial ganglia; au., auditory capsule; nc., notochord; Hy., tube of hypophysis; Or., oral cavity; u.l., upper lip; l.l. lower lip; vel., septum between oral and respiratory cavities; V., VII., IX., X., cranial nerves; x., nerve with four epibranchial ganglia.]

In accordance with the views put forward in this book, a possible interpretation of these plakodal ganglia would be given as follows:--

Beard, who, after Froriep, drew attention to this relation of the cranial ganglia to special skin-patches, has compared them with the parapodial ganglia of annelids, i.e. ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry in {284}vertebrate embryology (as has been so often asserted to be the case), seeing that undoubtedly the Arthropoda are an advanced stage of Annelida; and, indeed, the way is not a long one when we consider Beecher's evidence that the Trilobita belong to the Phyllopoda, certainly a primitive crustacean group, which Bernard derives directly from the annelid group Chætopoda. If, then, these plakodal ganglia indicate the former presence of appendages, we obtain this result:--The foremost ganglion on each side possesses one plakodal ganglion, and therefore indicates an anterior pair of appendages, possibly the cheliceræ. Then comes the peculiar nerve with four plakodal ganglia indicating on each side four appendages close together, possibly the endognaths. Then, finally, on each side, the second large ganglion with two plakodal ganglia, indicating two pairs of appendages, possibly the ectognaths and the metastoma.

SUMMARY.

The consideration of the history of the cranial segmentation shows that whereas, from the commencement of that history, the evidence for two ventral segments supplied by the trigeminal nerve is clear and unmistakable, later observers have tended more and more to increase the number of these segments, until at the present time the evidence is in favour of at least six, probably seven, as the number of segments supplied by the motor part of the trigeminal.

So, also, the original evidence for the number of dorsal or somatic segments limits the number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles which these three nerves supply are derived respectively from the walls of the premandibular, mandibular, and hyoid coelomic cavities.

Later evidence points strongly to the conclusion that the oculomotor nerve and the premandibular cavity represent not one segment but the fusion of four, while the mandibular cavity represents two segments. In addition to these, Miss Platt has discovered a still more anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental group, the only representative of which at present known in the mesosomatic region is the external rectus innervated by the VIth nerve.

These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities and muscles of the invertebrate to an extraordinary {285}degree, so that it becomes easy to see how the dorso-ventral muscles of the prosomatic segments of the latter have become converted into the eye-musculature of the former. The most powerful proof of all that such a conversion has taken place is that a natural and simple explanation is at once given of the extraordinary course taken by the IVth or trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens, each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For generations anatomists have wondered and found no solution, and yet, without any straining of hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair of muscles in the scorpion group, the solution is immediately apparent.

This pair of muscles alone, of all the musculature attached to the carapace, crosses the mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other side; by a continuation of the same process the relation of the trochlear to the superior oblique muscle can be explained.

The comparison of the eye-muscles of the vertebrate with the dorso-ventral segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much clearer.

{286}CHAPTER IX

THE PROSOMATIC SEGMENTS OF AMMOCOETES

The prosomatic region in Ammocoetes.--The suctorial apparatus of the adult Petromyzon.--Its origin in Ammocoetes.--Its derivation from appendages.--The segment of the lower lip or metastomal segment.--The tentacular segments.--The tubular muscles.--Their segmental arrangement.--Their peculiar innervation.--Their correspondence with the system of veno-pericardial muscles in Limulus.--The old mouth or palæostoma.--The pituitary gland.--Its comparison with the coxal gland of Limulus.--Summary.

In the last chapter it was seen not to be incompatible with both the anatomical and morphological evidence to look upon the trigeminal nerves as having originally supplied the seven prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ, and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their presence the nucleus masticatorius. Evidence in favour of this suggestion and of the nature of the dwindling process is afforded when we examine what the trigeminus does supply in Ammocoetes. In all vertebrates this nerve supplies the great muscles of mastication which, in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws; they take in their food by attaching themselves to their prey and by means of rasping teeth situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus, they suck the juices of the fish they feed upon. Not possessing jaws, they feed by suction on the living animal, a method of feeding which gives them no more claim to be classed as parasitic animals than the whole group of spiders which feed in a similar manner on living flies.

{287}THE ORIGIN OF THE SUCTORIAL APPARATUS OF PETROMYZON.

This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here in its muscular arrangements any original segmental arrangement of the muscles of mastication might be expected to be visible. It consists of a large rod or piston, to which are attached powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in producing a vacuum, and annular muscles around the circular lip.

Turn now to the full-grown larval form, Ammocoetes, an animal in the case of Petromyzon Planeri as large as the full-grown Petromyzon, and seek for this musculature. There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned, an oral chamber bounded by the lower and upper lips and the remains of the septum between it and the respiratory chamber--the velar folds. Attached to its walls a number of tentacles are situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a massive suctorial apparatus.

In order, then, to understand the origin of the muscles of mastication, it is necessary to study the changes which occur at transformation, and thus to find out how the suctorial apparatus of the adult arises. This most important investigation has been undertaken by Miss Alcock, and owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series in the transformation process than has ever been obtained before. Miss Alcock has not yet published her researches, but has allowed me to make use of some of her facts.

An enormous proliferation of muscular tissue takes place with great rapidity during this transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial apparatus. The starting point of this proliferation can be traced back in all cases to little groups of embryonic tissue found below the epithelial lining of the oral chamber in Ammocoetes. Of these groups the most conspicuous one is situated at the base of the large median ventral tentacles. Others are situated at the base of the tentacular ridge. Further, although this extraordinary change takes place in the {288}peripheral organ, no marked difference occurs in the arrangement of the nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding with the proliferation of the muscular elements.

The clue, then, to the origin of the suctorial apparatus and of the nature of the original organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of government, here is 'headquarters,' from which the answers to all our inquiries must originate.

ps. br., pseudo-branchial groove; met., nerve to lower lip, or metastomal nerve; t., nerve to tongue; tent., nerve to tentacles. The mandibular and internal maxillary nerves are coloured red; the purely sensory nerves to the external surface are coloured black.]

THE TRIGEMINAL NERVE OF AMMOCOETES.

Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure; the other two nerves, both {289}of which contain motor fibres, are called by Hatschek the mandibular and maxillary nerves. Of these the mandibular or velar nerve (met.) is a large, conspicuous nerve, which arises so separately from the rest of the trigeminal as almost to deserve the title of a separate nerve. When it leaves the large posterior ganglion, it passes into the anterior part of the velum, runs along with the tubular muscles, which it supplies, to the ventral surface as far as the junction of the lower lip with the thyroid plate, and has not been followed further by Hatschek. Miss Alcock, however, by means of serial sections, has traced it further, and shown that at this point it turns abruptly headwards to terminate in the muscles of the lower lip. If, then, as suggested, the lower lip represents the metastoma--the last pair of prosomatic appendages--then this mandibular or velar nerve represents that segmental nerve.

The other nerve--the maxillary nerve of Hatschek--which constitutes the larger part of the trigeminal, passes forwards from the ganglion, and at a point somewhere about the anterior region of the eyeball, divides into two, an external (black in Fig. 114) and an internal (red in Fig. 114) nerve. The external branch is apparently entirely sensory, and supplies the external surfaces of the upper and lower lips. The internal branch is mainly motor, and supplies the muscles of the upper lip; it contains also the nerves of the tentacles.

The nerve to the median ventral tentacle (t.) or tongue leaves the internal division of the maxillary immediately after its separation from the external; it runs ventralwards, and at the same time passes internally until it reaches a position between the muco-cartilage and the epithelium lining the cavity of the throat. It then turns, and passing posteriorly (towards the tail) to the point where the median ventral tentacle is attached to the lower lip, it supplies some very rudimentary-looking muscles which run from the tentacle to the adjoining surface, and no doubt serve to move the tentacle from side to side. A portion of the nerve still continues to run along the side of the median ventral ridge, as far back as the point where the muscles of the hyoid segment pass round to the ventral side between the velum and the thyroid; in fact, this small nerve passes along the whole length of the median ventral ridge.

This description shows that the trigeminal nerve divides itself into two groups: the one represented black in the figure, which is purely cutaneous and sensory, corresponding, in the main, according {290}to my theory, to the epimeral nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or splanchnic muscle-group, and contains also the nerves to the tentacles.

This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated nerves of the prosomatic appendages, and is clearly divisible into three distinct nerves--

1. The lower lip-nerve or the metastomal nerve (met.).

2. The tongue-nerve (t.).

3. The nerve (tent.) to the upper lip and tentacles.

Of these three pairs of nerves it is suggested that the first pair were derived from the nerves to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have supplied the pair of appendages immediately in front of the metastoma--that is, the pair of ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths, and therefore the third pair of nerves would represent the fused nerves to these concentrated endognaths, which, in the Eurypterids, stand aloof from the ectognaths.

Let us consider these three propositions separately. In the first place, have we any right to attribute segmental value to the mandibular nerve? What evidence is there of segments in this region in Ammocoetes?

THE SEGMENT OF THE LOWER LIP, OR METASTOMAL SEGMENT.

We have seen that in the branchial or mesosomatic region the segments corresponding to the mesosomatic appendages were mapped out by means of their supporting or skeletal structures, their segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ. Similarly, the segments in front of the branchial region, corresponding to the prosomatic appendages, ought to be definable by the same means, although, owing to the absence of branchiæ and the greater concentration in this region, the separate segments would probably not be so conspicuous.

The last segment considered was the segment belonging to the VIIth nerve corresponding to the opercular appendages of the {291}Eurypterid. The segment immediately in front of this is the next for consideration, viz. that corresponding to the chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with the basal part of the operculum, the one cannot be discussed without the other; therefore, the segment to which the lower lip belongs must be considered in connection with and not apart from the thyro-hyoid segments already dealt with.

In Chapter V., p. 188, I stated that the supporting bars of the foremost mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the branchial segments, in that they were composed of muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammocoetes which covers over the thyroid gland.

Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special interest, in connection with the segments indicated by such supporting structures, to find that this special tissue is entirely confined to the head-region, and disappears absolutely at transformation, thus indicating the ancestral nature of the segments marked out by its presence.

This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to be understood. It is of great importance, not only because it demonstrates the position of the segments in Ammocoetes which characterized its invertebrate ancestor, but also because it possesses a structure remarkably similar to that found in the head-plates of the most ancient fishes. For the present I will confine myself to the consideration of this muco-cartilaginous skeleton as evidence of the relationship of Ammocoetes to the Eurypterids, and in the next chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so that Ammocoetes is really a slightly modified Cephalaspid, the larval form of which was Eurypterid in character.

{292}[Illustration: FIG. 115.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.

Tr., trabeculæ; Pit., pituitary space; Inf., infundibulum; Ser., median serrated flange of velar folds.] {293}[Illustration: FIG. 116.--HORIZONTAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES, IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE.

sk1-sk5, skeletal bars; m1-m5, striated visceral muscles; mt1-mt4, tubular muscles; br1-br3, branchiæ; tr., trabeculæ; inf., infundibulum; ped., pedicle; V., trigeminal nerve. Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple.]

{294}[Illustration: FIG. 117.--SAGITTAL LATERAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES.

Lettering and colouring same as in Fig. 116. aud., auditory capsule; j.v., jugular vein.]

In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of an Ammocoetes cut in half horizontally. Such a section shows with great clearness the series of branchial appendages with their segmental muscles and cartilaginous bars which form the branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not of cartilage. If we follow this series of horizontal sections nearer to the origin of the cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain a picture, as in Fig. 116, in which each branchial segment is defined by the section of the branchial cartilaginous bar (sk4, sk5), by the section of the separate branchiæ (br2, br3), and by the separate segmental muscles arranged round each bar, these muscles being partly ordinary striated (m4, m5), partly tubular (mt3, mt4). The uppermost of these branchial segments shows the same arrangement; (sk3) is the branchial skeletal bar, which is now composed of muco-cartilage, not cartilage; (br1) is the branchiæ in the same situation as the others, but here composed of glandular rather than of respiratory epithelium, while the ordinary striated branchial muscles of this segment are marked as (m3), being separated from the tubular muscles of the segment (mt2), owing to the large size of the blood-space in which these latter muscles are lying. In front of this segment so defined we see again another well-marked skeletal bar (sk2) of muco-cartilage, evidently indicating a similar segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but again we see two sets of visceral muscles, the one ordinary striated, marked (m2), and the other tubular, marked (mt1_). Here, then, the section indicates the existence of a segment of the same character as the posteriorly situated branchial segments but belonging to a non-branchial region--a segment which would represent a non-branchial appendage, the last, therefore, of the prosomatic appendages. Let us, then, follow {295}out these two segmental muco-cartilaginous bars and their attendant muscles, and see to what sort of segments their investigation leads.

The bar which comes first for consideration (sk3) arises immediately behind the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its extension along the notochord at the place where the hard cartilage of the parachordal joins it, and in a sense it may be said to leave the notochord at this place and pass into the basal part of the first branchial bar. The most anterior continuation of this branchial system is this muco-cartilaginous bar (sk3), which passes forward and ventralwards, being separated from the axial line by the auditory capsule (cf. Fig. 118, A, B, C). Its position is well seen in a sagittal section, such as Fig. 117. It follows absolutely the line of the pseudo-branchial groove (ps. br., Fig. 114), and ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly. In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the cartilaginous bars with respect to their respective segments.

It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in Limulus, which also forms the termination of the branchial cartilaginous system, as fully explained in Chapter III.; it may therefore be called the opercular bar.

The next bar (sk2) is extremely interesting, as we are now out of the branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called by Parker the 'pedicle of the pterygoid'--a projection (ped.) which defines the posterior limit of the trabeculæ on each side, where they join on to the parachordals,--and winding round and below the auditory capsule, joins the opercular bar (cf. Fig. 118), to pass thence into and form part of the muco-cartilaginous plate of the lower lip. In the section figured (Fig. 116), this projection of hard cartilage is not directly continuous with (sk2), owing to a slight curvature in the bar; the next few sections show clearly the connection between (ped.) and (sk2_), and consequently the complete separation by means of this bar of the hyoid segment from the segment in front.

{296}[Illustration: FIG. 118.--SKELETON OF HEAD-REGION OF AMMOCOETES. A, LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple. sk1, sk2, sk3, skeletal bars; c.e., position of pineal eye; na. cart., nasal cartilage; ped., pedicle; cr., cranium; nc._, notochord.]

{297}In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one continuous ventral head-shield of muco-cartilage (Fig. 118, B), and also that this bar fuses with the opercular bar, we may conclude that the segment represented by the lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars, which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely enough, arose in connection with the posterior lateral horns of the plastron; these posterior lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in Ammocoetes the two lateral so-called pterygoid projections.

In the branchial region the muscles which marked out each branchial segment were of two kinds--ordinary striated visceral muscles and tubular muscles. Of these the former represented the dorso-ventral muscles of the branchial appendages, while the latter formed a separate group of dorso-ventral muscles with a separate innervation which may have been originally the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs. 116, 117, the grouping of these muscles in each branchial segment is well shown, and it is immediately seen that the hyoid segment possesses its group of striated visceral muscles (m3) supplied by the VIIth nerve in the same manner as the posterior groups, as has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in front, Fig. 116 shows that the group of visceral muscles (m2) corresponds in relative position with respect to the metastomal bar to the hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect to each branchial bar. What, then, is this muscular group? The series of sections show that these are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. 119 (M.), form a well-marked muscular sheet, whose fibres interlace across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive, for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium, just in front of the two lateral {298}eyes. In Fig. 117 the dorsal part is seen cut across on its way to its dorsal attachment. Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the primordial cranium and the tergal carapace, and suggests at once that when the chilarial appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles similar to those of the lower lip of Ammocoetes, by which the movements of the metastoma were effected. The posterior limit of these muscles ventrally marks out the junction of the segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma had fused ventrally with the operculum (Fig. 117).

Th., thyroid gland; M., lower lip, with its muscles.]

Besides the striated visceral muscles, each branchial segment possesses its own tubular muscles, shown in Fig. 116 (mt3) and (mt4). As the section shows, there is clearly a group of tubular muscle-fibres belonging to the hyoid segment (mt2), and also another group belonging to the segment in front of the hyoid (mt1); so that, judging from this section, each of these segments possesses its own tubular musculature just as do the branchial segments, the difference being that the tubular muscles are more separated from the striated visceral group than in the true branchial segments, owing to the size of the blood-spaces surrounding them. What, then, are these two groups of muscles? Tracing them in the series of sections, both groups are seen to belong to the system of velar muscles, forming an anterior and a posterior group respectively; and we see, further, that there is not the slightest trace of any tubular muscles anterior to these muscles of the velum.

In the living Ammocoetes the velar folds on each side can be seen {299}to move synchronously with the movements of respiration, contracting at each expiration, and thus closing the slit by which the oral and respiratory chambers communicate, and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a fact is clear evidence that these tubular muscles of the velar folds belong to the same series as the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter muscles were originally the veno-pericardial muscles of segments corresponding to the branchial appendages, then the former would represent the veno-pericardial muscles of the segments corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and how is it that the tubular muscles of these two segments become the velar muscles? I will consider, in the first instance, the posterior group of muscles (mt2_) in Fig. 116.

It has already been pointed out that the tubular muscles of the branchial segments are dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and running part of their course internally to and part externally to the ordinary constrictors. At first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at all. If, however, we follow the posterior group of velar muscles (mt2), we see (Fig. 117) that they pass between the auditory capsule and the opercular bar (sk3) of muco-cartilage to reach the region of the jugular vein (j.v.) posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective segments. Further, these muscles run along the length of the velar fold, and are attached ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular muscles as regards their respective segments.

This ventral attachment is shown in Fig. 119 on each side of the thyroid, and in Fig. 120 (mt2_); while in Fig. 117 the fibres are seen converging to this ventral position. In other words, this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial segments, if the velum were put back into its original position as the septum terminating the branchial chamber. Conversely, the presence of these {300}hyoid tubular muscles in the velum gives evidence that the opercular segment takes part in the formation of the septum, as already suggested.

Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the velar group as piercing the muco-cartilage (h. r. s.) in his figures 24 and 25, i.e. the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible that they represent a different group to those already considered, and suggest the possibility that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial muscles as well as in other respects.

The anterior group of tubular muscles (mt1, Figs. 116, 117) belonging to the same segment as the metastomal bar must now be taken into consideration. Very different is their origin to that of the posterior group: they arise close up against the eye, and have given rise to Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from these velar muscles, which form the foremost group of the segmental tubular muscles. They pass into the anterior part of the velar folds and run round to the ventral side just in the same way as does the posterior group. This anterior group of tubular muscles represents the veno-pericardial muscle of the segment immediately in front of the opercular, i.e._ the metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that the velar folds, formed as they were by the breaking down of the septum, are in reality part of two segments, viz. the opercular and the metastomal, which have fused together in their basal parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so apparent in the anatomy of the vertebrate cranial nerves.

A further piece of evidence that this anterior portion of the velum {301}belongs to the same segment as the lower lip is the fact that in addition to the tubular muscles a single ordinary striated muscle is found in the velum which, like the muscles of the lower lip, is innervated by this same mandibular nerve.

This muscle is attached laterally to the muco-cartilage of the metastomal bar (sk2_) at its junction with the muco-cartilage of the lower lip, and spreads out into a number of strands which are attached at intervals along the whole length of the free anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its contraction it draws the anterior portions of the velar folds apart from each other, and so opens the slit between them, through which the food and mud must pass. Clearly from its position it does not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have been one of the intrinsic muscles of the metastoma itself.

This anterior portion of the velar folds affords yet another striking hint of the correctness of my comparison of the lower lip segment of Ammocoetes with the chilaria of Limulus or the metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection marked Ser. in Fig. 115, and B in the accompanying Fig. 120. This is that part of the velar folds which comes together in the middle line and closes the entrance into the respiratory chamber. The epithelial surface here is most striking and suggestive, for it is markedly serrated, being covered with a large number of closely-set projections or serræ. The serration of the surface here is of so marked a character that Langerhans considered this part of the velar folds to act as a masticating organ, grinding and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases of the prosomatic appendages in Limulus or the Eurypteridæ.

This suggestion of Langerhans is surely most significant, considering that this somewhat separate portion of the velum, to which he assigns such a function, is in the very place where the gnathite portion of the metastomal appendages would have been situated if it were true that the lower lip and anterior portion of the velum of Ammocoetes were derived from the metastoma.

In addition to this marked serrated edge the whole surface of {302}the anterior portion of the velum is covered over with a scale-like or tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the ancient group Eurypteridæ. In Fig. 121 I give a picture of this surface-marking of the velum. It is striking to see that just as in the case of the invertebrate this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a surface which, according to Wolff, possibly contains chitin. The interpretation which I would give of the velar folds is therefore as follows:--

They represent the fused basal parts of the opercular and metastomal appendages, the gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because, owing to their position on each side of the opening into the respiratory chamber they were still able to manipulate the food as it passed by them after the closure of the old mouth.

tr., trabeculæ; vel., velum; B., anterior gnathic portion of velum; ps. br., pseudo-branchial groove; m2, muscles of lower lip segment; m3, muscles of thyro-hyoid segment; mt2_, insertion of tubular muscles of velum near thyroid.]

The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of the trigeminal does supply a splanchnic {303}segment which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal, and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded, therefore, to the chilarial or metastomal appendages.

A comparison of the ventral surface of Slimonia, as given in Fig. 8, p. 27, with that of Ammocoetes (Fig. 119), when the thyroid gland and lower lip muscles have been exposed to view, enables the reader to recognize at a glance the correctness of this conclusion.

THE TENTACULAR SEGMENTS AND THE UPPER LIP.

Anterior to this metastomal segment, Fig. 116 shows a group of visceral muscles, m1, and yet again a muco-cartilaginous bar, sk1, but, as already stated, no tubular muscles. These visceral muscles indicate the presence in front of the lower lip-segment of one or more segments of the nature of appendages. The muscles in question (m1) are the muscles of the upper lip, the skeletal elements form a pair of large bars of muco-cartilage (sk1), which start from the termination of the trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends in the middle line along the whole length of the lower lip, being separated from the muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which is strongly confirmed by the separate character of its nerve-supply, as already mentioned.

For three reasons, viz. the separateness both of their nerve-supply and of their skeletal tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it seems to me, be put {304}into a separate category from the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a single nerve which sends off a branch as it passes each one, together with the concentration of their skeletal elements into a single bar, with projections into each tentacle, points directly to the conclusion that these tentacles must be considered as a group, and not singly.

I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue representing the two ectognaths, and the four tentacles on each side the four pairs of endognaths.

As we see, this method of interpretation attributes segmental value to the tentacles, a conclusion which is opposed to the general opinion of morphologists, who regard them as having no special morphological importance, and certainly no segmental value. On the other hand, the importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in asserting that at transformation they are converted into that large and important organ the piston or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at transformation, being absorbed; they appear to me rather to take part in the formation of the sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial apparatus of Petromyzon is derived from the tentacles of Ammocoetes. In other words, on my view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at transformation, just as is frequently the case among the Arthropoda.

It is to the arrangement of the muscles that we look for evidence of segmental value. As long as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance, it was natural to deny segmental value to them. Matters are now, however, totally different since Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their development at transformation. If these muscles represent some of the appendage muscles belonging to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic muscles of those same segments, then we may expect ultimately to be able to give as good evidence of segmentation in their case as I have been able to give in the case of these latter muscles; for the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop until {305}transformation, but throughout the Ammocoetes stage remain in almost as rudimentary a condition as the tentacular muscles.

Another difficulty with respect to the tentacles is the determination of the number of them, owing to the fact that in addition to what may be called well-defined tentacles a large number of smaller tactile projections are found on the surface of the upper lip, as is seen in Fig. 115. In the very young condition, 7 or 8 mm. in length, it is easier to make sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior small group and a posterior larger group. The anterior group consists of a pair of very small tentacles and a very small median tentacle, all three situated quite dorsally in the front part of the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being fused together to form the large ventral median tentacle or tongue already mentioned. This pair, according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the very young condition the accessory projections above-mentioned are not present, or at all events are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the animal than in the older condition, where they have distinctly dwindled.

This posterior group of five conspicuous tentacles is the one which I suggest represents the four endognaths and one ectognath. What the significance of the small anterior group is, I know not. It is possible that the cheliceræ are represented here, for they are situated distinctly anterior to the other group; I know, however, of no sign of a markedly separate innervation to these most dorsal tentacles such as I should have expected to find if they represented the cheliceræ.

The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches its final destination. From their innervation, then, they must have belonged to the same appendages as the tentacles supplied by the tentacular nerve, i.e. to the endognaths. What conclusion can we form as to the probable origin of the upper lip of Ammocoetes? Since the oral chamber was formed by the forward growth of the metastoma, i.e. the lower lip of Ammocoetes, it follows that the upper {306}lip is the continuation forwards of the original ventral surface of such an animal as Limulus or a member of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up of two parts: (1) Of sternites composing the true median ventral surface of the body, called by Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents in triturating the food previously to its passage into the mouth. In Limulus, a conjoined pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present, forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic appendages play such a part, the pro-sternite not being present (cf. Fig. 97).

Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies the upper lip-muscles in Ammocoetes is the same as that supplying the tentacles which are attached to the upper lip, it appears to me more probable than not that the muscles in question are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what corresponds to the endostoma.

This interpretation of the meaning of the transformation process is in accordance with all the previous evidence both from the side of the palæostracan as from the side of the vertebrate, for it signifies that a dwindling process has taken place in the foremost of the original prosomatic appendages--the cheliceræ and the endognaths; while, on the contrary, the ectognath and the metastoma have continued to increase in importance right into the vertebrate stage. This process is simply a continuation of what was already going on in the invertebrate stage, for whereas in Eurypterus and other cases {307}the cheliceræ and endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage, and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.

The transformation of these foremost appendages into a suctorial apparatus is very common among the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is in accordance with the evidence that the main mass of that suctorial apparatus should be formed from appendages corresponding to the ectognath and metastoma rather than from the four endognaths. In all probability the nucleus masticatorius of the trigeminal nerve with its innervation of the great muscles of mastication is evidence of the continued development of the musculature of these two last prosomatic appendages, just as the descending root of the Vth demonstrates the further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however, as far as I know, the musculature of the head-region of Petromyzon has not been brought into line with that of other vertebrates, and until that comparative study has been completed it is premature to discuss the exact position of the masticating muscles of the higher vertebrates.

The analysis of these tentacular segments belonging to the trigeminal nerve presents greater difficulties than that of any of the other cranial segments, owing to the deficiency of our knowledge of what occurs at transformation. Light is required not only on the origin of the new muscles but also on the origin of the new and elaborate cartilages which are newly formed at this time.

Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we are not yet in a position to analyze the trigeminal supply in Petromyzon into its component appendage elements, an analysis which ought ultimately to enable us to determine from which appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the muscles are concerned, she gives me the following information:--

The tongue-nerve supplies in Ammocoetes the rudimentary muscles which pass laterally from the base of the large ventral tentacle to the wall of the throat, and even in Ammocoetes must possess some power of moving that tentacle.

At transformation these muscles proliferate and develop enormously, and form the bulk of the large basilar muscle which {308}surrounds the throat ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.

The velar or mandibular nerve supplies in Ammocoetes the muscles of the lower lip. In Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower lip region.

In Ammocoetes the tentacular nerve supplies the rudimentary muscles in the tentacles and the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived from the rudimentary tentacular muscles.

For the convenience of my reader I append here a table showing my conception of the manner in which the endognathal and ectognathal segments of the Palæostracan are represented in Ammocoetes. It shows well the uniform manner in which all the individual segmental factors have been fused together to represent the appearance of a single segment (van Wijhe's first segment) in the case of the four endognathal segments, but have retained their individuality in the case of the ectognathal segment.

+----------+----------+---------------------------+----------+----------+ | | | Appendages. | | | |V. Wijhe's|Eurypterid+-------------+-------------+ Appendage| Skeletal | |segments. |segments. | Eurypterid. | Ammocoetes. | nerves. | elements.| | | | | | | | +----------+----------+-------------+-------------+----------+----------+ | | | | | | | | | 2} | | | 1 | 1 | | 1 | 3} | 4 | 4 |Tentacular|Tentacular| | | 4} | Endognaths | Tentacles | to 4 | bar to 4 | | | 5} | | | tentacles| tentacles| | | | | | | | +----------+----------+-------------+-------------+----------+----------+ | | | | | | | | 2 | 6 | 1 | 1 | 1 Tongue | 1 Tongue | | | | Ectognath | Tongue | nerve | bar | | | | | | | | +----------+----------+-------------+-------------+----------+----------+

+----------+----------+---------+---------+----------+---------+ | | | | Dorso- | | | |V. Wijhe's|Eurypterid| Somatic | ventral |Coelomic| Coxal | |segments. |segments. | motor |segmental| cavities.| glands. | | | | nerves. | muscles.| | | +----------+----------+---------+---------+----------+---------+ | | | | | | 1 | | | 2} | 1 Oculo-| Sup. | 1 Pre- |Pituitary| | 1 | 3} | motor |inf. int.|mandibular| body; | | | 4} |supplying| rectus | fusion |fusion of| | | 5} | 4 | and inf.| of 4 | 4 coxal | | | | muscles | oblique | | glands | +----------+----------+---------+---------+----------+---------+ | | | 1 Troch-| | | | | 2 | 6 | learis | Sup. | 1 | | | | |supplying| oblique |Mandibular| | | | | 1 muscle| | | | +----------+----------+---------+---------+----------+---------+

{309}THE TUBULAR MUSCLES.

The only musculature innervated by the trigeminal nerve which remains for further discussion, consists of those peculiar muscles found in the velum, known by the name of striated tubular muscles. This group of muscles has already been referred to in Chapter IV., dealing with respiration and the origin of the heart.

It is a muscular group of extraordinary interest in seeking an answer to the question of vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival from a prevertebrate form, which is especially well marked in Ammocoetes. I have already suggested in this chapter that the homologues of these muscles are represented in Limulus by the veno-pericardial group of muscles. I will now proceed to deal with the evidence for this suggestion.

The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of striated fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of various forms of striated muscle found in various invertebrates, such as the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except in Ammocoetes. At transformation these muscles entirely disappear, becoming fattily degenerated and then absorbed.

A, portion of fibre seen longitudinally; B, transverse section of fibre (osmic preparation); the black dots are fat-globules.]

For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone would not make this tissue of any great importance, but when in addition these muscles are found to be arranged absolutely segmentally throughout the whole of the branchial region, then this tissue becomes a clue of the highest importance.

As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor muscle and the two constrictor muscles--the {310}striated constrictor and the tubular constrictor. Of these muscles, both the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton, whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are attached ventrally in the neighbourhood of the ventral aorta.

These segmental tubular muscles are found also in the velar folds--the remains of the septum or velum which originally separated the oral from the respiratory chamber. In the branchial region they act with the other constrictors as expiratory muscles, forcing the water out of the respiratory chamber. In the living Ammocoetes, the velar folds on each side can be seen to move synchronously with the movements of respiration, contracting at each expiration; they thus close the slit by which the oral and respiratory chambers communicate, and therefore, in conjunction with the respiratory muscles, force the water of respiration to flow out through the gill-slits, as described by Schneider.

These tubular muscles thus form a dorso-ventral system of muscles essentially connected with respiration; they belong to each one of the respiratory segments, and are also found in the velum; anterior to this limit they are not to be found. What, then, are these tubular muscles in the velar folds? Miss Alcock has worked out their topography by means of serial sections, and, as already fully explained, has shown that they form exactly similar dorso-ventral groups, which belong to the two segments anterior to the purely branchial segments, i.e. to the facial or hyoid segments and the lower lip-segment of the trigeminal nerve. If the velar folds could be put back into their original position as a septum, then the hyoid or facial group of tubular muscles would take up exactly the same position as those belonging to each branchial segment.

The presence of these two so clearly segmental groups of muscles in the velum--the one belonging to the region of the trigeminal, the other to the region of the facial--is strong confirmation of my contention that this septum between the oral and respiratory chambers was caused by the fusion of the last prosomatic and the first mesosomatic appendages, represented in Limulus by the chilaria and the operculum.

Yet another clue to the meaning of these muscles is to be found in their innervation, which is very extraordinary and unexpected. Throughout the branchial region the striated muscles of each segment {311}are strictly supplied by the nerve of that segment, and, as already described, each segment is as carefully mapped out in its innervation as it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the branchial region, giving off a branch to each segment as it passes. This nerve (Br. prof., Fig. 123) is known by the name of the ramus branchialis profundus of the facial, and its extraordinary course has always aroused great curiosity in the minds of vertebrate anatomists. Miss Alcock, by the laborious method of following its course throughout a complete series of sections, finds that each of the segmental branches which is given off, passes into the tubular muscles of that segment (Fig. 124). The tubular muscles which belong to the velum, i.e. those belonging to the lower lip-segment and to the hyoid segments, receive their innervation from the velar or mandibular nerve, and belong, therefore, to the trigeminal, not to the facial, system.

Motor branches, red; sensory branches, blue.]

The evidence presented by these muscles is as follows:--

In the ancestor of the vertebrate there must have existed a segmentally arranged set of dorso-ventral muscles of peculiar structure, concerned with respiration, and confined to the mesosomatic segments and to the last prosomatic segment, yet differing from the other dorso-ventral muscles of respiration in their innervation and their attachment.

Interpreting these facts with the aid of my theory of the origin of vertebrates, and remembering that the homologue of the vertebrate ventral aorta in such a palæostracan as Limulus is the longitudinal {312}venous sinus, while the opercular and chilarial segments are respectively the foremost mesosomatic and the last prosomatic segments; they signify that the palæostracan ancestor must have possessed a separate set of segmental dorso-ventral muscles confined to the branchial, opercular and chilarial or metastomal segments, which, on the one hand, were respiratory in function, and on the other were attached to the longitudinal venous sinus. Further, these muscles must all have received a nerve-supply from the neuromeres belonging to the chilarial and opercular segments, an unsymmetrical arrangement of nerves, on the face of it, very unlikely to occur in an arthropod.

Nerves coloured red are the motor nerves to the branchial muscles. Nerves coloured blue are the internal sensory nerves to the diaphragms and the external sensory nerves to the sense-organs of the lateral line system. Br. cart., branchial cartilage; M. con. str., striated constrictor muscles; M. con. tub., tubular constrictor muscles; M. add., adductor muscle; D.A., dorsal aorta; V.A., ventral aorta; S., sense-organs on diaphragm; n. Lat., lateral line nerve; X., epibranchial ganglia of vagus; R. br. prof. VII., ramus branchialis profundus of facial; J.v., jugular vein; Ep. pit., epithelial pit.]

{313}Is this prophecy borne out by the examination of Limulus? In the first place, these muscles were dorso-ventral and segmental, and, referring back to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral appendage muscles; and (3) the veno-pericardial muscles. Of these the first group is represented in the vertebrate by the muscles which move the eye, the second group by the striated constrictor and adductor muscles and the muscles for the lower lip. There is, then, the possibility of the third group for this system of tubular muscles.

Looking first at the structure of these muscles as previously described, so different are they in appearance from the ordinary muscles of Limulus, that Milne-Edwards, as already stated, called them "brides transparentes," and did not recognize their muscular character, while Blanchard called them in the scorpion, "ligaments contractils."

Consider their attachment and their function. They are attached to the longitudinal sinus, according to Lankester's observation, in such a way that the muscle-fibres form a hollow cone filled with blood; when they contract they force this blood towards the gills, and thus act as accessory or branchial hearts. According to Blanchard, in the scorpion they contract synchronously with the heart; according to Carlson, in Limulus they contract with the respiratory muscles. In Ammocoetes, where the respiration is effected after the fashion of Limulus, not of Scorpio, the tubular muscles are respiratory in function.

Look at their limits. The veno-pericardial muscles in Limulus are limited by the extent of the heart, they do not extend beyond the anterior limit of the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of the branchial region also attached to the longitudinal venous sinus, although in front of the gill-region. In Ammocoetes the upper limit of the tubular muscles is the group found in the velum; this most anterior group belongs to a region in front of the branchial region--that of the trigeminal.

Moreover, the supposition that the segmental tubular muscles belong throughout to the veno-pericardial group gives an adequate reason why they do not occur in front of the velum; for, as their existence is dependent upon the longitudinal collecting sinus in Limulus and Scorpio, which is represented by the ventral aorta in {314}Ammocoetes, they cannot extend beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in the spiracular artery, which exists only for a short time; and, in another place, speaking of this same termination of the ventral aorta, he states: "Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der betreffenden Seite einzumunden." These observations show that the vessel which in Ammocoetes represents the longitudinal collecting sinus in the Merostomata does not extend further forwards than the velum, and in consequence the representatives of the veno-pericardial muscles cannot extend into the segments anterior to the velum. One of the extraordinary characteristics of these tubular muscles which distinguishes them from other muscles, but brings them into close relationship with the veno-pericardial group, is the manner in which the bundles of muscle-fibres are always found lying freely in a blood-space; this is clearly seen in the branchial region, but most strikingly in the velum, the interior of which, apart from its muco-cartilage, is simply a large lacunar blood-space traversed by these tubular muscles.

All these reasons point to the same conclusion: the tubular muscles in Ammocoetes are the successors of the veno-pericardial system of muscles.

If this is so, then this homology ought to throw light on the extraordinary innervation of these tubular muscles by the branchialis profundus branch of the facial nerve and the velar branch of the trigeminal. We ought, in fact, to find in Limulus a nerve arising exclusively from the ganglia belonging to the chilarial and opercular segments, which, instead of being confined to those segments, traverses the whole branchial region on each side, and gives off a branch to each branchial segment; this branch should supply the veno-pericardial muscle of that side.

Patten and Redenbaugh have traced out the distribution of the peripheral nerves in Limulus, and have found that from each mesosomatic ganglion a segmental cardiac nerve arises which passes to the heart and there joins the cardiac median nerve, or rather the median heart-ganglion, for this so-called nerve is really a mass of ganglion-cells. In all the branchial segments the same plan exists, each cardiac nerve belonging to that neuromere is strictly segmental. {315}Upon reaching the opercular and chilarial neuromeres an extraordinary exception is found; the cardiac nerves of these two neuromeres are fused together, run dorsally, and then form a single nerve called the pericardial nerve, which runs outside the pericardium along the whole length of the mesosomatic region, and gives off a branch to each of the cardiac nerves of the branchial neuromeres as it passes them.

This observation of Patten and Redenbaugh shows that the pericardial nerve of Limulus agrees with the very nerve postulated by the theory, as far as concerns its origin from the chilarial and opercular neuromeres, its remarkable course along the whole branchial region, and its segmental branches to each branchial segment.

At present the comparison goes no further; there is no evidence available to show what is the destination of these segmental branches of the pericardial nerve, and so far all evidence of their having any connection with the veno-pericardial muscles is wanting. Carlson, at my request, endeavoured in the living Limulus to see whether stimulation of the pericardial nerve caused contraction of the veno-pericardial muscles, but was unable to find any such effect. On the contrary, his experimental work indicated that each veno-pericardial muscle received its motor supply from the corresponding mesosomatic ganglion. This is not absolutely conclusive, for if, as Blanchard asserts in the case of the scorpion, a close connection exists between the action of these muscles and of the heart, it is highly probable that their innervation conforms to that of the heart. Now Carlson has shown that this cardiac nerve from the opercular and chilarial neuromeres is an inhibitory nerve to the heart, while the segmental cardiac nerves belonging to the branchial ganglia are the augmentor nerves of the heart.

His experiments, then, show that the motor nerves of the heart and of the veno-pericardial muscles run together in the same nerves, but he says nothing of the inhibitory nerves to the latter muscles. If they exist and if they are in accordance with those to the heart, then they ought to run in the pericardial nerve, and would naturally reach the veno-pericardial muscles by the segmental branches of the pericardial nerve.

Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so that the nerve which corresponds {316}to the pericardial nerve, viz. the branchialis profundus of the facial, may be an inhibitory and sensory nerve, and not motor at all. Miss Alcock's observations are purely histological; no physiological experiments have been made.

At present, then, it does not seem to me possible to say that Carlson's experiments have disproved any connection of the pericardial nerve with the veno-pericardial muscles. We do not know what is the destination of its segmental branches; they may still supply the veno-pericardial muscles even if they do not cause them to contract; they certainly do not appear to pass directly into them, for they pass into the segmental cardiac nerves, and can only reach the muscles in conjunction with their motor nerves. Such a course would not be improbable when it is borne in mind how, in the frog, the augmentor nerves run with the inhibitory along the whole length of the vagus nerve.

Until further evidence is given both as to the function of the segmental branches of the pericardial nerve in the Limulus, and of the branchialis profundus in Ammocoetes, it is impossible, I think, to consider that the phylogenetic origin of these tubular muscles is as firmly established as is that of most of the other organs already considered. I must say, my own bias is strongly in favour of looking upon them as the last trace of the veno-pericardial system of muscles, a view which is distinctly strengthened by Carlson's statement that the latter system contracts synchronously with the respiratory movements, for undoubtedly in Ammocoetes their function is entirely respiratory. Then again, although at present there is no evidence to connect the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is extraordinarily significant that in such animals as Limulus and Ammocoetes, in both of which the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each case, extend through the whole region, giving off branches to each segment. Still more striking is it that this nerve should arise from the foremost mesosomatic and the last prosomatic neuromeres in Limulus--the opercular and chilarial segments--precisely the same neuromeres which give origin to the corresponding nerve in Ammocoetes, for according to my theory of the origin of vertebrates, the nerves which supplied the opercular and metastomal appendages have become the facial nerve and the lower lip-branch of the trigeminal nerve.

{317}With the formation of the vertebrate heart from the two longitudinal venous sinuses and the abolition of the dorsal invertebrate heart, the function of these tubular muscles as branchial hearts was no longer needed, and their respiratory function alone remained. The last remnant of this is seen in Ammocoetes, for the ordinary striated muscles were always more efficient for the respiratory act, and so at transformation the inferior tubular musculature was got rid of, there being no longer any need for its continued existence.

THE PALÆOSTOMA, OR OLD MOUTH.

The arrangement of the oral chamber in Ammocoetes is peculiar among vertebrates, and, upon my theory, is explicable by its comparison with the accessory oral chamber which apparently existed in Eurypterus. According to this explanation, the lower lip of the original vertebrate mouth was formed by the coalescence of the most posterior pair of the prosomatic appendages--the chilaria; from which it follows that the vertebrate mouth was not the original mouth, but a new structure due to such a formation of the lower lip.

It is very suggestive that the direct following out of the original working hypothesis should lead to this conclusion, for it is universally agreed by all morphologists that the present mouth is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately we see, as already explained, gill-slits must mean in this region the spaces between appendages which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of appendages to form a lower lip just as I have pointed out.

Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested, it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of Ammocoetes was originally the tube of the hypophysis, so that the following out of the theory points directly to the tube of the hypophysis as the place where the palæostoma must be looked for.

This conclusion is not only not at variance with the opinions of morphologists, but gives a straightforward, simple explanation why the palæostoma was situated in the very place where they are most inclined to locate it. Thus, if we trace the history of the question, {318}we see that Dohrn's original view of the comparison of the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis. This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in 1888, holding the view that the vertebrates were derived from annelids which had lost their supra-oesophageal ganglia, and that, therefore, there was no question of an oesophageal tube piercing the central nervous system of the vertebrate, explained the close connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis with the annelidan mouth, so that the infundibular or so-called nervous portion was a special nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening is the palæostoma. basing this view largely upon his observations on Ammocoetes and Acipenser.

Nc., neural canal with its infundibular termination; Nch., notochord; Al., alimentary canal with its anterior diverticulum; Hy., hypophysial or nasal tube; Or., oral chamber closed by septum.]

As is seen in Fig. 125, the position of this palæostoma is a very suggestive one. At this single point in Ammocoetes, four separate tubes terminate; here is the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal tube or tube {319}of the hypophysis, and the pre-oral elongation of the alimentary canal.

It is perfectly simple and easy for the olfactory tube to open into any one of the other three. By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by opening into the gut it produces the actual condition of things seen in Myxine and other vertebrates; by opening into the notochordal tube it would produce a transitional condition between the other two.

The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine. My view is that this tube originally opened into the infundibulum, in other words, into the original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube. When, with the breaking through of the septum between the oral and respiratory chambers, the external opening of the oral chamber became a new mouth, the old mouth was closed but the olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the higher vertebrates.

It is not, to my mind, at all improbable that during the transition stage, between its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind ending, as in Ammocoetes, the nasal tube opened into the tube of the notochord. This question will be discussed later on when the probable significance of the notochord is considered.

THE PITUITARY GLAND.

Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which represent respectively an imaginary sagittal section through an Eurypterus-like animal and through Ammocoetes at a larval stage, all the points for comparison mentioned on p. 244 have now been discussed with the exception of the suggested homology between the coxal glands of the one animal and the pituitary body of the other.

{320}This latter gland undoubtedly arises posteriorly to the hypophysial tube, or Rathke's pouch (as it is sometimes called), and, as already mentioned, is supposed by Kupffer to be formed from the posterior wall of this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who has investigated this matter, finds that the glandular hypophysis is not formed from the walls of Rathke's pouch, but from the tissue of the rudimentary connection or stalk between the two premandibular cavities, which becomes closely connected with the posterior wall of Rathke's pouch, and becoming cut off from the rest of the premandibular cavity on each side, becomes permanently a part of the 'Hypophysis Anlage.'

The importance of Nusbaum's investigation consists in this, that he derives the glandular hypophysis from the connecting stalk between the two premandibular cavities, and therefore from the walls of the ventral continuation of this cavity on each side.

This may be expressed as follows:--

The coelomic cavity, known as the premandibular cavity, divides into a dorsal and a ventral part; the walls of the dorsal part give origin to the somatic muscles belonging to the oculomotor nerve, while the walls of the ventral part on each side form the connecting stalk between the two cavities, and give origin to the glandular hypophysis.

Now, as already pointed out, the premandibular cavity is homologous with the 2nd prosomatic coelomic cavity of Limulus, and this 2nd prosomatic coelomic cavity divides, according to Kishinouye, into a dorsal and a ventral part; and, further, the walls of this ventral part form the coxal gland. Both in the vertebrate, then, and in Limulus, we find a marked glandular tissue in a corresponding position, and the conclusion is forced upon us that the glandular hypophysis was originally the coxal gland of the invertebrate ancestor. As in all other cases already considered, when the facts of topographical anatomy, of morphology and of embryology, all combine to the same conclusion as to the derivation of the vertebrate organ from that of the invertebrate, then there must be also a structural similarity between the two. What, then, is the nature of the coxal gland in the scorpions and Limulus? Lankester's paper gives us full information on this point as far as the scorpion and Limulus are concerned, and he shows that the coxal gland of Limulus differs markedly from that of Scorpio in the size of the cells and in the {321}arrangement of the tubes. In Fig. 126, A, I give a picture of a piece of the coxal gland of Limulus taken from Lankester's paper.

Turning now to the vertebrate, Bela Haller's paper gives us a number of pictures of the glandular hypophysis from various vertebrates, and he especially points out the tubular nature of the gland and its solidification in the course of development in some cases. In Fig. 126, B, I give his picture of the gland in Ammocoetes.

The striking likeness between Haller's picture and Lankester's picture is apparent on the face of it, and shows clearly that the histological structure of the glands in the two cases confirms the deductions drawn from their anatomical and morphological positions.

n.a., termination of nasal passage.]

The sequence of events which gave rise to the pituitary body of the vertebrate was in all probability somewhat as follows:--

Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata. Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor appendages. When these appendages became reduced in size and purely tactile they were compressed and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary consequence of the concentration of the coxæ of the endognaths, the coxal gland also became concentrated, {322}and took up a situation close against the pharynx, as represented in Fig. 106, B. When, then, the old mouth closed, and the pharynx became the saccus vasculosus, the coxal gland remained in close contact with the saccus vasculosus, and became the pituitary body, thus giving the reason why there is always so close a connection between the pituitary body and the infundibular region.

Whatever was the condition of the digestive tracts at the transition stage between the arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was ultimately closed. The method of its closure was exceedingly simple and evident. The membranous cranium was in process of formation by the extension of the plastron laterally and dorsally; a slight growth of the same tissue in the region of the mouth would suffice to close it and thus separate the infundibulum from the olfactory tube. As evidence that such was the method of closure, it is instructive to see how in Ammocoetes the glandular tissue of the pituitary body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the nasal tube is embedded in this same thickened mass of the cranial wall--how, in fact, both coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron, by means of which the mouth was closed.

I have now passed in review the nature of the evidence which justifies a comparison between the segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic segments of the palæostracan. For the convenience of my readers I have put these conclusions into tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however variable may be the number of branchial segments which the vagus supplies, the second branchial segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in the Pedipalpi) is never less than two.

{323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND IN AMMOCOETES (i.e. IN CEPHALASPIDS).

Key: So. = Supra-oesophageal. Si. = Supra-infundibular. Io. = Infra-oesophageal. Ii. = Infra-infundibular. Ps. = Prosomatic. Ms. = Mesosomatic.

+---+--------------------------------------------------------+ | | Median Eyes. | | +--------------------------------------------------------+ |So.| Lateral Eyes. | | +--------------------------------------------------------+ | | Camerostome. | +---+--------------------------------------------------------+ | | Invertebrate (Limulus or Eurypterid). | +---+---+---------+-----------------------------+------------+ | | | | Appendages. | Coelomic | | | |Segments.+-------------+---------------+ Cavities. | | | | | Limulus. | Eurypterid. | | | | +---------+-------------+---------------+------------+ | | | 1 | Cheliceræ | Cheliceræ | 1 | | | +---------+-------------+---------------+------------+ | | | 2 |1st Locomotor|} | | | | +---------+-------------+} | 2 | | | | 3 | 2nd " |} |Ventral part| | |Ps.+---------+-------------+}4 Endognaths | forms coxal| | | | 4 | 3rd " |} | gland. | | | +---------+-------------+} | | | | | 5 | 4th " |} | | | | +---------+-------------+---------------+------------+ |Io.| | 6 | 5th " | Ectognath | 3 | | | | | | | | | | +---------+-------------+---------------+------------+ | | | 7 | Chilaria | Metastoma | 4 | | | | | | | | | +---+---------+-------------+---------------+------------+ | | | 8 | Operculum |Genital } | 5 | | | |---------+-------------+ }Oper-+------------+ | | | 9 |1st Branchial| }culum| 6 | | | | | |Branchial} | | | |Ms.+---------+-------------+---------------+------------+ | | | 10 | 2nd " | 2nd Branchial | 7 | +---+---+---------+-------------+---------------+------------+

+-----------------------------------------------------+------+ | Pineal Eyes. | | +-----------------------------------------------------+ | | Lateral Eyes. | Si. | +-----------------------------------------------------+ | | Olfactory Organ. | | +-----------------------------------------------------+------+ | Vertebrate (Ammocoetes or Cephalaspid). | | +-------------+---------------+-------------+---------+------+ | | Splanchnic | Somatic | Somatic | | | Appendages. | Nerves. | Segmental | Nerves. | | | | | Muscles. | | | +-------------+---------------+-------------+---------+ | | ... | ... | ... | ... | | +-------------+---------------+-------------+---------+ | | | | | | | | | V | Muscles | | | |4 tentacles | Tentacular | supplied by | III | | | and upper | and upper | oculomotor | | | | lip. | lip nerve. | nerve. | | | | | | | | | | | | | | | +-------------+---------------+-------------+---------+ | | Tongue | V | Sup.oblique | IV | Ii. | | | Tongue nerve | | | | +-------------+---------------+-------------+---------+ | | Lower lip | V | | | | | |Lower lip nerve| | | | +-------------+---------------+-------------+---------+ | | Thyroid |} | | | | +-------------+} VII | Ext. rectus | VI | | | Hyoid or |} |Retract oculi| | | |1st Branchial|} | | | | +-------------+---------------+-------------+---------+ | |2nd Branchial| IX | ... | ... | | +-------------+---------------+-------------+---------+------+

+-------------------------+------+ | Pineal Nerve. | | +-------------------------+ | | II | Si. | +-------------------------+ | | I | | +-------------------------+------+ | Vertebrate. | | +-------------+-----------+------+ | Coelomic |V. Wijhe's | | | Cavities. | Segments. | | | | | | +-------------+-----------+ | | Anterior. | ... | | +-------------+-----------+ | | Premandi- | | | | bular | | | | Ventral | | | | part forms | 1 | | | pituitary | | | | body. | | | | pituitary | | | +-------------+-----------+ | | Mandibular |} | Ii. | | |} | | +-------------+} 2 | | | Mandibular |} | | | |} | | +-------------+-----------+ | | Hyoid1 | 3 | | +-------------+-----------+ | | Hyoid2 | 4 | | | | | | +-------------+-----------+ | |2nd Branchial| 5 | | +-------------+-----------+------+

{324}SUMMARY.

The general consideration of the evidence of the number of segments, and their nature in the pro-otic region of the vertebrate, as given in the last chapter, is not incompatible with the view that the trigeminal nerve originally supplied seven appendages, which appendages did not carry branchiæ, but were originally used for purposes of locomotion as well as of mastication.

Such appendages clearly no longer exist in the higher vertebrates, the muscles of mastication only remaining; but in the earliest fish-forms they must have existed, as, indeed, is seen in Pterichthys and Bothriolepis. Judging from all the previous evidence some signs of their existence may reasonably be expected still to remain in Ammocoetes. Such is indeed the case.

In the adult Petromyzon the trigeminal nerve innervates specially a massive suctorial apparatus, by means of which it holds on to other fishes, or to stones in the bottom of the stream. There is here no apparent sign of appendages. Very great, however, is the difference in the oral chamber of Ammocoetes; here there is no sign of any suctorial apparatus, but instead, a system of tentacles, together with the remains of the septum or velum, which originally closed off the oral from the respiratory chamber. These tentacles are the last remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the lateral eyes they do not develop until the transformation comes, but during the whole larval condition their musculature remains in an embryonic condition, and then from these embryonic muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus derived from the modification of appendages, as so frequently occurs in the arthropods.

The study of Ammocoetes indicates that the velum and lower lip correspond to the metastoma of the Eurypterid, i.e. the chilaria of Limulus, while the large ventral pair of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably to the oar-like appendages of Pterichthys and Bothriolepis. From these two splanchnic segments the suctorial apparatus in the main arises; the motor supply of these two segments forms the mass of the trigeminal nerve-supply, and the nerves supplying them, the velar nerve and the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.

The rest of the tentacles present much less the sign of independent segments. In their nerves, their muco-cartilaginous skeleton, and their rudimentary muscles, they indicate a concentration and amalgamation, such as might be expected from the concentrated endognaths. The continuation of the dwindling process, already initiated in the Eurypterid, would easily result in the tentacles of Ammocoetes.

The nasal tube of Ammocoetes, which originates in the hypophysial tube, corresponds absolutely in position and in its original structure, to the olfactory tube of a scorpion-like animal. From this homology two conclusions of importance follow: (1) the old mouth, or palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory tube from a ventral to a dorsal position, was originally formed by the foremost sternites or endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of the palæostracan ancestor.

In strict accordance with the rest of the comparisons made in this region, the pituitary body shows by similarity of structure, as well as of position, that it arose from the coxal glands, which were situated at the base of the four endognaths.

{325}One after another, when once the clue has been found, all these mysterious organs of the vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the remnants of corresponding important organs in the palæostraca.

Yet another clue is afforded by the tubular muscles of Ammocoetes, that strange set of non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which disappear at transformation, and are never found in any of the higher vertebrates, for the limits of their distribution correspond to the veno-pericardial muscles of Limulus.

Their nerve-supply in Ammocoetes is most extraordinary; for, although they are segmentally arranged throughout the whole respiratory region, which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are found in front of this region only in one segment, that of the lower lip, which is supplied by the velar branch of the Vth nerve, yet they are not supplied segmentally, but only by the velar nerve and a branch of the VIIth, the ramus branchialis profundus. This latter nerve extends throughout the respiratory region, and gives off segmental branches to supply these muscles.

It is also a curious coincidence that in such a markedly segmented animal as Limulus, a nerve--the pericardial nerve--which arises from the nerves of the chilarial and opercular segments, should pass along the whole respiratory region and give off branches to each mesosomatic segment. It is strange, to say the least of it, that the chilarial or metastomal and the opercular segments of Limulus should, on the theory advocated in this book, correspond to the lower lip and hyoid segments of the vertebrate. At present the homology suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles have anything to do with the pericardial nerve.

{326}CHAPTER X

THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE OSTRACODERMATA

The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and of Cephalaspis.--Ammocoetes the only living representative of these ancient fishes.--Formation of cranium.--Closure of old mouth.--Rohon's primordial cranium.--Primordial cranium of Phrynus and Galeodes.--Summary.

The shifting of the orifice of the olfactory passage, which led to the old mouth, from the ventral to the dorsal side, as seen in the transformation of the ventrally situated hypophysial tube of the young Ammocoetes, to the dorsally situated nasal tube of the full-grown Ammocoetes, affords one of the most important clues in the whole of this story of the origin of vertebrates; for, if Ammocoetes is the nearest living representative of the first-formed fishes, then we ought to expect to find that the dorsal head-shield of such fishes is differentiated from that of the contemporary Palæostraca by the presence of a median frontal opening anterior to the eyes. Conversely, if such median nasal orifice is found to be a marked characteristic of the group, in combination with lateral and median eyes, as in Ammocoetes, then we have strong reasons for interpreting these head-shields by reference to the head of Ammocoetes.

The oldest known fishes belong to a large group of strange forms which inhabited the Silurian and Devonian seas, classed together by Smith Woodward under the name of Ostracodermi. These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ, to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis, Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong to the Upper Silurian, while the third is Devonian.

THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI.

Of the three orders above-named, the Heterostraci and Osteostraci are the oldest, and among them the Cephalaspidæ have afforded the most numerous and best worked-out specimens. At Rootziküll, in the island of Oesel, the form known as Thyestes (Auchenaspis) verrucosus is especially plentiful, being found thickly present in among the masses of Eurypterid remains, which give the name to the deposit. Of late years this species has been especially worked at by Rohon, and many beautiful specimens have been figured by him, so that a considerable advance has been made in our knowledge since Pander, Eichwald, Huxley, Lankester, and Schmidt studied these most interesting primitive forms.

All observers agree that the head-region of these fishes was covered by a dorsal and ventral head-shield, while the body-region was in most cases unknown, or, as in Eichwald's specimens, and in the specimens figured in Lankester and Smith Woodward's memoirs, was made up of segments which were not vertebral in character, but formed an aponeurotic skeleton, being the hardened aponeuroses between the body-muscles. This body-skeleton, which possesses its exact counterpart in Ammocoetes, will be considered more fully when I discuss the origin of the spinal region of the vertebrate.

Of the two head-shields, ventral and dorsal, the latter is best known and characterizes the group. It consists of a dorsal plate, with characteristic horns, which in Thyestes verrucosus (Fig. 128), as described by Rohon, is composed of two parts, a frontal part and an occipital part (occ.), the occipital part being composed of segments, and possessing a median ridge--the crista occipitalis. In Lankester's memoir and in Smith Woodward's catalogue, a large number of known forms are described and delineated, and we may perhaps say that in some of the forms, such as Eukeraspis pustuliferus (Fig. 127, B), the frontal part of the shield only is capable of preservation as a fossil, while in Cephalaspis (Fig. 127, A) not only the frontal part but a portion of the occipital region is preserved, the latter being small in extent when compared with the occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and Didymaspis, the whole of both frontal {328}and occipital region is capable of preservation, the line of demarcation between these two regions being well marked in the latter species.

In the best preserved specimens of all this group of fishes a frontal median orifice is always present; it appears in some specimens obscurely partially divided into two parts. Perhaps the best specimen of all was obtained by Rohon at Rootziküll, and is thus described by him:--

The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits for the lateral eyes (l.e.), a marked frontal organ (fro.), and a median depression (gl.), to which he gives the name parietal organ. The occipital part (occ.) was clearly segmented, and carried, he thinks, the branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig. 129); he describes it as a deeply sunk pit, divided in the middle by a slit, which leads deeper in, he supposes, towards the central nervous system.

Fro., narial opening; l.e., lateral eyes; gl., glabellum or plate over brain; Occ., occipital region.]

{329}A similar organ was described by Schmidt in Tremataspis, and considered by him to be a median nose. Such also is the view of Jaekel, who points out that a median pineal eye exists between the two lateral eyes in this animal, as in all other of these ancient fishes, so that this frontal organ does not, as Patten thinks, represent the pineal eye. The whole of this group of fishes, then, is characterized by the following striking characteristics:--

1. Two well-marked lateral eyes near the middle line.

2. Between the lateral eyes, well-marked median eyes, very small.

3. In front of the eye-region a median orifice, single.

In addition, behind the eye-region a median plate is always found, frequently different in structure to the rest of the head-shield, being harder in texture--the so-called post-orbital plate.

STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES.

What is the structure of this head-shield? It has been spoken of as formed of bone because it possesses cells, being thus unlike the layers of chitin, which are formed by underlying cells but are not themselves cellular. At the same time, it is recognized on all sides that it has no resemblance to bone-structure as seen in fossil remains of higher vertebrates. The latest and best figure of the structure of this so-called bone is given in Rohon's paper already referred to. It is, so he describes, clearly composed of fibrillæ and star-shaped cells, arranged more or less in regular layers, with other sets of similar cells and fibrillæ arranged at right angles to the first set, or at varying angles. The groundwork of this tissue, in which these cells and fibrils are embedded, contained calcium salts, and so the whole tissue was preserved. In places, spaces are found in it, in the deepest layer large medullary spaces; more superficially, ramifying spaces which he considers to be vascular, and calls Haversian canals; the {330}star-like cells, however, are not arranged concentrically around these spaces, as in true Haversian canals.

This structure is therefore a calcareous infiltration of a tissue with cells in it. Where is there anything like it?

As soon as I saw Rohon's picture (Fig. 130), I was astounded at its startling resemblance to the structure of muco-cartilage as is seen in Fig. 131, taken from Ammocoetes. If such muco-cartilage were infiltrated with lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be preserved in the fossil condition, and be comparable with that of Cephalaspis, etc.

The whole structure is clearly remarkably like Rohon's picture of a section of the head-plate of a Cephalaspid (Fig. 130). In the latter case the matrix contains calcium salts, in the former it is composed of the peculiar homogeneous mucoid tissue which stains so characteristically with thionin. With respect to this calcification, it is instructive to recall the calcification in the interior of the branchial cartilages of Limulus, as described in Chapter III., for this example shows how easy it is to obtain a calcification in this chondro-mucoid material. With respect to the medullary spaces and smaller spaces in this tissue, as described by Rohon, I would venture to suggest that they need not all necessarily indicate blood-vessels, for similar spaces would appear in the head-shield of Ammocoetes if its muco-cartilage alone {331}were preserved. Of these, some would indicate the position of blood-vessels, such, for instance, as of the external carotid which traverses this structure; but the largest and most internal spaces, resembling Rohon's medullary spaces, would represent muscles, being filled up with bundles of the upper lip-muscles.

THE MUCO-CARTILAGINOUS HEAD-SHIELD OF AMMOCOETES.

The resemblance between the structure of the head-shield of Thyestes and the muco-cartilage of Ammocoetes, is most valuable, for muco-cartilage is unique, occurs in no other vertebrate, and every trace of it vanishes at transformation; it is essentially a characteristic of the larval form, and must, therefore, in accordance with all that has gone before, be the remnant of an ancestral skeletal tissue. The whole story deduced from the study of Ammocoetes would be incomplete without some idea of the meaning of this tissue. So also, as already mentioned, the skeleton of Ammocoetes is incomplete without taking this tissue into account. It is confined entirely to the head-region; no trace of it exists posteriorly to the branchial basket-work. It consists essentially of dorsal and ventral head-shields, connected together by the tentacular, metastomal, and thyroid bars, as already described. The ventral shield forms the muco-cartilaginous plate of the lower lip and the plate over the thyroid gland, so that the skeleton ventrally is represented by Fig. 118, B, which shows how the cartilaginous bars of the branchial basket-work are separated from each other by this thyroid plate. At transformation, with the disappearance of this muco-cartilaginous plate, the bars come together in the middle line, as in the more posterior portion of the branchial basket-work.

The dorsal head-shield of muco-cartilage covers over the upper lip, sends a median prolongation over the median pineal eyes and a lateral prolongation on each side as far as the auditory capsules, giving the shape of the head-shield of muco-cartilage, as in Fig. 118, C.

Not only then is the structure of the head-shield of a Cephalaspid remarkably like the muco-cartilage of Ammocoetes, but also its general distribution strangely resembles that of the Ammocoetes muco-cartilage.

Now, these head-shields in the Cephalaspidæ and Tremataspidæ {332}vary very much in shape, as is seen by the comparison of Tremataspis and Auchenaspis with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms belong to a single group, the Osteostraci.

The conception that Ammocoetes is the solitary living form allied to this group affords a clue to the meaning of this variation of shape, which appears to me to be possible, if not indeed probable. There is a certain amount of evidence given in the development of Ammocoetes which indicates that the branchial region of its ancestors was covered with plates of muco-cartilage as well as the prosomatic region.

The evidence is as follows:--

The somatic muscles of Ammocoetes form a continuous longitudinal sheet of muscles along the length of the body, which are divided up by connective tissue bands into a series of imperfect segments or myotomes. This simple muscular sheet can be dissected off along the whole of the head-region of the animal, with the exception of the most anterior part, without interfering with the attachments or arrangements of the splanchnic muscular system in the least. The reason why this separation can be so easily effected is to be found in the fact that the two sets of muscles are not attached to the same fascia. The sheet of fascia to which the somatic muscles are attached is separated from the fascia which encloses the branchial cavity by a space (cf. Figs. 63 and 64) filled with blood-spaces and cells containing fat, in which space is also situated the cartilaginous branchial basket-work. These branchial bars are closely connected with the branchial sheet of fascia, and have no connection with the somatic fascia, their perichondrium forming part of the former sheet. Upon examination, this space is seen to be mainly vascular, the blood-spaces being large and frequently marked with pigment; but it also possesses a tissue of its own, recognized as fat-tissue by all observers. The peculiarity of the cells of this tissue is their arrangement; they are elongated cells arranged at right angles to the plates of fascia, just as the fibres of the muco-cartilage are largely arranged at right angles to their limiting plates of perichondrium. These cells do not necessarily contain fat; and when they do, the fat is found in the centre of each cell, and does not push the protoplasm of the cell to the periphery, as in ordinary fat cells.

{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken from the plate of the lower lip; and in Fig. 133, A, a modification of the muco-cartilage taken from the velum, which shows the formation of a tissue intermediate between ordinary muco-cartilage and this branchial fat-tissue.

Further, in fully-grown specimens of Ammocoetes, in the region of undoubted muco-cartilage, a fatty degeneration of the cells frequently appears, together with an increase in the blood spaces,--the precursor, in fact, of the great change which overtakes this tissue soon afterwards, at the time of transformation, when it is invaded by blood, and swept away, except in those places where new cartilage is formed. I conclude, then, that the tissue of this vascular space was originally muco-cartilage, which has degenerated during the life of the Ammocoetes. The fact that in most cases undoubted muco-cartilage is to be found here and there in this space, is strong confirmation of the truth of this conclusion.

If this conclusion is correct, we may expect that it would be confirmed by the embryological history of the tissue, and we ought to find that in much younger stages a homogeneous tissue of the same nature as muco-cartilage fills up the spaces in the branchial {334}region, where in the Ammocoetes only blood and fat-containing cells are present. For this purpose Shipley kindly allowed me to examine his series of sections through the embryo at various ages. These specimens are very instructive, especially those stained by osmic acid, which preserves the natural thickness of this space better than other staining methods. At an age when the branchial cartilages are seen to be formed, when no fat-cells are present, a distinctive tissue (Fig. 133, B) is plainly visible in the velum and at the base of the tentacles, in the very position where in the more advanced Ammocoetes muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled with a similar material. At this stage a sheet of embryonic tissue occupies the position where, later on, blood-spaces and fat-cells are found, and this tissue resembles that seen in the velum and other places where muco-cartilage is afterwards found.

I conclude, therefore, that originally the branchial or mesosomatic region was covered with a dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the branchial basket-work, and sprang from the central core of skeletogenous tissue around the notochord; this plate was separated from the plate which covered this region ventrally by the lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface the ventral part of the branchial basket-work, and was originally continuous with the plate over the thyroid gland.

{335}[Illustration: FIG. 134.--SKELETON OF HEAD-REGION OF AMMOCOETES. A, LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.

Muco-cartilage, red; soft cartilage, blue; hard cartilage, purple. sk1, sk2, sk3, skeletal bars; c.e., position of pineal eye; na. cart., nasal cartilage; ped., pedicle; cr., cranium; nc._, notochord.]

{336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is represented from the dorsal, ventral, and lateral aspects. The muco-cartilage is coloured red, the branchial or soft cartilage blue, and the hard cartilage purple. The degenerated muco-cartilage of the branchial region is represented as an uncoloured plate, on which the branchial basket-work stands in relief. If it were restored to its original condition of muco-cartilage, it would represent a uniform plate, on the under surface of which the basket-work would be situated; and if it were calcified and made solid, the branchial basket-work would not show at all in these figures.

Is it possible to find the reason why this skeletal covering has degenerated so early before transformation, and why the thyroid plate remains intact until transformation? We see that all that part which has degenerated is covered over by the somatic muscles,--by, in fact, muscles which, being innervated by the foremost spinal nerves, belong naturally to the region immediately following the branchial. I suggest, therefore, that the original skeletal covering of muco-cartilage has remained intact only where it has not been invaded and covered over by somatic muscles, but has been invaded by blood and undergone the same kind of degenerative change as overtakes the great mass of this tissue at transformation wherever the somatic muscles have overgrown it.

The covering somatic muscles in the branchial region form a dorsal and ventral group, of which the latter is formed in the embryo much later than the former, the line of separation between the two groups being the lateral groove, with its row of branchial openings. This groove ends at the first branchial opening, but the ventral and dorsal somatic muscles continue further headwards. It is instructive to see that, although the lateral groove terminates, the separation between the two groups of muscles is still marked out by a ridge of muco-cartilage, represented in Fig. 134, A, which terminates anteriorly in the opercular bar.

Passing now to the prosomatic region, we find that here, too, the muco-cartilaginous external covering is divisible into a dorsal and a ventral head-plate, the ventral head-plate being the plate of the lower lip, and the dorsal head-plate the plate of muco-cartilage over the front part of the head. The staining reaction with thionin maps out this dorsal head-plate in a most beautiful manner, and shows that the whole of the upper lip-region in front of the nasal orifice is one large plate of muco-cartilage, obscured largely by the invasion of the crossing muscles of the upper lip, but left pure and uninvaded all around the nasal orifice, and where the upper and lower lips come together. In addition to this foremost plate, a median tongue of muco-cartilage covers over the pineal eye and fills up the {337}median depression between the two median dorsal somatic muscles. Also, two lateral cornua pass caudalwards from the main frontal mass of muco-cartilage over the lateral eyes, forming the well-known wedge which separates the dorsal and lateral portions of the dorso-lateral somatic muscle. In fact, similarly to what we find in the branchial region, the muco-cartilaginous covering can be traced with greater or less completeness only in those parts which are not covered by somatic muscles.

In Fig. 134, A, B, C, this striking muco-cartilaginous head-shield, both dorsal and ventral, is shown. Seeing that the upper lip wraps round the lower one on each side, and that this most ventral edge of the upper lip contains muco-cartilage, as is seen in Fig. 117, the dorsal head-shield of muco-cartilage ought, strictly speaking, to extend more ventrally in the drawings. I have curtailed it in order not to interfere with the representation of the lower lip and tentacular muco-cartilages.

From what has been said, it follows that the past history of the skeletal covering of the whole head-region of Ammocoetes, both frontal and occipital, can be conjectured by means of the ontogenetic history of the foremost myomeres.

Dohrn and all other observers are agreed that during the development of this animal a striking forward growth of the foremost somatic myomeres takes place, so that, as Dohrn puts it, the body-musculature has extended forwards over the gill-region, and at the same time the gill-region has extended backwards. It is therefore probable that in the ancestral form the myotomes, innervated by the first spinal nerves, immediately succeeded the branchial region. Judging from Ammocoetes, the forward growth was at first confined to the dorsal region, and therefore invaded the dorsal head-plate, the ventral musculature being distinctly a later growth. With respect to this dorsal part of the myotomes, the first myotome is originally situated some distance behind the auditory capsule, and then grows forward towards the nasal opening; the lateral part, according to Hatschek, grows forward more quickly than the dorsal part, and splits itself above and below the eye into a dorso-lateral part, which extends up to the olfactory capsule, and a ventro-lateral part (m. lateralis capitis anterior, superior, and inferior), thus giving rise to the characteristic appearance of the muco-cartilaginous head-shield of Ammocoetes.

According, then, to the extent of the growth of these somatic {338}muscles, the shape of the muco-cartilaginous head-shield will vary, and if it were calcified and then fossilized we should obtain fossil head-shields of widely differing configuration, although such fossils might be closely allied to each other. This is just what is found in this group. Let the muco-cartilage extend over the whole of the branchial region of Ammocoetes, the resulting head-shield would be as in Fig. 135, A; the branchial bars below the muco-cartilaginous shield might or might not be evident, and the line between the branchial and the trigeminal region might or might not be indicated. Such a head-shield would closely resemble those of Didymaspis and Tremataspis respectively. Now suppose the somatic musculature to encroach slightly on the branchial region and also laterally to the end of the anterior branchial region, then we should obtain a shape resembling that of Thyestes (Fig. 135, B). Continue the same process further, the lateral muscle always encroaching further than the median masses, until the whole or nearly the whole branchial region is invested, and we get the head-shield of Cephalaspis (Fig. 135, C); further still, that of Keraspis, and yet still further, that of Ammocoetes (Fig. 135, D).

A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocoetes.]

So close is this similarity, from the comparative point of view, between the dorsal head-shield of the Osteostraci and the dorsal cephalic region of Ammocoetes that it justifies us in taking Ammocoetes as the nearest living representative of such types; it is justifiable, therefore, to interpret by means of Ammocoetes the position of other organs in these forms. First and foremost is the hard plate {339}known as the post-orbital plate, so invariably found. In Fig. 134, C, I have inserted (cr.) the position of the membranous cranium of Ammocoetes, and it is immediately evident that the primordial cranium of the Osteostraci must occupy the exact position indicated by this median hard plate. For this very reason this median plate would be harder than the rest in order to afford a better protection to the brain underneath. This plate, because of its position, may well receive the same name as the similar plate in the trilobite and various palæostracans and be called the glabellum.

EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD--FORMATION OF CRANIUM.

We may thus conceive the position of the nose, lateral eyes, median eyes, and cranium in these old fishes. In addition, other indications of a segmentation in this head-region have been found. The most striking of all the specimens hitherto discovered are some of Thyestes verrucosus, discovered by Rohon, in which the dorsal shield has been removed, and so we are able to see what that dorsal shield covered.

In Fig. 136, I reproduce his drawing of one of his specimens from the dorsal and lateral aspects. These drawings show that the frontal part of the shield covered a markedly segmented part of the animal; five distinct segments are visible apart from the median most anterior region. This segmented region is entirely confined to the prosomatic region, i.e. to the region innervated by the trigeminal nerve. An indication of similar markings is given in Lankester's figure of Eukeraspis pustuliferus (see Fig. 127, B), and, indeed, evidence of a segmentation under the antero-lateral border of the head-shield is recognized at the present time, not only in the Cephalaspidæ, but also in the Pteraspidæ, as was pointed out to me by Smith Woodward in the specimens at the British Museum. Also, in Cyathaspis, Jaekel has drawn attention to markings of a similar segmental nature (Fig. 137).

There seems, then, little doubt but that these primitive fishes possessed something in this region which was of a segmental character, and indicated at least five segments, probably more.

Rohon entitles his discovery 'the segmentation of the primordial cranium.' It would, I think, be better to call it the segmentation of {340}the anterior region of the head, for that is in reality what his figures show, not the segmentation of the primordial cranium, which, to judge from Ammocoetes, was confined to the region of the glabellum.

What is the interpretation of this appearance?

A., lateral eyes; Ep., median eyes.]

Any segmentation in the head-region must be indicative of segments belonging to the trigeminal or prosomatic region, or of segments belonging to the vagus or mesosomatic region. Many palæontologists, looking upon segmentation as indicative of gills and gill-slits, have attempted to interpret such markings as branchial segments, regardless of their position. As the figures show, they extend in front of the eyes and reach round to the front middle line, a position which is simply impossible for gills, but points directly to a segmentation connected with the trigeminal nerve. Comparison with Ammocoetes makes it plain enough that the markings in question are prosomatic in position, and that the gill-region must be sought for in the place {341}where Schmidt and Rohon located it in Thyestes, viz. the so-called occipital region.

This discovery of Rohon's is, in my opinion, of immense importance, for it indicates that, in these early fishes, the prosomatic segmentation, associated with the trigeminal nerve, was much more well-marked than in any fishes living in the present day. Why should it be more well-marked? Turning to the palæostracan, it is very suggestive to compare the markings on their prosomatic carapace with these markings. Again and again we find indications of segmentation in these fossils similar to those seen in the ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan Bunodes and the fish Thyestes, both life size. In the latter I have indicated Rohon's segments; in the former the markings usually seen.

From the evidence of Phrynus, Mygale, etc., as already pointed out, such markings in the palæostracan fossils would indicate the position of the tergo-coxal muscles of the prosomatic appendages, even though such appendages have not yet been discovered, and it is significant that in all these cases there is a distinct indication of a median plate or glabellum in addition to the segmental markings. Especially instructive is the evidence of Phrynus, as is seen by a comparison of Figs. 107 and 108, which shows clearly that this median plate (glab.) covered the brain-region, a brain-region which is isolated and protected from the tergo-coxal muscles by the growth dorsalwards of the flanges of the plastron. In this way an incipient cranium of a membranous character is formed, which helps to give attachment to these tergo-coxal muscles. As such cranium is derived directly from the plastron, it is natural that it should ultimately become cartilaginous, just as occurs when Ammocoetes becomes Petromyzon and the cartilaginous cranium of the latter arises from the membranous cranium of the former. In Galeodes also the growth dorsalwards of the lateral flanges of the plastron to form an incipient cranium in which the brain lies is very apparent.

Both figures natural size.]

{342}I venture, then, to suggest that in the Osteostraci the median hard plate or glabellum protected a brain which was enclosed in a membranous cranium, very probably not yet complete in the dorsal region--certainly not complete if the median pineal eyes so universally found in these ancient fishes were functional--a cranium derived from the basal trabeculæ, in precisely the same manner as we see it already in its commencement in Phrynus and other scorpions. With the completion of this cranium and its conversion into cartilage, and subsequently into bone, an efficient protection was afforded to the most vital part of the animal, and thus the hard head-shield of the Palæostraca and of the earliest fishes was gradually supplanted by the protecting bony cranium of the higher vertebrates.

Step by step it is easy to follow in the mind's eye the evolution of the vertebrate cranium, and because it was evolved direct from the plastron, the impossibility of resolving it into segments is at once manifest; for although the plastron was probably originally segmented, as Schimkéwitsch thinks, all sign of such segmentation had in all probability ceased, before ever the vertebrates first made their appearance on the earth.

It follows further, from the comparison here made, that those antero-lateral markings indicative of segments, found so frequently in these primitive fishes, must be interpreted as due not to gills but to aponeuroses, due to the presence of muscles which moved prosomatic appendages, muscles which arose from the dorsal region in very much the same position as do the muscles of the lower lip in Ammocoetes; the latter, as already argued, represent the tergo-coxal muscles of the last pair of prosomatic appendages--the chilaria or metastoma. Such an interpretation of these markings signifies that the first-formed fishes must have possessed prosomatic appendages of a more definite character than the tentacles of Ammocoetes, something intermediate between those of the palæostracan and Ammocoetes.

For my part I should not be in the least surprised were I to hear that something of the nature of appendages in this region had been found, especially in view of the well-known existence of the pair of appendages in the members of the Asterolepidæ--large, oar-like appendages which may well represent the ectognaths.

{343}THE RELATIONSHIP OF THE OSTRACODERMS.

Of the three groups of fishes--the Heterostraci, the Osteostraci, and the Antiarcha--the last is Devonian, and therefore the latest in time of the three, while the earliest is the first group, as both Pteraspis and Cyathaspis have been found in lower levels of the Silurian age than any of the Osteostraci, and, indeed, Cyathaspis has been discovered in Sweden in the lower Silurian. This, the earliest of all groups of fishes, is confined to two forms only--Pteraspis and Cyathaspis,--for Scaphaspis is now recognized to be the ventral shield of Pteraspis.

Hitherto a strong tendency has existed in the minds both of the comparative anatomist and the palæontologist to look on the elasmobranchs as the earliest fishes, and to force, therefore, these strange forms of fish into the elasmobranch ranks. For this purpose the same device is often used as has been utilized in order to account for the existence of the Cyclostomata, viz. that of degeneration. The evidence I have put forward is very strongly in favour of a connection between the cyclostomes and the cephalaspids, and agrees therefore with all the rest of the evidence that the jawless fishes are more ancient than those which bore jaws--the Gnathostomata.

This is no new view. It was urged by Cope, who classified the Heterostraci, Osteostraci, and Antiarcha under one big group--the Agnatha--from which subsequently the Gnathostomata arose. Cope's arguments have not prevailed up to the present time, as is seen in the writings of Traquair, one of the chief authorities on the subject in Great Britain. He is still an advocate of the elasmobranch origin of all these earliest fishes, and claims that the latest discoveries of the Silurian deposits (Thelodus Pagei) and other members of the Coelolepidæ confirm this view of the question.

This view may be summed up somewhat as follows:--

Cartilaginous jaws would not fossilize, and the Ostracoderms may have possessed them.

They may have degenerated from elasmobranchs just as the cyclostomes are supposed to have degenerated.

Seeing that bone succeeds cartilage, the presence of bony shields in Cephalaspis, etc., indicates that their precursors were cartilaginous, presumably elasmobranch fishes.

Of these arguments the strongest is based on the supposed bony {344}covering of the Osteostraci, with the consequent supposition that their ancestors possessed a cartilaginous covering. This argument is entirely upset, if, as I have pointed out, the structure of the cephalaspid shield is that of muco-cartilage and not of bone. If these plates are a calcified muco-cartilage, then the whole argument for their ancestry from animals with a cartilaginous skeleton falls to the ground, for muco-cartilage is the precursor not only of bone, but also of cartilage itself.

The evidence, then, points strongly in favour of Cope's view that the most primitive fishes were Agnatha, after the fashion of cyclostomes, as is also believed by Smith Woodward, Bashford Dean, and Jaekel.

Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan type, and Ammocoetes alone gives a clue to the nature of the cephalaspid, i.e. the osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is it possible from their structure to obtain any clue as to the actual passage from the palæostracan to the vertebrate?

Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been supposed, for the following reasons:--

The latest discoveries in the Silurian and Devonian deposits have brought to light strange forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to Traquair, be included in the Heterostraci. It possessed, as seen in Fig. 139, large plates, after the fashion of Pteraspis, and also many smaller ones.

The former, from the upper Silurian, belongs to the Coelolepidæ, and was covered over with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis occurred, and, with still further fusion, Pteraspis.

There are always two ways of looking at a question, and it seems to me possible and more probable to turn the matter round and to argue that the original condition of the surface-covering was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a stage on the way to the formation of an elasmobranch, and not a backward stage from the elasmobranch towards Pteraspis.

{345}This method of looking at the problem seems to me to be more in consonance with the facts than the reverse; for, as pointed out by Jaekel, the fishes with large plates are the oldest, and in Cyathaspis, the very oldest of all, the size of the plates is most conspicuous; he considers, therefore, this preconceived view that large plates are formed by the fusion of small ones must give way to the opposite belief.

A., anus; E., lateral eyes.]

So also Rohon, as quoted by Traquair, who, in his first paper accepted Lankester's view that the ridges of the pteraspidian shield were formed by the fusion of a linear arrangement of numbers of placoid scales, suggests in his second paper that these ridges may have been the most primitive condition of the dermal skeleton of the vertebrate, out of which, by differentiation, the dermal denticles (placoid scales) of the selachian, as well as their modifications in the ganoids, teleosteans, and amphibians, have arisen.

One thing is agreed upon on all sides; no sign of bone-corpuscles is to be found in this dermal covering of Pteraspis. In the deeper layers are large spaces, the so-called pulp-cavities leading into narrow canaliculi, the so-called dentine canals. The structure is {346}looked upon as similar to that of the pulp and dentine canals of many fish-scales.

On the other hand, this dermal covering of Pteraspis has been compared by Patten with the arrangement of the chitinous structure of certain parts of the external covering of Limulus, a comparison which to my mind presents a great difficulty. The chitin-layers in Limulus are external to the epidermal cells, being formed by them; the layers in Pteraspis which look like chitin must have been internal to the epidermal layer; for each vascular canal which passes from a pulp-cavity on its way to be distributed into the dentine canals of the ridge gives off short side branches, which open directly into the groove between the ridges. If these canals were filled with blood they could not possibly open directly into the open grooves between the ridges; these openings must, therefore, have been covered over with an epithelial layer which covered over the surface of the animal, and consequently the chitin-like structure must have been internal to the epidermis, and not external, as on Patten's view. The comparison of this structure with the dentine of fish-scales signifies the same thing, for in the latter the epidermis is external to the dentine-plates, the hard skeletal structure is in the position of the cutis, not of the cuticle.

The position appears to me to be this: the dermal cranial skeleton of vertebrates, whether it takes the form of a bony skull or of the dorsal plates of a cephalaspid or a pteraspid is, in all cases, not cuticular, i.e. is not an external formation of the epidermal cells, but is formed in tissue of the nature of connective tissue underlying the epidermis. On the contrary, the hard part of the head-carapace of the palæostracan is an external formation of the epidermal cells.

If, then, this tissue of Pteraspis is not to be looked upon as chitin, how can we imagine its formation? It is certainly not bone, for there are no bone-corpuscles; it is a very regular laminated structure resembling in appearance chitin rather than anything else.

As in all cases of difficulty, turn to Ammocoetes and let us see what clue there is to be found there. The skin of Ammocoetes is peculiar among vertebrates in many respects. It consists of a number of epidermal cells, as in Fig. 140, the varying function of which need not be considered here, covered over with a cuticular layer which is extraordinarily thick for the cuticle of a vertebrate skin; this cuticular layer is perforated with fine canaliculi, through which the {347}secretion of the underlying cells passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the chitinous covering of the arthropod, and like it is perforated with canaliculi, and, according to Lwoff, possibly contains chitin. The epidermal cells rest on a thick layer of most striking appearance (Fig. 141), for it resembles, in an extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at right angles to each other, each fibre running a straight course and possessing its own nucleus. Although the fibres in each layer are packed close together, they are sufficiently apart to form with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus the surface view of this layer shows a regular network of very fine spaces through which nerve-fibres and fluid pass. This layer is easily dissolved in a solution of hypochlorite of soda, a fluid which dissolves chitin. Any one looking at Ammocoetes would say that the only part of its skin which resembles chitin is this laminated layer, and therefore the only part of its skin which would afford an indication of the nature of the skeleton of Pteraspis is this laminated layer, which belongs to the cutis, and not to the cuticle. Yet another significant peculiarity of this layer is its entire disappearance at transformation. Miss Alcock, in a research not yet published, has shown that this layer is completely broken up and absorbed at transformation; the cutis of Petromyzon is formed entirely anew, and no longer presents any regular laminated character, but resembles rather the sub-epidermal connective tissue layer of the skin of higher vertebrates. This laminated layer, then, just like the muco-cartilage, shows, by its complete disappearance at transformation, its ancestral character.

Very suggestive is the arrangement of the different skeletal {348}tissues in the head-region of Ammocoetes. Fig. 141 represents a section through the head near the pineal eye. Most internally is a, a section of the membranous cranium, then comes b, the muco-cartilaginous skeleton, then c, the laminated layer, and finally d, the external cuticle. If in Ammocoetes we possess an epitome of the history of the vertebrate, how would these layers be represented in the past ages, supposing they could be fossilized?

a, cranial wall; b, muco-cartilage; c, laminated layer; d, external cuticular layer.]

The most internal layer a, by the formation of cartilage and then bone, represents the great mass of vertebrate fossils; the next layer b, by a process of calcification, as previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular layer d, no longer thin, is the remnant of the Palæostracan head-carapace. Between these two layers, b and d, lies the laminated layer c. Intermediate to the Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield--a head-shield whose origin is more easily conceivable as arising from something of the nature of the laminated layer than from any other structure represented in Ammocoetes.

My present suggestion, then, is this: the transition from the skeletal covering of the Palæostracan to that of the highest vertebrates was brought about by the calcification of successive layers from without inwards, all of which still remain in Ammocoetes and show how the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal bony cranium of the higher vertebrates.

In Ammocoetes the layer which represents the covering of the {349}Palæostracan has already almost disappeared. At transformation the layers representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the stage representing the higher vertebrates, far from disappearing, by the formation of cartilage reaches a higher stage and prepares the way for the ultimate stage of all--the formation of the bony cranium.

So much for the evidence as to the nature of the structure of the head-shield of the Pteraspidæ.

It suggests that these fishes were covered anteriorly with armoured plates derived from the cutis layer of the skin, a layer which was specially thickened and very vascular, apparently, to enable respiration to be very largely, if not entirely, effected by the surface of the body. It is difficult to understand how the sea-scorpions breathed, and it is easy to see how the formation of ventral and dorsal plates enclosing the mesosomatic appendages may at the outset have hindered the action of the branchiæ. The respiratory chamber, according to my view, had at first the double function of respiration and digestion. A new digestive apparatus was the pressing need at the time; it would, therefore, be of distinct advantage to remove, as much as possible, the burden of respiration from this incipient alimentary canal.

What can be said as to the shape of these ancient forms of fishes? Certain parts of them are absolutely known, other parts are guesswork. They are known to have possessed a dorsal shield, a ventral shield formerly looked upon as belonging to a separate species, called Scaphaspis, and a spine attached to the dorsal shield. The rest of their configuration, as given in Smith Woodward's restoration (Fig. 142) is guesswork; the fish-like body with its scales, the heterocercal tail, is based on the most insufficient evidence of something of the nature of scales having being found near the head-plates.

The dorsal shield is characterized by a pair of lateral eyes situated on the edge of the shield, not as in Cephalaspis near the middle line. In the middle line, where the rostrum meets the large dorsal plate, median eyes were situated. But the slightest sign of any median single nasal opening, such as is so characteristic of the head-shield of the Osteostraci and of Ammocoetes has never been discovered. The olfactory organ must have been situated on the ventral side as in the larval stage of Ammocoetes, or in the Palæostraca. Many of these head-shields are remarkably well preserved, {350}and it is difficult to believe that an olfactory opening would not be seen if any such had existed, as it does in Thyestes.

The difficulty of interpreting these types is the difficulty of understanding their method of locomotion; that is largely the reason why the spine has been placed as if projecting from the back, and a fish-like body with a heterocercal tail-fin added. If, on the contrary, the spine is a terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by means of {351}appendages. At present we have not sufficient evidence to decide this question.

That the animal crawled about in the mud by means of free appendages is by no means an impossible view, seeing how difficult it is to find the remains of appendages in the fossils of this far-back time, even when we are sure that they existed. Thus, for many generations, the appendages of trilobites, which occur in such countless numbers, and in such great variety of form, were absolutely unknown, until at last, in consequence of a fortunate infiltration by pyrites, they were found by Beecher preserved down to the minutest detail. Even to this day no trace of appendages has been found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia.

The whole question of the evidence of any prosomatic appendages in these ancient fishes is one of very great interest, and of late years has been investigated by Patten. It has long been known that forms such as Pterichthys and Bothriolepis possessed two large, jointed locomotor appendages, and Patten has lately obtained better specimens of Bothriolepis than have ever been found before, which show not only the general configuration of the fish, but also the presence of mandibles or gnathites in the mouth-region resembling those of an arthropod. These mandibles had been seen before (Smith Woodward), but Patten's specimens are more perfect than any previously described, and cause him to conclude that these ancient fish were of the nature of arthropods rather than of vertebrates.

Patten has also been able to obtain some excellent specimens of the under surface of the head of Tremataspis, which, as evident in Fig. 143, show the presence of a series of holes, ranging on each side from the mouth-opening, in a semicircular fashion towards the middle line. He considers that these openings indicate the attachments of appendages, in opposition to other observers, such as Jaekel, who look upon them as gill-slits. To my mind, they are not in the right position for gill-slits; they are certainly in a prosomatic rather than in a mesosomatic position, and I should not be at all surprised if further research justified Patten's position. So convinced is he of the presence of appendages in all these old forms, that he considers them to be arthropods rather than vertebrates, although, at the same time, he looks upon them as indicating the origin of vertebrates from arthropods. Here, perhaps, it is advisable to say a few words on Patten's attitude towards this question.

{352}Two years after I had put forward my theory of the derivation of vertebrates from arthropods, Patten published, in the Quarterly Journal of Microscopical Science, simultaneously with my paper in that journal, a paper entitled "The Origin of Vertebrates from Arachnids." In this paper he made no reference to my former publications, but he made it clear that there was an absolutely fundamental difference between our treatment of the problem; for he took the old view that of necessity there must be a reversal of surfaces in order that the internal organs should be in the same relative positions in the vertebrate and in the invertebrate. He simply, therefore, substituted Arachnid for Annelid in the old theory. Because of this necessity for the reversal of surfaces he discarded the terms dorsal and ventral as indicative of the surfaces of an animal, and substituted hæmal and neural, thereby hopelessly confusing the issue and making it often very difficult to understand his meaning.

He still holds to his original opinion, and I am still waiting to find out when the reversal of surfaces took place, for his investigations lead him, as must naturally be the case, to compare the dorsal (or, as he would call it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and of the Pteraspidæ with the dorsal surface of the Palæostraca.

All these ancient fishes are, according to him, still in the arthropod stage, have not yet turned over, though in a peculiarly unscientific manner he argues elaborately that they must have swum on their back rather than on their front, and so indicated the coming reversal. Because they were arthropods they cannot have had a {353}frontal nose-organ; therefore, Patten looks upon the nose and the two lateral eyes of the Osteostraci as a complex median eye, regardless of the fact that the median eyes already existed.

Every atom of evidence Patten has brought forward, every new fact he has discovered, confirms my position and makes his still more hopelessly confused. Keep the animal the right side uppermost, and the evidence of the rocks confirms the transition from the Palæostracan to the Cyclostome; reverse the surfaces, and the attempt to derive the vertebrate from the palæostracan becomes so confused and hopelessly muddled as to throw discredit on any theory of the origin of vertebrates from arthropods. For my own part, I fully expect that appendages will be found not only in the Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue his researches with increasing success. I feel sure, however, his task will be much simplified if he abandons his present position and views the question from my standpoint.

SUMMARY.

The shifting of the nasal tube from a ventral to a dorsal position, as seen in Ammocoetes, is, perhaps, the most important of all clues in connection with the comparison of Ammocoetes to the Palæostracan on the one hand, and to the Cephalaspid on the other; for, whereas the exact counterpart of the opening of such a tube is always found on the dorsal head-shield in all members of the latter group, nothing of the kind is ever found on the dorsal carapace of the former group.

The reason for this difference is made immediately evident in the development of Ammocoetes itself, for the olfactory tube originates as a ventral tube--the tube of the hypophysis--in exactly the same position as the olfactory tube of the Palæostracan, and later on in its development takes up a dorsal position.

In fact, Ammocoetes in its development indicates how the Palæostracan head-shield became transformed into that of the Cephalaspid.

In another most important character Ammocoetes indicates its relationship to the Cephalaspidæ, for it possesses an external skeleton or head-shield composed of muco-cartilage, which is the exact counterpart of the so-called bony head-shield of the latter group; and still more strikingly the structure of the cephalaspidian head-shield is remarkably like that of muco-cartilage. In the one case, by the deposition of calcium salts, a hard external skeleton, capable of being preserved as a fossil, has been formed; in the other, by the absence of the calcium salts, a soft chondro-mucoid matrix, in which the characteristic cells and fibrils are embedded, distinguishes the tissue.

The recognition that the head-shields of these most primitive fishes were not composed of bone, but of muco-cartilage, the precursor of both cartilage and bone, immediately clears up in the most satisfactory manner the whole {354}question of their derivation from elasmobranch fishes; for the main argument in favour of the latter derivation is the exceedingly strong one that bone succeeds cartilage--not vice versâ--therefore, these forms, since their head-shield is bony, must have arisen from some other fishes with a cartilaginous skeleton, most probably of an elasmobranch nature. Seeing, however, that the structure of their shields resembles muco-cartilage much more closely than bone, and that Ammocoetes forms a head-shield of muco-cartilage closely resembling theirs, there is no longer any necessity to derive the jawless fishes from the gnathostomatous; but, on the contrary, we may look with certainty upon the Agnatha as the most primitive group from which the others have been derived.

The history of the rocks shows that the group of fishes, Pteraspis and Cyathaspis, are older than the Cephalaspidæ--come, therefore, phylogenetically between the Palæostraca and the latter group. In this group the head-shields are of a very different character, without any sign of any structure comparable with that of bone, and although they possessed both lateral and median eyes, there is never in any case any trace of a dorsal nasal orifice. Their olfactory passage, like that of the Palæostraca, must have been ventral.

The remarkable comparison which exists between the head-shields of Ammocoetes and Cephalaspis, enables us to locate the position of the brain and cranium of the latter with considerable accuracy, and so to compare the segmental markings found in many of these fossils with the corresponding markings, found either in fossil Palæostraca or on the head-carapaces of living scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial region was covered with a median plate, often especially hard, which corresponded to the glabellum of the trilobite; the growth of the cranium can be traced from its beginnings as the upturned lateral flanges of the plastron to the membranous cranium of Ammocoetes.

From such a comparison it follows that the segments, found in the antero-lateral region of the head-shield, were not segments of the cranium, but of parts beyond the region of the cranium, and from their position must have been segments supplied by the trigeminal nerve, and not by the vagus group; segments, therefore, which did not indicate gills and gill-slits, but muscles, innervated by the trigeminal nerve; muscles which, as indicated by the corresponding markings on the carapace of Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic appendages.

The discovery of the nature of these appendages in the Pteraspidæ and Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and Bothriolepis), is a problem of the future, though in the latter, not only have the well-known oar-like appendages been long since discovered, but Patten has recently found specimens of Bothriolepis which throw light on the anterior masticating gnathite-like appendages which these ancient forms possessed.

{355}CHAPTER XI

THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE

Lateral line organs.--Function of this group of organs.--Poriferous sense-organs on the appendages in Limulus.--Branchial sense-organs.--Prosomatic sense organs.--Flabellum.--Its structure and position.--Sense-organs of mandibles.--Auditory organs of insects and arachnids.--Poriferous chordotonal organs.--Balancers of Diptera.--Resemblance to organs of flabellum.--Racquet-organs of Galeodes.--Pectens of scorpions.--Large size of nerve to all these special sense-organs.--Origin of parachordals and auditory capsule.--Reason why VIIth nerve passes in and out of capsule.--Evidence of Ammocoetes.--Intrusion of glandular mass round brain into auditory capsule.--Intrusion of generative and hepatic mass round brain into base of flabellum.--Summary.

When speaking of the tripartite arrangement of the cranial nerves, an arrangement which gave the clue to the meaning of the cranial segments, I spoke of the trigeminal as supplying the sensory nerves to the skin in the head-region, and I compared this dorsal system of afferent nerves to the system of epimeral nerves in Limulus which supply the prosomatic and mesosomatic carapaces of Limulus with sensory fibres. I compared the ventral system of eye-muscle nerves with the system of nerves supplying the segmental dorso-ventral somatic muscles of the prosomatic region, and I compared the lateral system of mixed nerves with the nerves supplying the prosomatic and mesosomatic appendages of Limulus. I compared, also, the optic nerves and the olfactory nerves with the corresponding nerves in the same invertebrate group. My readers will see at once that one well-marked group of nerves--the auditory and lateral line system--has been entirely omitted up to the present, it has not even been mentioned in the scheme of the cranial segments; I have purposely reserved its consideration until now, because the organs these nerves supply, though situated in the skin, are of such a special character {356}as to form a category by themselves. These nerves cannot be classed among the afferent nerves of the skin any more than the nerves of the optic and olfactory apparatus; they require separate consideration. A very extensive literature has grown up on the subject of this system of lateral line sense-organs and their innervation, the outcome of which is decisively in favour of this system being classed with the sense-organs supplied by the auditory nerve, so that in endeavouring to understand the position of the auditory nerve, we must always bear in mind that any theory as to its origin must apply to the system of lateral line nerves as well.

Now, although the auditory apparatus is common to all vertebrates, the lateral line system is not found in any land-dwelling animals; it belongs essentially to the fishes, and is, therefore, an old system so far as concerns the vertebrate group. Its sense-organs are arranged along the lateral line of the fish, and, in addition, on the head-region in three well-marked lines known as the supra-orbital, infra-orbital, and mandibular line systems. These sense-organs lie in the skin in a system of canals, and are innervated by a special nervous system different to that innervating adjacent skin-areas. The great peculiarity of their innervation consists in the fact that their nerves all belong to the branchial system of nerves; no fibres arise in connection with the trigeminal, but all of them in connection with the facial, glossopharyngeal and vagus nerves. In other words, although organs in the skin, their nerve-supply belongs to the lateral nervous system which supplies splanchnic and not somatic segments, a system which, according to the theory advanced in this book, originated in the nerves supplying appendages. The conclusion, therefore, is that in order to obtain some clue as to the origin of the sense-organs of this system in the assumed palæostracan ancestor, we must examine the mesosomatic appendages and see whether they possess any special sense-organs of similar function.

Further, considering that the auditory organ is to be regarded as a specially developed member of this system, we must especially look for an exceptionally developed organ in the region supplied by the auditory nerve.

The question of the origin of this system of lateral line sense-organs possesses a special interest for all those who attempt to obtain a solution of the origin of vertebrates, for the upholders of the view that the vertebrates have descended from annelids have always {357}found its strongest support in the similarity of two sets of segmental organs found in annelids and vertebrates. On the one hand, great stress was laid upon the similarity of the segmental excretory organs in the two groups of animals, as will be discussed later; on the other, of the similarity of the segmentally arranged lateral sense-organs.

These lateral sense-organs of the annelids have been specially described by Eisig in the Capitellidæ, and, according to Lang, "there are many reasons for considering these lateral organs to be homologous with the dorsal cirri of the ventral parapodia of other Polychæta, and in the family of the Glyceridæ we can follow, almost step by step, the transformation of the cirri into lateral organs." Eisig describes them in the thoracic prebranchial region as slightly different from those in the abdominal branchial region; in the latter region, the ventral parapodia are gill-bearing, so that these lateral organs are in the branchial region closely connected with the branchiæ, just as is also the case in the vertebrates. It is but a small step from the gill-bearing ventral parapodia of the annelid to the gill-bearing appendages of the phyllopod-like protostracan; so that if we assume that this is the correct line along which to search for the origin of the vertebrate auditory apparatus, then, on my theory of the origin of the vertebrates from a group resembling the Protostraca, it follows that special sense-organs must have existed either on or in close connection with the branchial and prebranchial appendages of the protostracan ancestor of the vertebrates, which would form an intermediate link between the lateral organs of the annelids and the lateral and auditory organs of the vertebrates.

Further, these special sense-organs could not have been mere tactile hairs, but must have possessed some special function, and their structure must have been compatible with that function. Can we obtain any clear conception of the original function of this whole system of sense-organs?

A large amount of experimental work has been done to determine the function of the lateral line organs in fishes, and they have been thought at one time or another to be supplementary organs for equilibration, organs for estimating pressure, etc. The latest experimental work done by Parker points directly to their being organs for estimating slow vibrations in water in contradistinction to the quicker vibrations constituting sound. He concludes that surface wave-movements, whether produced by air moving on the water or {358}solid bodies falling into the water, are accompanied by disturbances which are stimuli for the lateral line organs.

One of these segmental organs has become especially important and exists throughout the whole vertebrate group, whether the animal lives on land or in water--this is the auditory organ. Throughout, the auditory organ has a double function--the function of hearing and the function of equilibration. If, then, this is, as is generally supposed, a specialized member of the group, it follows that the less specialized members must possess the commencement of both these functions, just as the experimental evidence suggests.

In our search, then, for the origin of the auditory organ of vertebrates, we must look for special organs for the estimation of vibrations and for the maintenance of the equilibrium of the animal, situated on the appendages, especially the branchial or mesosomatic appendages; and, further, we must specially look for an exceptional development of such segmental organs at the junction of the prosomatic and mesosomatic regions.

Throughout this book the evidence which I have put forward has in all cases pointed to the same conclusion, viz. that the vertebrate arose by way of the Cephalaspidæ from some arthropod, either belonging to, or closely allied to, the group called Palæostraca, of which the only living representative is Limulus. If, then, my argument so far is sound, the appendages of Limulus, both prosomatic and mesosomatic, ought to possess special sense-organs which are concerned in equilibration or the appreciation of the depth of the water, or in some modification of such function, and among these we might expect to find that somewhere at the junction of the prosoma and mesosoma such sense-organs were specially developed to form the beginning of the auditory organ.

Now, it is a striking fact that the appendages of Limulus do possess special sense-organs of a remarkable character, which are clearly not simply tactile. Thus Gegenbaur, as already stated, has drawn attention to the remarkable branchial sense-organs of Limulus; and Patten has pointed out that special organs, which he considers to be gustatory in function, are present on the mandibles of the prosomatic appendages. I myself, as mentioned in my address to the British Association at Liverpool in 1896, searched for some special sense-organ at the junction of the prosoma and mesosoma, and was rewarded by finding that that extraordinary adjunct to the {359}last locomotor appendage, known as the flabellum, was an elaborate sense-organ. I now propose to show that all these special sense-organs are constructed on a somewhat similar plan; that the structure of the branchial sense-organs suggests that they are organs for the estimation of water pressures; that among air-breathing arthropods sense-organs, built up on a somewhat similar plan, are universally found, and are considered to be of the nature of auditory and equilibration organs; and, what is especially of importance, in view of the fact that the most prominent members of the Palæostraca were the sea-scorpions, that the remarkable sense-organs of the scorpions known as the pectens belong apparently to the same group.

THE PORIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS.

On all the branchial appendages in Limulus, special sense-organs are found of a most conspicuous character. They form in the living animal bluish convex circular patches, the situation of which on the appendages is shown in Fig. 58. These organs are not found on the non-branchial operculum. Gegenbaur, who was the first to describe them, has pointed out how the surface of the organ is closely set with chitinous goblets shaped as seen in Fig. 144, A, which do not necessarily project free on the surface, but are extruded on the slightest pressure. Each goblet fits into a socket in the chitinous covering, and is apparently easily protruded by variations of pressure from within. The whole surface of the organ on the appendage is slightly bulged in the living condition, and the chitin is markedly softer here than in the surrounding part of the limb. Each of these organs is surrounded by a thick protection of strongly branching spines. On the surface of the organ itself no spines are found, only these goblets, so that the surface-view presents an appearance as in Fig. 144, B. Each goblet possesses a central pore, which is the termination of a very fine, very tortuous, very brittle chitinous tubule (ch.t.), which passes from the goblet through the layers of the chitin into the subjacent tissue. The goblets vary considerably in size, a few very large ones being scattered here and there. The fine chitinous tubule is especially conspicuous in connection with these largest goblets. In the smaller ones there is the same appearance of a pore and a commencing tube, but I have not been able to trace the tube through the chitinous layers, as in the case of the larger goblets.

{360}[Illustration: FIG. 144.--A, A GOBLET FROM ONE OF THE BRANCHIAL SENSE-ORGANS OF LIMULUS (ch.t., chitinous tubule); B, SURFACE VIEW OF A PORTION OF A BRANCHIAL SENSE-ORGAN.]

Gegenbaur, in his picture, draws a straight tubule passing from every goblet among the fine canaliculi of the chitin. He says they are difficult to see, except in the case of the larger goblets. The tubule from the larger goblets is most conspicuous, and is in my sections always tortuous, never straight, as represented by Gegenbaur. A special branch of the appendage-nerve passes to these organs, and upon the fine branches of this nerve groups of ganglion-cells are seen, very similar in appearance to the groups described by Patten on the terminal branches of the nerves which supply the mandibular organs. At present I can see no mechanism by which the goblets are extruded or returned into place. In the case of the Capitellidæ, Eisig describes retractor muscles by means of which the lateral sense-organs are {361}brought below the level of the surface, and he imagines that the protrusion is effected by hydraulic means, by the aid of the vascular system. In the branchial sense-organs of Limulus there are no retractor muscles, and it seems to me that both retraction and protrusion must be brought about by alterations of pressure in the vascular fluids. Certainly the cavity of the organ is very vascular. If this be so, it seems likely enough that such an organ should be a very delicate organ for estimating changes in the pressure of the external medium, for the position of the goblets would depend on the relation between the pressure of the fluid inside the organ and that on the surface of the appendage. Whether the chitinous tubule contains a nerve-terminal or not I am unable to decide from my specimens, but, judging from Patten's description of the similar chitinous tubules belonging to the mandibular organs, it is most highly probable that these tubules also contain a fine terminal nerve-fibre.

These organs, then, represent segmental branchial sense-organs, of which it can be said their structure suggests that they may be pressure-organs; but the experimental evidence is at present wanting.

Passing now from the branchial to the prosomatic region, the first thing that struck me was the presence of that most conspicuous projection at the base of the last locomotor appendage, which is usually called the flabellum, and has been described by Lankester as an exopodite of this appendage. It is jointed on to the most basal portion of the limb (cf. Fig. 155), and projects dorsally from the limb into the open slit between the prosomatic and mesosomatic carapace, as is seen in Fig. 145 (fl.). Of its two surfaces, the undermost is very convex and the uppermost nearly flat from side to side, the whole organ being bent, so that when the animal is lying half buried in sand, entirely covered over by the prosomatic and mesosomatic carapaces except along this slit between the two, the upper flat or slightly convex surface of the flabellum is exposed to any movement of water through this slit, and owing to its possessing a joint, the direction of the whole organ can be altered to a limited extent. The whole of this flat upper surface is one large sense-organ of a striking character, thus forming a great contrast to the convex under surface, which is remarkably free from tactile spines or special sense-organs.

The nerve going to the flabellum is very large, almost as large as the nerve to the rest of the appendage, and the very large majority {362}of the nerve-fibres turn towards the flat, uppermost side, where the sense-organ is situated. Between the nerve-fibres (n.) and the chitinous surface containing the special sense-tubes masses of cells (gl.) are seen, as in Fig. 146, apparently nerve-cells, which form a broad border between the nerve-fibres and the pigmented chitinogenous layer (p.). On the opposite side, nothing of the sort intervenes between the pigmented layer and the blood-spaces and nerve-fibres which constitute the central mass of the flabellum.

ch., chitinous layers; s.o., sense-organs; sp., spike-organ; p., pigment layer; gl., ganglion cell layer; bl. and n., blood-spaces and nerves.]

At present I am inclined to look upon this mass of cells as constituting a large ganglion, which extends over the whole length and breadth of the upper surface of the flabellum. At the same {363}time, my preparations are not sufficiently clear to enable me to trace out the connections of these cells, especially their connections with the special sense-organs.

bl., blood-spaces; n., nerve; gl., layer of ganglion-cells; p., pigment layer; ch., 1, 2, 3, the three layers of chitin; ch.t., chitinous tubule in large tube of sense-organ; cap., capitellum or swollen extremity of large tube; can., very fine porous canals or canaliculi of chitin.]

In Fig. 148 I give a magnified representation of a section through three of these flabellar sense-organs. As is seen, the section divides itself into four zones: (1) the chitinous layer (ch.); (2) the layer of pigment (p.) and hypodermal cells; (3) the layer of ganglion-cells (gl.); and (4) the layer of nerve-fibres (n.) and blood-spaces (bl.). The chitinous layer is composed of the usual three zones of the Limulus surface--externally (Fig. 148), a thin homogeneous layer, followed by a thick layer of chitin (3), in which the fine vertical tubules or canaliculi are well marked; the external portion (2) of this layer is differentiated from the rest by the presence of well-marked horizontal layers in addition to the canaliculi.

In this chitinous layer the special sense-organs are found. They consist of a large tube which passes through all the layers of the chitin except the thin homogeneous most external layer. {364}This tube is conical in shape, its base, which rests on the pigmented layer, being so large and the organs so crowded together that a section of the chitin across the base of the tubes gives the appearance of a honeycomb, the septa of which is all that remains of the chitin. This large tube narrows down to a thin elongated neck as it passes through the chitin, and then, at its termination, bulges out again into an oval swelling (cap.) situated always beneath the homogeneous most external layer of chitin. Within this tube a fine chitinous tubule (ch. t.) is situated similar to that seen in the branchial sense-organs; it lies apparently free in the tube, not straight, but sinuous, and it passes right through all the chitinous layers to open at the surface as a pore; in the last part of its course, where it passes through the most external layer (1) of chitin, it lies always at right angles to the surface.

If the flabellum be stained with methylene blue and acid fuchsin, then all the canaliculi in the chitin show up as fine red lines, and present the appearance given in Fig. 148, and it is seen that each of the terminations of the tubules is surrounded in the homogeneous layer of chitin by a thick-set circular patch of canaliculi which pass to the very surface of the chitin, while the canaliculi in other parts terminate at the commencement of the homogeneous layer and do not reach the surface. Further, the contents of the oval swelling, and, indeed, of the tube as a whole, are stained blue, the chitinous tubule being either unstained or slightly pink in colour. We see, then, that the chitinous tubule alone reaches the surface, while the large tube, which contains the tubule, terminates in an oval swelling, which often presents a folded or wrinkled appearance, as in Fig. 149 (see also Patten's Fig. 1, Plate I.). This terminal bulging of the tube is reminiscent of the bulging in the chitinous tubes of the lyriform organs of the Arachnida, as described by Gaubert, and of the poriferous chordotonal organs in insects, as described by Graber (see Fig. 150). This terminal swelling is filled with a homogeneous refringent mass staining blue with methylene blue, in which I have seen no trace of a nucleus; through this the chitinous tubule makes its way without any sign of bulging on its part. Patten, in his description of the sense-organs on the mandibles of Limulus, which are evidently the same in structure as those on the flabellum, refers to this homogeneous mass as a coagulum. I doubt whether this is an adequate description; it appears to me to stain rather more {365}readily than a blood-coagulum, yet in the sense of being structureless it resembles a coagulum.

The enormous number of these organs crowded together over the whole flat surface of the flabellum produces a very striking appearance when viewed on the surface. Such a view presents an appearance resembling that of the surface-view of the branchial sense-organs; in both cases the surface is covered with a great number of closely set circular plaques, in the centre of each of which is seen a well-marked pore. The circular plaques in the case of the flabellum are much smaller than those of the branchial sense-organs, and clearly are not protrusible as in the latter organs, the appearance as of a plaque being due to the ring of thickly-set canaliculi round the central tubule, as already described. When stained with methylene blue, the surface view of the flabellum under a low power presents an appearance of innumerable circular blue masses, from each of which springs a fine bent hair, terminating in a pore at the surface. The blue masses are the homogeneous substance (cap.) of the bulgings seen through the transparent external layer of chitin, and the hairs are the terminal part of the chitinous tubules. Patten has represented their appearance in the mandibles in his Fig. 2, Plate I.

The large tubes in the chitin alter in shape according to their position. Those in the middle of the sensory surface of the flabellum, in their course through the chitinous layers, are hardly bent at all; as they approach the two lateral edges of this surface, their long thin neck becomes bent more and more, the bending always being directed towards the middle of the surface (see Fig. 146); in this way the chitinous tubules increase more or less regularly in length from the centre of the organ to the periphery. The large basal part of the conical tube contains, besides the chitinous tubule, a number of nuclei which are confined to this part of the tube; some of these nuclei look like those belonging to nerve-fibres, others are apparently the nuclei of the chitinogenous membrane lining the tube. I have never seen any sign of nerve-cells in the tube itself.

The only other kind of sense-organ I have found in connection with these sense-organs are a few spike-like projections, the appearance of which is given in Fig. 149. I have always seen these in the position given in Fig. 146 (sp.), i.e. at the junction of the surface which contains the sense-organs and the surface which is free from them. They are, so far as I have seen, not very numerous; I have {366}not, however, attempted to examine the whole sense-organ for the purpose of estimating their number and arrangement.

As is seen in Fig. 149, they possess a fine tubule of the same character as that of the neighbouring sense-organs, which apparently terminates at the apex of the projecting spike. They appear to belong to the same group as the other poriferous sense-organs, and are of special interest, because in their appearance they form a link between the latter and the poriferous sense-organs which characterize the pecten of the scorpion (cf. Fig. 152, C).

ch.t., chitinous tubule.]

Such, then, is the structure of this remarkable sense-organ of the flabellum, as far as I have been able to work it out with the materials at my disposal. It is evident that the flabellar organs, apart from the spike-organs, are of the same kind as those described by Patten on the mandibles and chelæ of Limulus, and therefore it is most probable that the nerve-terminals in the chitinous tubules, and the origin of the latter, are similar in the two sets of organs.

These organs, as Patten has described them, are situated in lines on the spines of the mandibles of the prosomatic locomotor appendages, and are grouped closely together to form a compact sense-organ on the surface of the inner mandible (Lankester's epicoxite) (i.m. in Fig. 155), so that a surface-view of the organ here gives the characteristic appearance of these poriferous sense-patches. Precisely similar organs are found on the chilaria, which are, in function at all events, simply isolated mandibles, to use Patten's terminology.

On the digging appendage (ectognath), as the comparison of Fig. 155, A and C, shows, the mandibular spines are almost non-existent, and the inner mandible or epicoxite is not present, so that {367}the special sense-organ of this appendage is represented solely by the flabellum.

This sketch of the special sense-organs of Limulus shows that all the appendages of Limulus possess special sense-organs, with the exception of the operculum. All these sense-organs are formed on the same plan, in that they possess a fine chitinous tubule passing through the layers of chitin into the underlying hypodermal and nervous tissues, which terminates on the surface in a pore. The surface of the chitin where these pores are situated is perfectly smooth, although, in the case of the branchial sense-organs, the goblet-shaped masses of chitin, each of which contains a pore, are able to be pressed out beyond the level of the surface.

As to their functions, we unfortunately do not know much that is definite. Patten considers that he has evidence of a gustatory function in the case of the mandibular organs, and suggests also a temperature-sense in the case of some of these organs. The large organ of the flabellum and the branchial organs he has not taken into consideration. The situation of these organs puts the suggestion of any gustatory function, as far as they are concerned, out of the question; and I do not think it probable that such large specialized organs would exist only for the estimation of temperature, when one sees how, in the higher animals, the temperature-nerves and the nerves of common sensation are universally distributed over the body. As already stated, the structure of the branchial organs seems to me to point to organs for estimating varying pressures more than anything else, and I am strongly inclined to look upon the whole set of organs as the derivatives of the lateral sense-organs of annelids, such as are described by Eisig in the Capitellidæ. This is Patten's opinion with respect to the mandibular organs; and from what I have shown, these organs cannot be separated in type of structure from those of the flabellum and the branchial sense-organs.

In our search, then, for the origin of the vertebrate auditory organ in Limulus and its allies, we see so far the following indications:--

1. The auditory organ of the vertebrate is regarded as a special organ belonging to a segmentally arranged set of lateral sense-organs, whose original function was co-ordination and equilibration.

2. Such a set of segmentally arranged lateral sense-organs is found in annelids in connection with the dorsal cirri of the ventral parapodia.

{368}3. If, as has been supposed, there is a genetic connection between (1) and (2) and if, as I suppose, the vertebrates did not arise from the annelids directly, but from a protostracan group, then it follows that the lateral sense-organs, one of which gave rise to the auditory organ, must have been situated on the protostracan appendages.

4. In Limulus, which is the sole surviving representative of the palæostracan group, such special sense-organs are found on both the prosomatic and mesosomatic appendages, and therefore may be expected to give a direct clue to the origin of the vertebrate auditory organ.

5. Both from its position, its size, and its specialization, the flabellum, i.e. an organ corresponding to the flabellum, must be looked upon as more likely to give a direct clue to the origin of the auditory organ than the sense-organs of the branchial appendages, or the so-called gustatory organs of the mandibles.

THE AUDITORY ORGANS OF ARACHNIDS AND INSECTS.

The difficulty of the investigating these organs consists in the fact that so little is known about them in those Arthropoda which live in the water; the only instance of any organ apparently of the nature of an auditory organ, is the pair of so-called auditory sacs at the base of the antennæ in various decapods. We are in a slightly better position when we turn to the land-living arthropods; here the presence of stridulating organs in so many instances carries with it the necessity of an organ for appreciating sound. It has now been shown that such stridulating organs are not confined to the Insecta, but are present also in the scorpion group, and I myself have added to their number by the discovery of a distinct stridulating apparatus in various members of the Phrynidæ. We may then take it for granted that arachnids as well as insects hear. Where is the auditory organ?

Many observers believe that certain surface-organs found universally among the spiders, to which Gaubert has given the name of lyriform organs, are auditory in function. His investigations show that they are universally present on the limbs and pro-meso-sternite of all spiders; that they are present singly, not in groups, on the limbs of Thelyphonus, and that a group of them exists on the second segment of each limb in the members of the Phrynus tribe. In the latter case this organ is the most elaborate of all described by him.

{369}It is especially noticeable that they do not exist in Galeodes or in the scorpions, but in the former special sense-organs are found in the shape of the so-called 'racquet-organs,' on the basal segments of the most posterior pair of appendages, and also, according to Gaubert, on the extremity of the palps and the first pair of feet, while in the latter they occur in the shape of the pectens.

This observation of Gaubert suggests that the place of the lyriform organs in other arachnids is taken in Galeodes by the racquet-organs, and in the scorpions by the pectens. Bertkau, Schimkéwitsch, and Wagner, as quoted by Gaubert, all suggest that the lyriform organs of the arachnids belong to the same group of sense-organs as the porous chordotonal organs of the Insecta, sense-organs which have been found in every group of Insecta, and are generally regarded as auditory organs. Gaubert does not agree with this, and considers the lyriform organs to be concerned with the temperature-sense rather than with audition.

The chordotonal organs of insects have been specially studied by Graber. He divides them into two groups, the poriferous and the non-poriferous, the former being characterized by the presence of pores on the surface arranged in groups or lines. These poriferous chordotonal organs are remarkably constant in position, being found only at the base of the wings on the subcostal ridge, in marked contrast to the other group of chordotonal organs which are found chiefly on the appendages in various regions. The striking character of this fixity of position of these organs and the universality of their presence in the whole group, led Graber to the conclusion that in these poriferous chordotonal organs we are studying a form of auditory apparatus which characterized the ancestor of the insect-group. These organs are always well developed on the hind wings, and in the large group of Diptera the auditory apparatus has usurped the whole of the function of the wing; for the balancers or 'halteres,' as they are called, are the sole representatives of the hind wings, and they are usually considered to be of the nature of auditory organs. It is instructive to find that such an auditory organ serves not only for the purpose of audition, but also as an organ of equilibration; thus Lowne gives the evidence of various observers, and confirms it himself, that removal of the balancers destroys the power of orderly flight in the animal.

A striking peculiarity of these organs in the Insecta, as described {370}by Graber, is the bulging of the porous canal near its termination (Fig. 150, C). This bulging is filled with a homogeneous, highly refractive material, from which, according to Lowne, a chordotonal thread passes, to be connected with a ganglion-cell and nerve. This sphere of refractive material he calls the 'capitellum' of the chordotonal thread. The presence of this material produces in a surface view an appearance as of a halo around the terminal plaque with its central pore; Graber has attempted to represent this by the white area round the central area (in Fig. 150, B). A very similar appearance is presented by the surface view of the flabellum in those parts where the tube runs straight to the surface, so that the refractive material which fills the oval bulging shines through the overlying chitin and appears to surround the terminal plaque with a translucent halo.

Such a peculiarity must have a very definite meaning, and suggests that the canals in the flabellum of Limulus and in the hind wings of insects belong to the same class of organ, the chitinous tubule with its nerve-terminal in the former corresponding to the chordotonal thread in the latter. One wonders whether this sphere of refractive material or 'capitellum' (to use Lowne's phraseology) is so universally present in order to act as a damper upon the vibrations of the chordotonal thread in the one case and of the {371}chitinous tubule in the other, just as the membrana tectoria and the otoliths act in the case of the vertebrate ear.

Patten says that the only organs which seem to him to be comparable with the gustatory porous organs of Limulus are the sense-organs in the extremities of the palps and of the first pair of legs of Galeodes, as described by Gaubert. I imagine that he was thinking only of arachnids, for the comparison of his drawings with those of Graber show what a strong family resemblance exists between the poriferous sense-organs of Limulus and those of the insects. On the course of the terminal nerve-fibres, between the nerve-cell and their entrance into the porous chitinous canal, Graber describes the existence of rods or scolophores. On the course of the terminal fibres in the Limulus organ, between the nerve-cells and their entrance into the porous chitinous canal, Patten describes a spindle-shaped swelling, containing a number of rod-like thickenings among the fibrils in the spindle, which present an appearance reminiscent of the rods described by Graber.

It appears as though a type of sense-organ, characterized by the presence of pores on the surface and a fine chitinous canal which opens at these pores, was largely distributed among the Arthropoda. According to Graber, this kind of organ represents a primitive type of sense-organ, which was probably concerned with audition and equilibration, and he expresses surprise that similar organs have not been discovered among the Crustacea. It is, therefore, a matter of great interest to find that so ancient a type of animal as Limulus, closely allied to the primitive crustacean stock, does possess poriferous sense-organs upon its appendages which are directly comparable with these poriferous chordotonal organs of the Insecta.

THE PECTENS OF SCORPIONS.

Among special sense-organs such as those with which I am now dealing, the pectens of scorpions and the 'racquet-organs' of Galeodes must, in all probability, be classed. I have given my reasons for this conclusion in my former paper. At present such reasons are based entirely upon the structure of the organs; experimental {372}evidence as to their function is entirely wanting. With respect to the pectens of the scorpion (Fig. 151), it has been suggested that they are of the nature of copulatory organs, a suggestion which may be dismissed without hesitation, for they are not constructed after the fashion of claspers, but are simply elaborate sense-organs, and, as such, are found equally in male or female. The only observer who has hitherto specially studied the structure of the sense-organs in the pecten is, as far as I know, Gaubert, and he describes their structure together with that of the sense-organs of the racquets of Galeodes, in connection with the lyriform organs of arachnids, as though he recognized a family resemblance between the three sets of organs.

The operculum is marked out with dots, and on each side of it is seen one of the pectens.]

The pecten of the scorpions is an elaborate sense-organ, or rather group of sense-organs, the special organ being developed on each tooth of the comb; its surface, which is frequently flattened, being directed backwards and inwards, when the axis of the pecten is horizontal at right angles to the length of the body. The surface view of this part of the tooth resembles that of the branchial organs or of the flabellum in Limulus, in that it is thickly covered with circular patches, in the centre of which an ill-defined appearance as of a fine pore is seen. In Fig. 152, B, I give a sketch of the surface view of a part of the organ.

Transverse sections of a tooth of the comb of Scorpio Europæus present the appearance given in Fig. 152, A, and show that each of these circular patches is the surface-view of a goblet-shaped chitinous organ, Fig. 152, C, from the centre of which a short, somewhat cylindrical chitinous spike projects. Within this spike, and running through the goblet into the subjacent tissue, is a fine tubule. The series of goblets gives rise to the appearance of the circular plaques on the surface-view, while the spike with its tubule {373}is the cause of the ill-defined appearance of the central pore, just as the terminal pore is much less conspicuous on surface-view in the spike-organs of the flabellum than in the purely poriferous organs, no part of which projects beyond the level of the chitinous surface.

bl. and n., region of blood-spaces and nerves; gl., ganglion-cell layer; ch., modified chitinous layer; s.o., sense-organ.]

The fine tubule is soon lost in the thickened but soft modification of the chitinous layer (ch.) which is characteristic of the sense-organ; at all events, I have not succeeded in tracing it through this layer with any more success than in the corresponding case of the tubules belonging to the smaller goblets of the branchial sense-organ of Limulus already described.

At the base of the modified chitinous layer a series of cells is seen, many, if not all, of which belong to the chitinogenous layer. Next to these is the marked layer of ganglion-cells (gl.), similar to those seen in the flabellum of Limulus. The rest of the space in the section of the tooth is filled up with nerves (n.) and blood-spaces (bl.) just as in the section, Fig. 146, of the flabellum of Limulus.

Gaubert does not appear to have seen the goblets at all clearly; {374}he describes them simply as conical eminences, and states that they "recouvrent un pore analogue a celui des poils mais plus petit; il est rempli par le protoplasma de la couche hypodermique." From the ganglion, according to him, nervous prolongations pass, which traverse the chitinogenous layer and terminate at the base of the conical eminences. Each of these prolongations "présente sur son trajet, mais un peu plus près du ganglion que de sa terminaison périphérique, une cellule nerveuse fusiforme (g.) offrant, comme celles du ganglion, un gros noyau." He illustrates his description with the following, Fig. 153, taken from his paper.

n., nerve; gl., ganglion.]

I have not been able to obtain any evidence of a fusiform nerve-cell on the course of the terminal nerve-fibres as depicted by him; fusiform cells there are in plenty, as depicted in my drawing, but none with a large nucleus resembling those of the main ganglion. In no case, either in the flabellum or in the branchial organs of Limulus, or in the pecten-organs, have I ever seen a ganglion-cell within the chitin-layer; all the nuclei seen there resemble those of the cells of the hypodermis or else the elongated nuclei characteristic of the presence of nerve-fibres. Gaubert's drawing is a striking one, and I have looked through my specimens to see whether there was anything similar, but have hitherto failed to obtain any definite evidence of anything of the kind.

I feel, myself, that an exhaustive examination of the structure and function of the pecten of scorpions ought to be undertaken. At present I can only draw the attention of my readers to the similarity of the arrangement of parts, and of the nature of the end-organs, in the sense-organs of the flabellum of Limulus and of the pecten of the scorpion. In both cases the special nerve-fibres terminate in a massive ganglion, situated just below the chitinogenous layer. In both cases the terminal fibres from these ganglion-cells pass through the modified chitinous layer to supply end-organs of a striking character; and although the end-organ of the pecten of the scorpion does {375}not closely resemble the majority of the end-organs of the flabellum, yet it does resemble, on the one hand, the isolated poriferous spikes found on the flabellum (Fig. 149) and, on the other, the poriferous goblets found on the sense-patches of the branchial appendages of Limulus (Fig. 144, A), so that a combination of these two end-organs would give an appearance very closely resembling that of the pecten of the scorpion.

Finally, the special so-called 'racquet-organs' of Galeodes, which are found on the most basal segments of the last pair of prosomatic appendages, ought also to be considered here. Gaubert has described their structure, and shown how the nerve-trunk in the handle of the racquet splits up into a great number of separate bundles, which spread out fan-shaped to the free edge of the racquet; each of these separate bundles supplies a special sense-organ, which terminates as a conical eminence on the floor of a deep groove, running round the whole free edge of the racquet. This groove is almost converted into a canal, owing to the projection of its two sides. Gaubert imagines that the sense-organs are pushed forward out of the groove to the exterior by the turgescence of the whole organ; each of the nerve-fibres forming a bundle is, according to Gaubert, connected with a nerve-cell before it reaches its termination.

This sketch of the special sense-organs on the appendages of Limulus, of the scorpions, of Galeodes, and other arachnids, and their comparison with the porous chordotonal organs of insects, affords reason for the belief that we are dealing here with a common group of organs, which, although their nature is not definitely known, have largely been accredited with the functions of equilibration and audition, a group of organs among which the origin of the auditory organ of vertebrates must be sought for, upon any theory of the origin of vertebrates from arthropods.

Whenever in any animal these organs are concentrated together to form a special organ, it is invariably found that the nerve going to this organ is very large, out of all proportion to the size of the organ, and also that the nerve possesses, close to its termination in the organ, large masses of nerve-cells. Thus, although the whole hind wing in the blow-fly has been reduced to the insignificant balancers or 'halteres,' yet, as Lowne states, the nerves to them are the largest in the body.

The pectinal nerve in the scorpion is remarkable for its size, and {376}so, also, is the nerve to the flabellum in Limulus, while the large size of the auditory nerve in the vertebrate, in distinction to the size of the auditory apparatus, has always aroused the attention of anatomists.

Throughout this book my attention has been especially directed to both Limulus and the scorpion group in endeavouring to picture to myself the ancestor of the earliest vertebrates, because the Eurypteridæ possessed such marked scorpion-like characteristics; so that in considering the origin of a special sense-organ, such as the vertebrate auditory organ near the junction of the prosoma and mesosoma, it seems to me that the presence of such marked special sense-organs as the flabellum on the one hand and the pecten on the other, must both be taken into account, even although the former is an adjunct to a prosomatic appendage, while the latter represents, according to present ideas, the whole of a mesosomatic appendage.

From the point of view that the VIIIth nerve represents a segment immediately posterior to that of the VIIth, it is evident that an organ in the situation of the pecten, immediately posterior to the operculum, i.e. according to my view, posterior to the segment originally represented by the VIIth nerve, is more correctly situated than an organ like the flabellum, which belongs to a segment anterior to the operculum.

On the other hand, from the point of view of the relationship between the scorpions and the king-crabs, it is a possibly debatable question whether the pecten really belongs to a segment posterior to the operculum. The position of any nerve in a series depends upon its position of origin in the central nervous system, rather than upon the position of its peripheral organ. Now, Patten gives two figures of the brain of the scorpion built up from serial sections. In both he shows that the main portion of the pectinal nerve arises from a swelling, to which he gives the name ganglion nodosum. This swelling arises on each side in close connection with the origin of the most posterior prosomatic appendage-nerve, according to his drawings, and posteriorly to such origin he figures a small nerve which he says supplies the distal parts of the sexual organs. This nerve is the only nerve which can be called the opercular nerve, and apparently arises posteriorly to the main part of the pectinal nerve. If this is so, it would indicate that the pectens arose from sense-organs which were originally, like the flabella, pre-opercular in position, but have shifted to a post-opercular position.

{377}THE ORIGIN OF THE PARACHORDALS AND AUDITORY CARTILAGINOUS CAPSULE.

In addition to what I have already said, there is another reason why a special sense-organ such as the pecten is suggestive of the origin of the vertebrate auditory organ, in that such a suggestion gives a clue to the possible origin of the parachordals and auditory cartilaginous capsules.

In the lower vertebrates the auditory organ is characterized by being surrounded with a cartilaginous capsule which springs from a special part of the axial cartilaginous skeleton on each side, known as the pair of parachordals. The latter, in Ammocoetes, form a pair of cartilaginous bars, which unite the trabecular bars with the branchial cartilaginous basket-work. They are recognized throughout the Vertebrata as distinct from the trabecular bars, thus forming a separate paired cartilaginous element between the trabeculæ and the branchial cartilaginous system, which of itself indicates a position for the auditory capsule between the prosomatic trabeculæ and the mesosomatic branchial cartilaginous system.

The auditory capsule and parachordals when formed are made of the same kind of cartilage as the trabeculæ, i.e. of hard cartilage, and are therefore formed from a gelatin-containing tissue, and not from muco-cartilage. Judging from the origin already ascribed to the trabeculæ, viz. their formation from the great prosomatic entochondrite or plastron, this would indicate that a second entochondrite existed in the ancestor of the vertebrate in the region of the junction of the prosoma and mesosoma, which was especially connected with the sense-organ to which the auditory organ owes its origin. This pair of entochondrites becoming cartilaginous would give origin to the parachordals, and subsequently to the auditory capsules, their position being such that the nerve to the operculum would be surrounded at its origin by the growth of cartilage.

On this line of argument it is very significant to find that the scorpions do possess a second pair of entochondrites, viz. the supra-pectinal entochondrites, situated between the nerve-cord and the pectens, so that if the ancestor of the Cephalaspid was sufficiently scorpion-like to have possessed a second pair of entochondrites and at the same time a pair of special sense-organs of the nature either of {378}the pectens or flabella, then the origin of the auditory apparatus would present no difficulty.

It is also easy to see that the formation of the parachordals from entochondrites homologous with the supra-pectinal entochondrites, would give a reason why the VIIth or opercular nerve is involved with the VIIIth in the formation of the auditory capsule, especially if the special sense-organ which gave origin to the auditory organ was originally a pre-opercular sense-organ such as the flabellum, which subsequently took up a post-opercular position like that of the pecten.

THE EVIDENCE OF AMMOCOETES.

As to the auditory apparatus itself, we see that the elaborate organ for hearing--the cochlea--has been evolved in the vertebrate phylum itself. In the lowest vertebrates the auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a simple sac forms the early stage of the auditory vesicle in Ammocoetes, according to Shipley; subsequently, by a series of foldings and growings together, the chambers of the ear of the adult Petromyzon, as figured and described by Retzius, are formed. Further, we see that throughout the Vertebrata this sac was originally open to the exterior, the auditory vesicle being first an open pit, which forms a vesicle by the approximating of its sides, the last part to close being known as the recessus labyrinthicus; in many cases, as in elasmobranchs, this part remains open, or communicates with the exterior by means of the ductus endolymphaticus.

Judging, therefore, from the embryological evidence, it would appear that the auditory organ originated as a special sense-organ, formed by modified epithelial cells of the surface, which epithelial surface becoming invaginated, came to line a closed auditory vesicle under the surface. This special sense-organ was innervated from a large ganglionic mass of nerve-cells, situated close against the peripheral sense-cells, the axis-cylinder processes of which formed the sensory roots of the nerve.

Yet another peculiarity of striking significance is seen in connection with the auditory organ of Ammocoetes. The opening of the cartilaginous capsule towards the brain is a large one (Fig. 154), and {379}admits the passage not only of the auditory and facial nerves, but also of a portion of the peculiar tissue which surrounds the brain. The large cells of this tissue, with their feebly staining nuclei and the pigment between them, make them quite unmistakable; and, as I have already stated, nowhere else in the whole of Ammocoetes is such a tissue found. When I first noticed these cells within the auditory capsule, it seemed to me almost impossible that my interpretation of them as the remnant of the generative and hepatic tissue which surrounds the brain of animals such as Limulus could be true, for it seemed too unlikely that a part of the generative system could ever have become included in the auditory capsule. Still, they are undoubtedly there; and, as already argued with respect to the substance round the brain, they must represent some pre-existing tissue which was functional in the ancestor of Ammocoetes. If my interpretation is right, this tissue must be generative and hepatic tissue, and its presence in the auditory capsule immediately becomes a most important piece of evidence, for it proves that the auditory organ must have been originally so situated that a portion of the generative and hepatic mass surrounding the cephalic region of the nervous system followed the auditory nerve to the peripheral sense-organ.

Au., auditory organ; VIII, auditory nerve; gl., ganglion cells of VIIIth nerve; Au. cart., cartilaginous auditory capsule; gen., cells of old generative tissue round brain and in auditory capsule; bl., blood-vessels]

{380}Here there was a test of the truth of my theory ranking second only to the test of the median eyes; the strongest possible evidence of the truth of any theory is given when by its aid new and unexpected facts are brought to light. The theory said that in the group of animals from which the vertebrates arose, a special sense-organ of the nature of an auditory organ must have existed on the base of one of the appendages situated at the junction of the prosoma and mesosoma, and that into this basal part of the appendage a portion of the cephalic mass of generative and hepatic material must have made its way in close contiguity to the nerve of the special organ.

The only living example which nearly approaches the ancient extinct forms from which, according to the theory, the vertebrates arose, is Limulus, and, as has already been shown, in this animal, in the very position postulated by the theory, a large special sense-organ--the flabellum--exists, which, as already stated, may well have given rise to a sense-organ concerned with equilibration and audition. If, further, it be found that a diverticulum of the generative and hepatic material does accompany the nerve of the flabellum in the basal part of the appendage, then the evidence becomes very strong that the auditory organ of Ammocoetes, i.e. of the ancient Cephalaspids, was derived from an organ homologous with the flabellum; that, therefore, the material round the brain of Ammocoetes was originally generative and hepatic material; that, in fact, the whole theory is true, for all the parts of it hang together so closely that, if one portion is accepted, all the rest must follow. As pointed out in my address at Liverpool, and at the meeting of the Philosophical Society at Cambridge, it is a most striking fact that a mass of the generative and hepatic tissue does accompany the flabellar nerve into the basal part of this appendage. Into no other appendage of Limulus is there the slightest sign of any intrusion of the generative and hepatic masses; nowhere, except in the auditory capsule, is there any sign of the peculiar large-celled tissue which surrounds the brain and upper part of the spinal cord of Ammocoetes. The actual position of the flabellum on the basal part of the ectognath is shown in Fig. 155, A, and in Fig. 155, B, I have removed the chitin, to show the generative and hepatic tissue (gen.) lying beneath.

The reason why, to all appearance, the generative and hepatic mass penetrates into the basal part of this appendage only is apparent {381}when we see (as Patten and Redenbaugh have pointed out) to what part of the appendage the flabellum in reality belongs.

fl., flabellum; cox., coxopodite; ent., entocoxite; m., mandible; i.m., inner mandible or epicoxite.]

Patten and Redenbaugh, in their description of the prosomatic appendages of Limulus, describe the segments of the limbs as (1) the dactylopodite, (2) the propodite, (3) the mero- and carpo-podites, (4) the ischiopodite, (5) the basipodite, and (6) the coxopodite (cox. in Fig. 155). Still more basal than the coxopodite is situated the entocoxite (ent. in Fig. 155), which is composed of three sclerites {382}or sensory knobs, to use Patten's description. The middle one of these three sclerites enlarges greatly in the digging appendage, and grows over the coxopodite to form the base from which the flabellum springs. Thus, as they have pointed out, the flabellum does not belong to the coxopodite of the appendage, but to the middle sensory knob of the entocoxite. Upon opening the prosomatic carapace, it is seen that the cephalic generative and hepatic masses press closely against the internal surface of the prosomatic carapace and also of the entocoxite, so that any enlargement of one of the sensory knobs of the entocoxite would necessarily be filled with a protrusion of the generative and hepatic masses. This is the reason why the generative and hepatic material apparently passes into the basal segment of the ectognath, and not into that of the endognaths; it does not really pass into the coxopodite of the appendage, but into an enlarged portion of the entocoxite, which can hardly be considered as truly belonging to the appendage. Kishinouye has stated that a knob arises in the embryo at the base of each of the prosomatic locomotor appendages, but that this knob develops only in the last or digging appendage (ectognath) forming the flabellum. Doubtless the median sclerites of the entocoxites of the endognaths represent Kishinouye's undeveloped knobs.

I conclude, therefore, that the flabellum, together with its basal part, is an adjunct to the appendage rather than a part of it, and might, therefore, easily remain as a separate and well-developed entity, even although the appendage itself dwindled down to a mere tentacle.

The evidence appears to me very strong that the flabellum of Limulus and the pecten of scorpions are the most likely organs to give a clue to the origin of the auditory apparatus of vertebrates. At present both the Eurypterids and Cephalaspids have left us in the lurch; in the former there is no sign of either flabellum or pecten; in the latter, no sign of any auditory capsule beyond Rohon's discovery of two small apertures situated dorsally on each side of the middle line in Tremataspis, which he considers to be the termination of the ductus endolymphaticus on each side. In both cases it is probable, one might almost say certain, that any such special sense-organ, if present, was not situated externally, but was sunk below the surface as in Ammocoetes.

The method by which such a sense-organ, situated externally on {383}the surface of the animal, comes phylogenetically to form the lining wall of an internally situated membranous capsule is given by the ontogeny of this capsule, which shows step by step how the sense-organ sinks in and forms a capsule, and finally is entirely removed from the surface except as regards the ductus endolymphaticus.

SUMMARY.

The special apparatus for hearing is of a very different character from that for vision or for smell, for its nerve belongs to the infra-infundibular group of nerves, and not to the supra-infundibular, as do those of the other two special senses. Of the five special senses the nerves for touch, taste, and hearing, all belong to the infra-infundibular segmental nerve-groups. The invertebrate origin, then, of the vertebrate auditory nerve must be sought for in the infra-oesophageal segmental group of nerves, and not in the supra-oesophageal.

The organs supplied by the auditory nerve are only partly for the purpose of hearing; there is always present also an apparatus--the semicircular canals--concerned with equilibration and co-ordination of movements. Such equilibration organs are not confined to the auditory nerve, but in the water-living vertebrates are arranged segmentally along the body, forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral line organs, which has been specially developed.

These lateral line organs have been compared to similar segmental organs found in connection with the appendages in worms, especially the respiratory appendages. In accordance with this suggestion we see that they are all innervated from the region of the respiratory nerves--the vagus, glosso-pharyngeal, and facial--nerves which originally supplied the respiratory appendages of the palæostracan ancestor.

The logical conclusion is that the appendages of the Palæostraca possessed special sense-organs concerned with the perception of special vibrations, especially in the mesosomatic or respiratory region, and that somewhere at the junction of the prosoma and mesosoma, one of these sense-organs was specially developed to form the origin of the vertebrate auditory apparatus.

Impressed by this reasoning I made search for some specially striking sense-organ at the base of one of the appendages of Limulus, at the junction of the prosoma and mesosoma, and was immediately rewarded by the discovery of the extraordinary nature of the flabellum, which revealed itself as an elaborate sense-organ supplied with a nerve out of all proportion to its size. Up to this time no one had the slightest conception that this flabellum was a special sense-organ; the discovery of its nature was entirely due to the logical following out of the theory of the origin of vertebrates described in this book.

The structure of this large sense-organ is comparable with that of the sense-organs of the pectens of the scorpion, and of many other organs found on the appendages of various members of the scorpion group, of arachnids and {384}other air-breathing arthropods. Many of these organs, such as the lyriform organs of arachnids, and the 'halteres' or balancers of the Diptera, are usually regarded as auditory and equilibration organs.

On all the mesosomatic appendages of Limulus very remarkable sense-organs are found, apparently for estimating pressures, which, when the appendages sank into the body to form with their basal parts the branchial diaphragms of Ammocoetes, could easily be conceived as remaining at the surface, and so giving rise to the lateral line organs.

Further confirmation of the view that an organ, such as the flabellum, must be looked upon as the originator of the vertebrate auditory organ, is afforded by the extraordinary coincidence that in Limulus a diverticulum of the generative and hepatic mass accompanies the flabellar nerve into the basal part of the digging appendage, while in Ammocoetes, accompanying the auditory nerve into the auditory capsule, there is seen a mass of cells belonging to that peculiar tissue which fills up the space between the brain and the cranial walls, and has already, on other grounds, been homologized with the generative and hepatic masses which fill up the encephalic region of Limulus.

For all these reasons special sense-organs, such as are found in the flabellum of Limulus and in the pectens of scorpions, may be looked upon as giving origin to the vertebrate auditory apparatus. In such case it is highly probable that the parachordals, with the auditory capsules attached, arose from a second entochondrite of the same nature as the plastron; a probability which is increased by the fact that the scorpion does possess a second entochondrite, which, owing to its special relations to the pecten, is known as the supra-pectinal entochondrite.

{385}CHAPTER XII

THE REGION OF THE SPINAL CORD

Difference between cranial and spinal regions.--Absence of lateral root.--Meristic variation.--Segmentation of coelom.--Segmental excretory organs.--Development of nephric organs; pronephric, mesonephric, metanephric.--Excretory organs of Amphioxus.--Solenocytes.--Excretory organs of Branchipus and of Peripatus, appendicular and somatic.--Comparison of coelom of Peripatus and of vertebrate.--Pronephric organs compared to coxal glands.--Origin of vertebrate body-cavity (metacoele).--Segmental duct.--Summary of formation of excretory organs.--Origin of somatic trunk-musculature.--Atrial cavity of Amphioxus.--Pleural folds.--Ventral growth of pleural folds and somatic musculature.--Pleural folds of Cephalaspidæ and of Trilobita.--Significance of the ductless glands.--Alteration in structure of excretory organs which have lost their duct in vertebrates and in invertebrates.--Formation of lymphatic glands.--Segmental coxal glands of arthropods and of vertebrates.--Origin of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless glands.--Summary.

The consideration of the auditory nerve and the auditory apparatus terminates the comparison between the cranial nerves of the vertebrate and the prosomatic and mesosomatic nerves of the arthropod, and leaves us now free to pass on to the consideration of the vertebrate spinal nerves and the organs they supply. Before doing so, it is advisable to pass in review the conclusions already attained.

Starting with the working hypothesis that the central nervous system of the vertebrate has arisen from the central nervous system of the arthropod, but has involved and enclosed the alimentary canal of the latter in the process, so that there has been no reversal of surfaces in the derivation of the one form from the other, we have been enabled to compare closely all the organs of the head-region in the two groups of animals, and in no single case have we been compelled to make any startling or improbable assumptions. The simple following out of this clue has led in every case in the most natural {386}manner to the interpretation of all the organs in the head-region of the vertebrate from the corresponding organs of the arthropod.

That it is possible to bring together all the striking resemblances between organs in the two classes of animals, such as I have done in preceding chapters, has been ascribed to a perverted ingenuity on my part--a suggestion which is flattering to my imaginative powers, but has no foundation of fact. There has been absolutely no ingenuity on my part; all I have done is to compare organs and their nerve-supply, as they actually exist in the two groups of animals, on the supposition that there has been no turning over on to the back, no reversal of dorsal and ventral surfaces. The comparison is there for all to read; it is all so simple, so self-evident that, given the one clue, the only ingenuity required is on the part of those who fail to see it.

The great distinction that has arisen between the two head-regions is the disappearance of appendages as such, never, however, of important organs on those appendages. If the olfactory organs of the one group were originally situated on antennules, the olfactory organs still remain, although the antennules as such have disappeared. The coxal excretory organs at the base of the endognaths remain and become the pituitary body. A special sense-organ, such as the flabellum of Limulus or the pecten of scorpion, remains and gives rise to the auditory organ. A special glandular organ, the uterus in the base of the operculum, remains, and gives rise to the thyroid gland. The branchiæ and sense-organs on the mesosomatic appendages remain, and even the very muscles to a large extent. As will be seen later, the excretory organs at the base of the metasomatic appendages remain. It is merely the appendage as such which vanishes either by dwindling away, or by so great an alteration as no longer to be recognizable as an appendage.

This dwindling process was already in full swing before the vertebrate stage; it is only a continuation of a previous tendency, as is seen in the dwindling of the prosomatic appendages in the Merostomata and the inclusion of the branchiæ within the body of the scorpion. Already among the Palæostraca, swimming had largely taken the place of crawling. The whole gradual transformation from the arthropod to the vertebrate is associated with a transformation from a crawling to a swimming animal--with the concomitant loss of locomotor appendages as such, and the alteration of the shape of {387}the animal into the lithe fish-like form. The consideration of the manner in which this latter change was brought about, takes us out of the cranial into the spinal region.

If we take Limulus as the only living type of the Palæostraca, we are struck with the fact that the animal consists to all intents and purposes of prosomatic and mesosomatic regions only; the metasoma consisting of the segments posterior to the mesosoma is very insignificant, so that the large mass of the animal consists of what has become the head-region in the vertebrate; the spinal region, which has become in the higher vertebrates by far the largest region of the body, can hardly be said to exist in such an animal as Limulus. As to the Eurypterids and others, similar remarks may be made, though not to the same extent, for in them a distinct metasoma does exist.

In this book I have considered up to the present the cranial region as a system of segments, and shown how such segments are comparable, one by one, with the corresponding segments in the prosoma and mesosoma of the presumed arthropod ancestor.

In the spinal region such direct comparison is not possible, as is evident on the face of it; for even among vertebrates themselves the spinal segments are not comparable one by one, so great is the variation, so unsettled is the number of segments in this region. This meristic variation, as Bateson calls it, is the great distinctive character of the spinal region, which distinguishes it from the cranial region with its fixed number of nerves, and its substantive rather than meristic variation. At the borderland, between the two regions, we see how the one type merges into the other; how difficult it is to fix the segmental position of the spino-occipital nerves; how much more variable in number are the segments supplied by the vagus nerves than those anterior to them.

This meristic variation is a sign of instability, of want of fixedness in the type, and is evidence, as already pointed out, that the spinal region is newer than the cranial. This instability in the number of spinal segments does not necessarily imply a variability in the number of segments of the metasoma of the invertebrate ancestor; it may simply be an expression of adaptability in the vertebrate phylum itself, according to the requirements necessitated by the conversion of a crawling into a swimming animal, and the subsequent conversion of the swimming into a terrestrial or flying animal.

{388}However many may have been the original number of segments belonging to the spinal region, one thing is certain--the segmental character of this region is remarkably clearly shown, not only by the presence of the segmental spinal nerves, but also by the marked segmentation of the mesoblastic structures. The question, therefore, that requires elucidation above all others is the origin of the spinal mesoblastic segments, i.e. of the coelomic cavities of the trunk-region, and the structures derived from their walls.

Proceeding on the same lines as in the case of the cranial segments, it is necessary in the first instance to inquire of the vertebrate itself as to the scope of the problem in this region. In addition to the variability in the number of segments so characteristic of the spinal region, the complete absence in each spinal segment of a lateral root affords another marked difference between the two regions. Here, except, of course, at the junction of the spinal and cranial regions, each segmental nerve arises from two roots only, dorsal and ventral, and these roots are separately sensory and motor, and not mixed in function as was the lateral root of each cranial segment. Now, these lateral roots were originally the nerves supplying the prosomatic and mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral roots in the spinal region implies that in the vertebrate none of the musculature belonging to the metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to the origin of the spinal segments must be sought for in the segmentation of the body-muscles.

Here, in contradistinction to the cranial region, the segmentation is most marked, for the somatic spinal musculature of all vertebrates can be traced back to a simple sheet of longitudinal ventral and dorsal muscles, such as are seen in all fishes. This sheet is split into segments or myotomes by transverse connective tissue septa or myo-commata; each myotome corresponding to one spinal segment.

In addition to the evidence of segmentation afforded by the body-musculature in all the higher vertebrates, similar evidence is given by the segmental arrangement of parts of the supporting tissue to form vertebræ. Such segments have received the name of sclerotomes, and each sclerotome corresponds to one spinal segment.

Yet another marked peculiarity of this region is the segmental arrangement of the excretory organs. Just as our body-musculature {389}has arisen from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the excretory organs.

The origin of all these three separate segmentations may, in accordance with the phraseology of the day, be included in the one term--the origin of the spinal mesoblastic segments--i.e. of the coelomic cavities of the trunk-region and the structures derived from their walls.

THE ORIGIN OF THE SEGMENTAL EXCRETORY ORGANS.

Of these three clues to the past history of the spinal region, the segmentation manifested by the presence of vertebræ is the least important, for in Ammocoetes there is no sign of vertebræ, and their indications only appear at transformation. Especially interesting is the segmentation due to the excretory organs, for the evidence distinctly shows that such excretory organs have steadily shifted more and more posteriorly during the evolution of the vertebrate.

In Limulus the excretory organs are in the prosomatic region--the coxal glands; these become in the vertebrate the pituitary body.

In Amphioxus the excretory organs are in the mesosomatic region, segmentally arranged with the gills.

In vertebrates the excretory organs are in the metasomatic region posterior to the gills, and are segmentally arranged in this region. Their investigation has demonstrated the existence of three distinct stages in these organs: 1. A series of segmental excretory organs in segments immediately following the branchial segments. This is the oldest of the three sets, and to these organs the name of the pronephros is given. 2. A second series which extends more posteriorly than the first, overlaps them to an extent which is not yet settled, and takes their place; to them is given the name of the mesonephros. 3. A third series continuous with the mesonephric is situated in segments still more posterior, supplants the mesonephros and forms the kidneys of all the higher vertebrates. This forms the metanephros.

These three sets of excretory organs are not exactly alike in their origin, in that the pronephric tubules are formed from a different portion of the coelomic walls to that from which the meso- and {390}metanephric tubules are formed, and the former alone gives origin to a duct, which forms the basis for the generative and urinary ducts, and is called the segmental duct. The mesonephric tubules, called also the Wolffian body, open into this duct.

In order to make the embryology of these excretory organs quite clear, I will make use of van Wijhe's phraseology and also of his illustrations. He terms the whole coelomic cavity the procoelom, which is divisible into a ventral unsegmented part, the body-cavity or metacoelom, and a dorsal segmented part, the somite. This latter part again is divided into a dorsal part--the epimere--and a part connecting the dorsal part with the body-cavity, to which therefore he gives the name of mesomere.

The cavity of the epimere disappears, and its walls form the muscle and cutis plates of the body. The part which forms the muscles is known as the myotome, which separates off from the mesomere, leaving the latter as a blind sac--the mesocoelom--communicating by a narrow passage with the body cavity or metacoelom. At the same time, from the mesomere is formed the sclerotome, which gives rise to the skeletal tissues of the vertebræ, etc., so that van Wijhe's epimere and mesomere together correspond to the original term, protovertebra, or somite of Balfour; and when the myotome and sclerotome have separated off, there is still left the intermediate cell-mass of Balfour and Sedgwick, i.e. the sac-like mesocoele of van Wijhe, the walls of which give origin to the mesonephrotome or mesonephros. Further, according to van Wijhe, the dorsal part of the unsegmented metacoelom is itself segmented, but not, as in the case of the mesocoele, with respect to both splanchnopleuric and somatopleuric walls. The segmentation is manifest only on the somatopleuric side, and consists of a distinct series of hollow somatopleuric outgrowths, called by him hypomeres, which give rise to the pronephros and the segmental duct.

Van Wijhe considers that the whole metacoelom was originally segmented, because in the lower vertebrates the segmentation reaches further ventral-wards, so that in Selachia the body-cavity is almost truly segmental. Also in the gill-region of Amphioxus the cavities which are homologous with the body-cavity arise segmentally.

{391}[Illustration: FIG. 156.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE VERTEBRATE COELOM. (After VAN WIJHE.)

N., central nervous system; Nc., notochord; Ao., aorta; Mg., midgut. A, My., myocoele; Mes., mesocoele; Met., metacoele; Hyp., hypomere (pronephric). B and C, My., myotome; Mes., mesonephros; S.d., segmental duct (pronephric); Met., body cavity.]

{392}As is well known, Balfour and Semper were led, from their embryological researches, to compare the nephric organs of vertebrates with those of annelids, and, indeed, the nature of the vertebrate segmental excretory organs has always been the fact which has kept alive the belief in the origin of vertebrates from a segmented annelid. These segmental organs thus compared were the mesonephric tubules, and doubts arose, especially in the mind of Gegenbaur, as to the validity of such a comparison, because the mesonephric tubules did not open to the exterior, but into a duct--the segmental duct--which was an unsegmented structure opening into the cloaca; also because the segmental duct, which was the excretory duct of the pronephros, was formed first, and the mesonephric tubules only opened into it after it was fully formed. Further, the pronephros was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited number of metameres, and was not segmental, but continuous. Gegenbaur and others therefore argued that the original prevertebrate excretory organ was the pronephros and its duct, not the mesonephros, from which they concluded that the vertebrate must have been derived from an unsegmented type of animal, and not from the segmented annelid type.

Such a view, however, has no further reason for acceptance, as it was based on wrong premises, for Rückert has shown that the pronephros does arise as a series of segmental nephric tubules, and is not unsegmented. He also has pointed out that in Torpedo the anterior part of the pronephric duct shows indications of being segmented, a statement fully borne out by the researches of Maas on Myxine, who gives the clearest evidence that in this animal the anterior part of the pronephric duct is formed by the fusion of a series of separate ducts, each of which in all probability once opened out separately to the exterior.

Rückert therefore concludes that Balfour and Semper were right in deriving the segmental organs of vertebrates from those of annelids, but that the annelid organs are represented in the vertebrate, not by the mesonephric tubules, but by the pronephric tubules and their ducts, which originally opened separately to the exterior. By the fusion of such tubules the anterior part of the segmental duct was formed, while its posterior part either arose by a later coenogenetic lengthening, or is the only remnant of a series of pronephric tubules which originally extended the whole length of the body, as suggested also by Maas and Boveri. Rückert therefore supposed that the mesonephric tubules were a secondary set of nephric organs, which were not necessarily directly derived from the annelid nephric organs.

{393}At present, then, Rückert's view is the one most generally accepted--the original annelid nephric organs are represented by the pronephric tubules and the pronephric duct, not by the mesonephric tubules, which are a later formation. This latter statement would hold good if the mesonephric tubules were found entirely in segments posterior to those containing the pronephric tubules; such, however, is said not to be the case, for the two sets of organs are said to overlap in some cases; even when they exist in the same segments, the former are said always to be formed from a more dorsal part of the coelom than the pronephros, always to be a later formation, and never to give any indication of communicating with the exterior except by way of the pronephric duct.

The recent observations of Brauer on the excretory organs of the Gymnophiona throw great doubt on the existence of mesonephric and pronephric tubules in the same segment. He criticizes the observations on which such statements are based, and concludes that, as in Hypogeophis, the nephrotome which is cut off after the separation of the sclero-myotome gives origin to the pronephros in the more anterior regions, just as it gives origin to the mesonephros in the more posterior regions. In fact, the observations of van Wijhe and others do not in reality show that two excretory organs may be formed in one segment, the one mesonephric from the remains of the mesomere and the other pronephric from the hypomere, but rather that in such cases there is only one organ--the pronephros--part of which is formed from the mesomere and part from the hypomere. Brauer goes further than this, and doubts the validity of any distinction between pronephros and mesonephros, on the ground of the former arising from a more ventral part of the procoelom than the latter; for, as he says, it is only possible to speak of one part of the somite as being more ventral than another part when both parts are in the same segment; so that if pronephric and mesonephric organs are never in the same segment, we cannot say with certainty that the former arises more ventrally than the latter.

These observations of Brauer strongly confirm Sedgwick's original statement that the pronephric and mesonephric organs are homodynamous organs, in that they are both derived from the original serially situated nephric organs, the differences between them being of a subordinate nature and not sufficient to force us to believe that the mesonephros is an organ of quite different origin to the {394}pronephros. So, also, Price, from his investigations of the excretory organs of Bdellostoma, considers that in this animal both pronephros and mesonephros are derived from a common embryonic kidney, to which he gives the name holonephros.

Brauer also is among those who conclude that the vertebrate excretory organs were derived from those of annelids; he thinks that the original ancestor possessed a series of similar organs over the whole pronephric and mesonephric regions, and that the anterior pronephric organs, which alone form the segmental duct, became modified for a larval existence--that their peculiarities were adaptive rather than ancestral. This last view seems to me very far-fetched, without any sufficient basis for its acceptance. According to the much more probable and reasonable view, the pronephros represents the oldest and original excretory organs, while the mesonephros is a later formation. Brauer's evidence seems to me to signify that the pronephros, mesonephros, and metanephros are all serially homologous, and that the pronephros bears much the same relation to the mesonephros that the mesonephros does to the metanephros. The great distinction of the pronephros is that it, and it alone, forms the segmental duct.

We may sum up the conclusions at which we have now arrived as follows:--

1. The pronephric tubules and the pronephric duct are the oldest part of the excretory system, and are distinctly in evidence for a few segments only in the most anterior part of the trunk-region immediately following the branchial region. They differ also from the mesonephric tubules by not being so clearly segmental with the myotomes.

2. The mesonephric tubules belong to segments posterior to those of the pronephros, are strictly segmental with the myotomes, and open into the pronephric duct.

3. All observers are agreed that the two sets of excretory organs resemble each other in very many respects, as though they arose from the same series of primitive organs, and, according to Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs. Other observers, however, consider that the pronephric organs, in part at all events, arise from a part of the nephrocoele more ventral than that which gives origin to the mesonephric organs, and that this difference in position of origin, combined {395}with the formation of the segmental duct, does constitute a true morphological distinction between the two sets of organs.

4. All the recent observers are in agreement that the vertebrate excretory organs strongly indicate a derivation from the segmental organs of annelids.

The very strongest support has been given to this last conclusion by the recent discoveries of Boveri and Goodrich upon the excretory organs of Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into the dorsal coelom by one or more funnels. Around each funnel are situated groups of peculiar cells, called by him 'Fadenzellen,' each of which sends a long process across the opening of the funnel. Goodrich has examined these 'Fadenzellen,' and found that they are typical pipe-cells, or solenocytes, such as he has described in the nephridial organs of various members of the annelid group Polychæta. Also, just as in the Polychæta, the ciliated nephric tubule has no internal funnel-shaped opening into the coelom, but terminates in these groups of solenocytes. "Each solenocyte consists of a cell-body and nucleus situated at the distal free extremity of a delicate tube; the proximal end of the tube pierces the wall of the nephridial canal and opens into its lumen. A single long flagellum arising from the cells works in the tube and projects into the canal."

The exceedingly close resemblance between the organs of Amphioxus and those of Phyllodoce, as given in his paper, is most striking, and, as he says, leads to the conclusion that the excretory organs of Amphioxus are essentially identical with the nephridia of certain polychæte worms.

It is to me most interesting to find that the very group of annelids, the Polychæta, which possess solenocytes so remarkably resembling those of the excretory organs of Amphioxus, are the highest and most developed of all the Annelida. I have argued throughout that the law of evolution consists in the origination of successive forms from the dominant group then alive, dominance signifying the highest type of brain-power achieved up to that time. The highest type among Annelida is found in the Chætopoda; from them, therefore, the original arthropod type must have sprung. This original group of Arthropoda gave rise to the two groups of Crustacea and Arachnida, in my opinion also to the Vertebrata, and, as already mentioned, it is convenient to give it a generalized {396}name, the Protostraca, from which subsequently the Palæostraca arose.

The similarity between the excretory organs of Amphioxus and those of Phyllodoce suggests that the protostracan ancestor of the vertebrates arose from the highest group of the Chætopoda--the Polychæta. The evidence which I have already given points, however, strongly to the conclusion that the vertebrate did not arise from members of the Protostraca near to the polychæte stock, but rather from members in which the arthropod characters had already become well developed--members, therefore, which were nearer the Trilobita than the Polychæta. Such early arthropods would very probably have retained in part excretory organs of the same character as those found in the original polychæte stock, and thus account for the presence of solenocytes in the excretory organs of Amphioxus.

In connection with such a possibility, I should like to draw attention to the observations of Claus and Spangenberg on the excretory organs of Branchipus--that primitive phyllopod, which is recognized as the nearest approach to the trilobites at present living. According to Claus, an excretory apparatus exists in the neighbourhood of each nerve-ganglion, and Spangenberg finds a perfectly similar organ in the basal segment of each appendage--a system, therefore, of excretory organs as segmentally arranged as those of Peripatus. Claus considers that although these organs formed an excretory system, it is not possible to compare them with the annelid segmental organs, because he thought the cells in question arose from ectoderm. Now, the striking point in the description of the excretory cells in these organs, as described both by Claus and Spangenberg, is that they closely resemble the pipe-cells or solenocytes of Goodrich; each cell possesses a long tube-like projection, which opens on the surface. They appear distinctly to belong to the category of flame-cells, and resemble solenocytes more than anything else. According to Goodrich, the solenocyte is probably an ectodermal cell, so that even if it prove to be the case, as Claus thought, that these pipe-cells of Branchipus are ectodermal, they would still claim to be derived from the segmental organs of annelids, especially of the Polychæta, being, to use Goodrich's nomenclature, true nephridial organs, as opposed to coelomostomes.

These observations of Claus and Spangenberg suggest not only that the primitive arthropod of the trilobite type possessed segmental {397}organs in every segment directly derived from those of a polychæte ancestor, but also that such organs were partly somatic and partly appendicular in position. Such a suggestion is in strict accord with the observations of Sedgwick on the excretory organs of the most primitive arthropod known, viz. Peripatus, where also the excretory organs, which are true segmental organs, are partly somatic and partly appendicular. Further, the excretory organs of the Scorpion and Limulus group are again partly somatic and partly appendicular, receiving the name of coxal glands, because there is a ventral projection of the gland into the coxa of the corresponding appendage.

Judging from all the evidence available, it is probable that when the arthropod stock arose from the annelids, simultaneously with the formation of appendages, the segmental somatic nephric organs of the latter extended ventrally into the appendage, and thus formed a segmental set of excretory organs, which were partly somatic, partly appendicular in position, and might therefore be called coxal glands.

As already stated, all investigators of the origin of the vertebrate excretory organs are unanimous in considering them to be derived from segmental organs of the annelid type. I naturally agree with them, but, in accordance with my theory, would substitute the words "primitive arthropod" for the word "annelid," for all the evidence I have accumulated in the preceding chapters points directly to that conclusion. Further, the most primitive of the three sets of vertebrate segmental organs--the pronephros, mesonephros, and metanephros--is undoubtedly the pronephros; consequently the pronephric tubules are those which I consider to be more directly derived from the coxal glands of the primitive arthropod ancestor. Such a derivation appears to me to afford an explanation of the difficulties connected with the origin of the pronephros and mesonephros respectively, which is more satisfactory than that given by the direct derivation from the annelid.

The only living animal which we know of as at all approaching the most primitive arthropod type is, as pointed out by Korschelt and Heider, Peripatus; and Peripatus, as is well known, possesses a true coelom and true coelomic excretory organs in all the segments of the body. Sedgwick shows that at first a true coelom, as typical as that of the annelids, is formed in each segment of the body, and that then this coelom (which represents in the vertebrate van Wijhe's pro-coelom) {398}splits into a dorsal and a ventral part. In the anterior segments of the body the dorsal part disappears (presumably its walls give origin to the mesoblast from which the dorsal body-muscles arise), while the ventral part remains and forms a nephrocoele, giving origin to the excretory organs of the adult. According to von Kennel, the cavity becomes divided into three spaces, which for a time are in communication--a lateral (I.), a median (II.), and a dorso-median (III.). The dorso-median portion becomes partitioned off, and this, as well as the greater part of the lateral portion, which lies principally in the foot, is used up in providing elements for the formation of the body- and appendage-muscles respectively and the connective tissue.

In Fig. 157 I reproduce von Kennel's diagram of a section across a Peripatus embryo, in which I. represents the lateral appendicular part of the coelom, II. the ventral somatic part, and III. the dorsal part which separates off from the ventral and lateral parts, and, as its walls give origin largely to the body-muscles, may be called the myocoele. The muscles of the appendages are formed from the ventral part of the original procoelom, just as I have argued is the case with the muscles of the splanchnic segmentation in vertebrates.

Sedgwick states that the ventral part of the coelom extends into the base of each appendage, and there forms the end-sac of each nephric tubule, into which the nephric funnel opens, thus forming a coxal gland; this end-sac or vesicle in the appendage is called by him the internal vesicle (i.v.), because later another vesicle is formed from the ventral coelom in the body itself, close against the nerve-cord on each side, which he calls the external vesicle (e.v.). (Cf. Fig. 158, taken from Sedgwick.) This second vesicle is, according to him, formed later in the development from the nephric tubule of the internal vesicle, so that it discharges its contents to the exterior by the same opening as the original tubule. Of course, as he points out, the whole system of internal and external vesicles and nephric tubules are all simply derivatives of the original ventral part of the coelom or nephrocoele.

{399}[Illustration: FIG. 157.--TRANSVERSE SECTION OF PERIPATUS EMBRYO. (After VON KENNEL.)

Al., alimentary canal; N., nerve-cord; App., appendage; I, II, III, the three divisions (lateral, median, and dorso-median) of the coelom.]

Al., alimentary canal; N., nerve-cord; App., appendage; i.v., internal, and e.v., external vesicles of the segmented excretory tubule (coxal gland).]

Here, then, in Peripatus, and presumably, therefore, in members of the Protostraca, we see that the original segmental organs of the annelid have become a series of nephric organs, which extended into the base of the appendages, and may therefore be called coxal glands; also it is clear, from Sedgwick's description, that if the appendages disappeared, the nephric organs would still remain, not as coxal glands, but as purely somatic excretory glands. They would still be homologous with the annelid segmental organs, or with the coxal glands, but would arise in toto from a part of the ventral coelom or nephrocoele, more dorsal than the former appendicular part, because the appendages and their enclosed coelom are always situated ventrally to the body. Again, according to Sedgwick, the nephric tubules are connected with two coelomic vesicles, the one in the appendage the internal vesicle, and the other, the so-called bladder, or the external vesicle, in the body itself, close against the nerve-cord. Sedgwick appears to consider that either of these vesicles may form the end-sac of a nephric tubule, for he discusses the question whether the single vesicle, which in each case gives origin to the nephridia of the first three legs, corresponds to the internal or external vesicle. He {400}decides, it is true, in favour of the internal vesicle, and therefore considers the excretory organ to be appendicular, i.e. a coxal gland, in these segments as well as in those more posterior. Still, the very discussion shows that in his opinion, at all events, the external vesicle might represent the end-sac of the tubule, in the absence of the internal or appendicular vesicle.

Such an arrangement as Sedgwick describes in Peripatus is the very condition required to give rise to the pronephric and mesonephric tubules, as deduced by me from the consideration of the vertebrate, and harmonizes and clears up the controversy about the mesonephros and pronephros in the most satisfactory manner. Both pronephros and mesonephros are seen to be derivatives of the original annelid segmental organs, not directly from an annelid, but by way of an arthropodan ancestor; the difference between the two is simply that the pronephric organs were coxal glands, and indicate, therefore, the presence of the original metasomatic appendages, while the mesonephric organs were homologous organs, formed in segments of later origin which had lost their appendages. For this reason the pronephros is said to be formed, in part at least, from a portion of the coelom situated more ventrally than the purely somatic part which gives rise to the mesonephros. For this reason Sedgwick, Brauer, etc., can say that the mesonephros is strictly homodynamous with the pronephros; while equally Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far that the two organs are not derived strictly from absolutely homologous parts of the coelom. For this reason Semon can speak of the mesonephros as a dorsal derivative of the pronephros, just as Sedgwick says that the external or somatic vesicle of Peripatus is a derivative of the appendicular nephric organ. For this reason the pronephros, or rather a part of it, is always derived from the somatopleuric layer, for, as is clear from Miss Sheldon's drawing, the part of the coelom in Peripatus which dips into the appendage is derived from the somatopleuric layer alone.

Such a coelom as that of Peripatus, Fig. 157, would represent the origin of the vertebrate coelom, and would therefore represent the procoelom of van Wijhe. In strict accordance with this, we see that it separates into a dorsal part, the walls of which give origin to the somatic muscles, or at all events to the great longitudinal dorsal muscles of the animal, and a ventral part, which forms a nephrocoele, {401}dips into the appendage, and gives origin to the muscles of the appendage. In the vertebrate, after the somatic dorsal part or myocoele has separated off, a ventral part is left, which forms a nephrocoele in the trunk-region, and gives origin to the splanchnic striated muscles in the cranial region, i.e. to the muscles which, according to my theory, were once appendicular muscles. This ventral nephrocoelic part is divisible in the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part, the metacoele or true body-cavity of the vertebrate.

This comparison of the procoelom of the vertebrate and arthropod signifies that the vertebrate metacoele was directly derived by ventral downgrowth from the arthropod nephrocoele, so that if, as I suppose, the vertebrate nervous system represents the conjoined nervous system and alimentary canal of the arthropod, then the vertebrate metacoele, or body-cavity, must have been originally confined to the region on each side of the central nervous system, and from this position have spread ventrally, to enclose ultimately the new-formed vertebrate gut. This means that the body-cavity (metacoele) of the vertebrate is not the same as the body-cavity of the annelid, but corresponds to a ventral extension of the nephrocoele, or ventral part of such body-cavity.

Such a phylogenetic history is most probable, because it explains most naturally and simply the facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always appears first in that region, and then extends laterally and ventrally on each side until it reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. Thus Shipley, in his description of the formation of the mesoblastic plates which line the body-cavity in Ammocoetes, describes them as commencing in two bands of mesoblast situated on each side, close against the commencing nervous system:--

"These two bands are separated dorsally by the juxtaposition of the dorsal wall of the mesenteron and the epiblast, and ventrally by the hypoblastic yolk-cells which are in contact with the epiblast over two-thirds of the embryo. Subsequently, but at a much later date, the mesoblast is completed ventrally by the downgrowth on {402}each side of these mesoblastic plates. The subsequent downward growth is brought about by the cells proliferating along the free ventral edge of the mesoblast, these cells then growing ventralwards, pushing their way between the yoke-cells and epiblast."

The derivation of the vertebrate pronephric segmental organs from the metasomatic coxal glands of a primitive arthropod would mean, if the segmental organs of Peripatus be taken as the type, that such glands opened to the exterior on every segment, either at the base of the appendage or on the appendage itself. It is taken for granted by most observers that the pronephric segmental organs once opened to the exterior on each segment, and then, from some cause or other, ceased to do so, and the separate ducts, by a process of fusion, came to form a single segmental duct, which opened into the cloaca. Many observers have been led to the conclusion that the pronephric duct is epiblastic in origin, although from its position in the adult, it appears far removed from all epiblastic formations. However, at no time in the developmental history is there any clear evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the epidermis to warrant the statement that the epiblastic cells take part in the formation of the duct. All that can be said is, that the formation of the duct takes place at a time when the pronephric diverticulum is in close propinquity to the epidermis, before the ventral downgrowth of the myotome has taken place.

The formation of the anterior portion of the pronephric duct is, according to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by the fusion of a number of pronephric tubules, which, according to Maas, are clearly seen in the youngest specimens as separate segmental tubes; each of these tubules is supplied by a capillary network from a segmental branch of the aorta, as in the tubules of Amphioxus according to Boveri, and does not possess a glomerulus.

The posterior part of the duct into which the mesonephric tubules enter possesses also a capillary network, which Maas considers to represent the original capillary network of a series of pronephric tubules, the only remnant of which is the duct into which the mesonephric tubules open. He therefore argues that the pronephric duct indicates a series of pronephric tubules, which originally extended {403}along the whole length of the body, and were supplanted by the mesonephric tubules, which also belonged to the same segments.

I also think that the paired appendages which have left the pronephric tubules as signs of their past existence, existed originally, in the invertebrate stage, on every segment of the body. But I do not consider that such a statement is at all equivalent to saying that such pairs of tubules must have existed upon every one of the segments existing at the present day; for it seems to me that Rückert is much more likely to be right when he says that in Selachians the duct clearly does grow back, and is not formed throughout in situ; so that he gives a double explanation of the formation of the duct--a palingenetic anterior part formed by the fusion of the extremities of the original excretory tubules, to which a posterior coenogenetic lengthening has been added.

It does not seem to me at all necessary that the immediate invertebrate ancestor of the vertebrate should have possessed excretory organs which opened out separately to the exterior on each segment; a fusion may already have taken place in the invertebrate stage, and so a single duct have been acquired for a number of organs. Such a suggestion has been made by Rückert, because of the fact discovered by Cunningham and E. Meyer, that the segmental organs of Lanice conchilega are on each side connected together by a single strong longitudinal canal. I would, however, go further than this and say, that even although the nephric organs of the polychæte ancestor opened out on every segment, and although the primitive arthropodan ancestor derived from such polychæte possessed coxal glands which opened out either on to or at the base of each appendage, similarly to those of Peripatus, yet the immediate arthropodan ancestor, with its palæostracan affinities, may already have possessed metasomatic coxal glands, all of which opened into a single duct, with a single opening to the exterior.

Judging from Limulus, such was very probably the case, for Patten and Hazen have shown (1) that the coxal glands of Limulus are segmental organs belonging to the prosomatic segments; (2) that the organs belonging to the cheliceral and ectognathal segments are not developed; (3) that the four glands belonging to the endognaths become connected together by a stolon, which communicates with a single nephric duct, opening to the exterior on the basal segment of the 5th prosomatic appendage (the last endognath). At {404}no time is there any evidence of any separate openings or any fusion with the ectoderm, such as might indicate separate openings of these prosomatic coxal segmental organs. Thus we see that in Limulus, which is presumably much nearer the annelid condition than the vertebrate, all evidence of separate nephric ducts opening to the exterior on each prosomatic segment has entirely disappeared, just as is the case in the metasomatic coxal glands (i.e. the pronephros) of the vertebrate. What is seen in the prosomatic region of Limulus, and doubtless also of the Eurypterids, may very probably have occurred in the metasomatic region of the immediate invertebrate ancestors of the vertebrate, and so account for the single pronephric duct belonging to a number of pronephric organs.

The interpretation of these various embryological investigations may be summed up as follows:--

1. The ancestor of the vertebrates possessed a pair of appendages on each segment; into the base of each of these appendages the segmental excretory organ sent a diverticulum, thus forming a coxal gland.

2. Such coxal glands, even in the invertebrate stage, may have discharged into a common duct which opened to the exterior most posteriorly.

3. Then, from some cause, the appendages were rendered useless, and dwindled away, leaving only the pronephric organs to indicate their former presence. At the end of this stage the animal possessed vertebrate characteristics.

4. For the purpose of increasing mobility, of forming an efficient swimming instead of a crawling animal, the body-segments increased in number, always, as is invariably the case, by the formation of new ones between those already formed and the cloacal region, and so of necessity caused an elongation of the pronephric duct. Into this there now opened the ducts of the segmental organs formed by recapitulation, those, therefore, belonging to the body-segments--mesonephric--having nothing to do with appendages, for the latter had already ceased to exist functionally, and would not, therefore, be repeated with each meristic repetition.

This, so to speak, passive lengthening of the pronephric duct in consequence of the lengthening of the early vertebrate body by the addition of metameres, each of which contained only mesonephric and no pronephric tubules, is, to my mind, an example of a principle {405}which has played an important part in the formation of the vertebrate, viz. that the meristic variation by which the spinal region of even the lowest of existing vertebrates has been formed, has largely taken place in the vertebrate phylum itself, and that such changes must be eliminated before we can picture to ourselves the pre-vertebrate condition. As an example, I may mention the remarkable repetition of similar segments pictured by Bashford Dean in Bdellostoma. Such repetition leads to passive lengthening of such parts as are already formed but are not meristically repeated: such are the notochord, the vertebrate intestine, the canal of the spinal cord, and possibly the lateral line nerve. The fuller discussion of this point means the discussion of the formation of the vertebrate alimentary canal; I will therefore leave it until I come to that part of my subject, and only say here that the evidence seems to me to point to the conclusion that at the time when the vertebrate was formed, the respiratory and cloacal regions were very close together, the whole of the metasoma being represented by the region of the pronephros alone.

Here, as always, the evidence of Ammocoetes tends to give definiteness to our conceptions, for Wheeler points out that up to a length of 7 mm. the pronephros only is formed; there is no sign of the more posteriorly formed mesonephros. Now we know, as pointed out in Chapter VI., p. 228, this is the time of Kupffer's larval stage of Ammocoetes. This is the period during which the invertebrate stage is indicated in the ontogeny, so that, in accordance with all that has gone before, this means that the metasoma of the invertebrate ancestor was confined to the region of the pronephros.

Again, take Shipley's account of the development of Petromyzon. He says--

"The alimentary canal behind the branchial region may be divided into three sections. Langerhans has termed these the stomach, midgut, and hindgut, but as the most anterior of these is the narrowest part of the whole intestine, it would, perhaps, be better to call it oesophagus. This part of the alimentary canal lies entirely in front of the yolk, and is, with the anterior region, which subsequently bears the gills, raised from the rest of the egg when the head is folded off. It is supported by a dorsal mesentery, on each side of which lies the head-kidney (pronephros)."

Further on he says--

{406}"The hindgut is smaller than the midgut; its anterior limit is marked by the termination of the spiral valve, which does not extend into this region. The two segmental ducts open into it just where it turns ventrally to open to the exterior by a median ventral anus. Its lumen is from an early stage lined with cells which have lost their yolk, and it is in wide communication with the exterior from the first. This condition seems to be, as Scott suggests, connected with the openings of the ducts of the pronephros, for this gland is completed and seems capable of functioning long before any food could find its way through the midgut, or, indeed, before the stomodæum has opened."

Is there no significance in this statement of Shipley? Even if it be possible to find some special reason why the branchial and cloacal parts of the gut are freed from yolk and lined with serviceable epithelium a long time before the midgut, why should a bit of the midgut, which Shipley calls the oesophagus, which is connected with the region of the pronephros and not of the branchiæ, differ so markedly from the rest of the midgut? Surely the reason is that the branchial region of the gut, the pronephric region of the gut, and the cloacal region of the gut, belong to a different and earlier phase in the phylogenetic history of the Ammocoetes than does the midgut between the pronephric and cloacal regions. This observation of Shipley fits in with and emphasizes the view that the original animal from which the vertebrate arose consisted of a cephalic and branchial region, followed by a pronephric and cloacal region; the whole intermediate part of the gut, which forms the midgut, with its large lumen and spiral valve, and belongs to the mesonephric region, being a later formation brought about by the necessity of increasing the length of the body.

THE ORIGIN OF THE SOMATIC TRUNK-MUSCULATURE AND THE FORMATION OF AN ATRIAL CAVITY.

Next comes the question, why was the pronephros not repeated in the meristic repetition that took place during the early vertebrate stage? What, in fact, caused the disappearance of the metasomatic appendages, and the formation of the smooth body-surface of the fish?

The embryological evidence given by van Wijhe and others of the manner in which the original superficially situated pronephros is {407}removed from the surface and caused to assume the deeper position, as seen in the later embryo, is perfectly clear and uniform in all the vertebrate groups. The diagrams at the end of van Wijhe's paper, which I reproduce here, illustrate the process which takes place. At first the myotome (Fig. 159, A) is confined to the dorsal region on each side of the spinal cord and notochord. Then (Fig. 159, B) it separates from the rest of the somite and commences to extend ventrally, thus covering over the pronephros and its duct, until finally (Fig. 159, C) it reaches the mid-ventral line on each side, and the foundations of the great somatic body-muscles are finally laid.

In order, therefore, to understand how the obliteration of the appendages took place, we must first find out what is the past history of the myotomes. Why are they confined at first to the dorsal region of the body, and extend afterwards to the ventral region, forcing by their growth an organ that was originally external in situation to become internal?

In the original discussion at Cambridge, I was accused of violating the important principle that in phylogeny we must look at the most elementary of the animals whose ancestors we seek, and was told that the lowest vertebrate was Amphioxus, not Ammocoetes; that therefore any argument as to the origin of vertebrates must proceed from the consideration of the former and not the latter animal. My reply was then, and is still, that I was considering the cranial region in the first place, and that therefore it was necessary to take the lowest vertebrate which possessed cranial nerves and sense-organs of a distinctly vertebrate character, a criterion evidently not possessed by Amphioxus. Such argument does not apply to the spinal region, so that, now that I have left the cranial region and am considering the spinal, I entirely agree with my critics that Amphioxus is likely to afford valuable help, and ought to be taken into consideration as well as Ammocoetes. The distinction between the value of the spinal (including respiratory) and cranial regions of Amphioxus for drawing phylogenetic conclusions is recognized by Boveri, who says that, in his opinion, "Amphioxus shows simplicity and undifferentiation rather than degeneration. If truly Amphioxus is somewhat degenerated, then it is so in its prehensile and masticatory apparatus, its sense organs, and perhaps its locomotor organs, owing to its method of living."

{408}[Illustration: FIG. 159.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF THE VERTEBRATE COELOM. (After VAN WIJHE.)

N., central nervous system; Nc., notochord; Ao., aorta; Mg., midgut. A, My., myocoele; Mes., mesocoele; Met., metacoele; Hyp., hypomere (pronephric). B and C, My., myotome; Mes., mesonephros; S.d., segmental duct (pronephric); Met., body-cavity.]

{409}Hatschek describes in Amphioxus how the coelom splits into a dorsal segmented portion, the protovertebra, and a ventral unsegmented portion, the lateral plates. He describes in the dorsal part the formation of myotome and sclerotome, as in the Craniota. Also, he describes how the myotome is at first confined to the dorsal region in the neighbourhood of the spinal cord and notochord, and subsequently extends ventrally, until, just as in Ammocoetes, the body is enveloped in a sheet of somatic segmented muscles, the well-known myomeres.

The conclusion to be drawn from this is inevitable. Any explanation of the origin of the somatic muscles in Ammocoetes must also be an explanation of the somatic muscles in Amphioxus, and conversely; so that if in this respect Amphioxus is the more primitive and simpler, then the condition in Ammocoetes must be looked upon as derived from a more primitive condition, similar to that found in Amphioxus. Now, it is well known that a most important distinction exists between Amphioxus and Ammocoetes in the topographical relation of the ventral portion of this muscle-sheet, for in the former it is separated from the gut and the body-cavity by the atrial space, while in the latter there is no such space. Fürbringer therefore concludes, as I have already mentioned, that this space has become obliterated in the Craniota, but that it must be taken into consideration in any attempt at formulating the nature of the ancestors of the vertebrate.

Kowalewsky described this atrial space as formed by the ventral downgrowth of pleural folds on each side of the body, which met in the mid-ventral line and enclosed the branchial portion of the gut. According to this explanation, the whole ventral portion of the somatic musculature of the adult Amphioxus belongs to the extension of the pleural folds, the original body-musculature being confined to the dorsal region. This is expressed roughly on the external surface of Amphioxus by the direction of the connective tissue septa between the myotomes (cf. Fig. 162, B). These septa, as is well known, bend at an angle, the apex of which points towards the head. The part dorsal to the bend represents the part of the muscle belonging to the original body; the part ventral to the bend is the pleural part, and represents the extension into the pleural folds.

Lankester and Willey have attempted to give another explanation of the formation of the atrial cavity; they look upon it as originating from a ventral groove, which becomes a canal by the meeting of two {410}outgrowths from the metapleure on each side. This canal then extends dorsalwards on each side, and so forms the atrial cavity; the metapleure still remains in the adult; the somatic muscles in the epipleure of the adult are the original body-muscles, and not extensions into an epipleuric fold, for there is no such fold.

This explanation is a possible conception for the post-branchial portion of the atrium, but is impossible for the branchial region; for, as Macbride points out, as must necessarily be the case, the point of origin of the atrial wall is, in all stages of development, situated at the end of the gill-slit. It shifts in position with the position of the gill-slit, but there can be no backwards extension of the cavity. Macbride therefore agrees with Kowalewsky that the atrial cavity is formed by the simultaneous ventral extension of pleural folds, and of the branchial part of the original pharynx. Thus, in his summing up, he states: "In the larva practically the whole sides and dorsal portion of the pharynx represent merely the hyper-pharyngeal groove and the adjacent epithelium of the pharynx of the adult, the whole of the branchial epithelium of the adult being represented by a very narrow strip of the ventral wall of the pharynx of the larva. The subsequent disproportionate growth of this part of the pharynx of the larva, and of the adjacent portion of the atrial cavity, has given the impression that the atrial cavity grew upwards and displaced other structures, which is not the case."

Further, van Wijhe states that the atrium extends beyond the atriopore right up to the anus, just as must have been the case if the pleural folds originally existed along the whole length of the body. His words are: "Allerdings hat sich das Atrium beim Amphioxus lanceolatus eigenthümlich ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus, d.h. bis an die Wurzel des Schwanzes ausdehnt."

We get, therefore, this conception of the origin of the somatic musculature of the vertebrate. The invertebrate ancestor possessed on each side, along the whole length of its body, a lateral fold or pleuron which was segmented with the body, and capable of movement with the body, because the dorsal longitudinal somatic muscles extended segmentally into each segment of the pleuron. By the ventral extension of these pleural folds, not only was the smooth body-surface of the vertebrate attained, but also the original appendages obliterated as such, leaving only as signs of their existence the {411}branchiæ, the pronephric tubules, and the sense-organs of the lateral line system.

Such an explanation signifies that the somatic trunk-musculature of the vertebrate was derived from the dorsal longitudinal musculature of the body of the arthropod, and not from the ventral longitudinal musculature, and that therefore in the primitive arthropod stage the equivalent of the myotome of the vertebrate did not give origin to the ventral longitudinal muscles of the invertebrate ancestor. Now, as I have said, von Kennel states that in the procoelom of Peripatus a dorsal part (III. in Fig. 157) is cut off which gives origin to the dorsal body-musculature, while the ventral part which remains (I. and II. in Fig. 157) gives origin in its appendicular portion (I.) to the muscles of the appendage, and presumably in its ventral somatic portion (II.) to the ventral longitudinal muscles of the body. This dorsal cut-off part might be called the myotome, in the same sense as the corresponding part of the procoelom in the vertebrate is called the myotome. In both cases the muscles derived from it form only a part of the voluntary musculature of the animal, and in both cases the muscles in question are the dorsal longitudinal muscles of the body, to which must be added the dorso-ventral body-muscles. Now, the whole of my theory of the origin of vertebrates arose from the investigation of the structure of the cranial nerves, which led to the conception that their grouping is not, like the spinal, a dual grouping of motor and sensory elements, but a dual grouping to supply two sets of segments, characterized especially by the different embryological origin of their musculature. The one set I called the somatic segmentation, because the muscles belonging to it were the great longitudinal body-muscles; the other I called the splanchnic segmentation, because its muscles were those connected with the branchial and visceral arches. According to my theory, this latter segmentation was due to the segmentation of the appendages in the invertebrate ancestor; and in previous chapters, dealing as they do with the cranial region, attention was especially directed to the way in which the position of the striated splanchnic musculature could be explained by a transformation of the prosomatic and mesosomatic appendages. Now, I am dealing with the metasomatic region, in which it is true the appendages take a very subordinate place, but still something corresponding to the splanchnic segments of the cranial region might fairly be expected to exist, and I therefore {412}desire to emphasize what appears to me to be the fact, that the musculature, which in the region of the trunk would correspond to that derived from the ventral segmentation of the mesoblast in the region of the head, may have arisen not only from the musculature of the appendages, but also from the ventral longitudinal musculature of the body of the invertebrate ancestor, for it seems probable that this latter musculature had nothing to do with the origin of the great longitudinal muscles of the vertebrate body, either dorsal or ventral.

The way in which I imagine the obliteration of the atrial cavity to have taken place is indicated in Fig. 160, B, which is a modification of a section across a trilobite-like animal as represented in Fig. 160, A. As is seen, the pleural folds on each side have nearly met the bulged-out ventral body-surface. A continuation of the same process would give Fig. 160, C, which is, to all intents and purposes, the same as Fig. 159, C, taken from van Wijhe, and shows how the segmental duct is left in the remains of the atrial cavity. The lining walls of the atrial cavity are represented very black, in order to indicate the presence of pigment, as indeed is seen in the corresponding position in Ammocoetes. In these diagrams I have represented the median ventral surface as a large bulged-out bag, without indicating any structures in it except the ventral extension of the procoelom to form the metacoelom. At present I will leave the space between the central nervous system and the ventral mesentery blank, as in the diagrams; in my next chapter I will discuss the possible method of formation within this blank space of the notochord and midgut. Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not complete, but that its presence is still visible in the shape of the pronephric duct. The evidence of Maas and others that the duct is formed by the fusion of the pronephric tubules is, it seems to me, conclusive against Boveri's view; but yet, as may be seen from my diagrammatic figures, the very place where one would expect to find the last remnant of the atrial cavity is exactly where the pronephric duct is situated. For my own part I should expect to find evidence of a former existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the duct itself.

{413}[Illustration: FIG. 160.--A, DIAGRAM OF SECTION THROUGH A TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE SUGGESTED OBLITERATION OF APPENDAGES AND THE FORMATION OF AN ATRIAL CAVITY BY THE VENTRAL EXTENSION OF THE PLEURAL FOLDS; C, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE AND THE OBLITERATION OF THE ATRIAL CAVITY.

Al., alimentary canal; N., nervous system; My., myotome; Pl., pleuron; App., appendage; Neph., nephrocoele; Met., metacoele; S.d., segmental duct; At., atrial chamber; V.Mes., ventral mesentery; Mes., mesonephros. The dotted line represents the splanchnopleuric mesoblast in all figures.]

{414}The conception that Amphioxus shows us how to account for the great envelope of somatic muscles which wraps round the vertebrate body, in that the ancestor of the vertebrate possessed on each side the body a segmented pleuron, is exactly in accordance with the theory of the origin of vertebrates deduced from the study of Ammocoetes, as already set forth in previous chapters. For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is the presence of segmented pleural flaps on each side of the main part of the body; and if we pass further back to the great group of trilobites, we find in the most manifold form, and in various degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced from the embryological evidence, might very well represent a cross-section of a trilobite, provided only that each appendage of the trilobite possessed an excretory coxal gland.

The earliest fishes, then, ought to have possessed segmented pleural folds, which were moved by somatic muscles, and enveloped the body after the fashion of Ammocoetes and Amphioxus, and I cannot help thinking that Cephalaspis shows, in this respect also, its relation to Ammocoetes. It is well known that some of the fossil representatives of the Cephalaspids show exceedingly clearly that these animals possessed a very well-segmented body, and it is equally recognized that this skeleton is a calcareous, not a bony skeleton, and does not represent vertebræ, etc. It is generally called an aponeurotic skeleton, meaning thereby that what is preserved represents not dermal plates alone, or a vertebrate skeleton, but the calcified septa or aponeuroses between a number of muscle-segments or myomeres, precisely of the same kind as the septa between the myomeres in Ammocoetes. The termination of such septa on the surface would give rise to the appearance of dermal plates or scutes, or the septa may even have been attached to something of the nature of dermal plates. The same kind of picture would be represented if these connective tissue dissepiments of Ammocoetes were calcified, and the animal then fossilized. In agreement with this interpretation of the spinal skeleton of Cephalaspis, it may be noted that again and again, in parts of these dissepiments, I have found in old specimens of Ammocoetes nodules of cartilage formed, and at transformation it is in this very tissue that the spinal cartilages are formed.

{415}[Illustration: FIG. 161.--A, FACSIMILE OF WOODWARD'S DRAWING OF A SPECIMEN OF Cephalaspis Murchisoni, AS SEEN FROM THE SIDE. THE CEPHALIC SHIELD IS ON THE RIGHT AND CAUDAL TO IT THE PLEURAL FRINGES ARE WELL SHOWN; B, ANOTHER SPECIMEN OF Cephalaspis Murchisoni TAKEN FROM THE SAME BLOCK OF STONE, SHOWING THE DERMOSEPTAL SKELETON AND IN ONE PLACE THE PLEURAL FRINGES, bc.]

Now, the specimens of Cephalaspis all show, as seen in Fig. 161, that the skeletal septa cover the body regularly, and then along one line are bent away from the body to form, as it were, a fringe, or rather a free pleuron, which has been easily pushed at an angle to the body-skeleton in the process of fossilization. Patten thinks that this fringed appearance is evidence of a number of segmental appendages which were jointed to the corresponding body-segments, and in the best specimen at the South Kensington Natural History Museum he thinks such joints are clearly visible. He concludes, therefore, that the cephalaspids were arthropods, and not vertebrates. I have also carefully examined this specimen, and do not consider that what is seen resembles the joint of an arthropod appendage; the appearance is rather such as would be produced if the line of attachment of Patten's appendages to the body were the place where the pleural body folds became free from the body, and so with any pressure a {416}bending or fracture of the calcified plates would take place along this line. There is, undoubtedly, an appearance of finish at the termination of these skeletal fringes, as though they terminated in a definitely shaped spear-like point, just as is seen in the trilobite pleuræ. This, again, to my mind, is rather evidence of pleural fringes than of true appendages.

As already argued, I look upon Ammocoetes as the only living fish at all resembling the cephalaspids; it is therefore instructive to compare the arrangement of this spinal dermo-septal skeleton of Cephalaspis with that of the septa between the myomeres in the trunk-region of Ammocoetes and Amphioxus. Such a skeleton in Ammocoetes would be represented by a series of plates overlapping each other, arranged as in Fig. 162, A, and in Amphioxus as in Fig. 162, B. I have lettered the corresponding parts of the two structures by similar letters, a, b, c. Ammocoetes differs in configuration from Amphioxus in that it possesses an extra dorsal (a, d) and an extra ventral bend. Ammocoetes is a much rounder animal than Amphioxus, and both the dorsal and ventral bends are on the extreme ventral and dorsal surfaces--surfaces which can hardly be said to exist in Amphioxus. The part, then, of such an aponeurotic skeleton {417}in Ammocoetes which I imagine corresponds to b, c in Amphioxus, and therefore would represent the pleural fold, is the part ventral to the bend at b. In both the animals this bend corresponds to the position of the notochord NC.

The skeleton of Cephalaspis compares more directly with that of Ammocoetes than that of Amphioxus, for there is the same extra dorsal bend (Fig. 161, a, d) as in Ammocoetes; the lateral part of the skeleton again gives an angle a, b, c; the part from b to c would therefore represent the pleural fold. I picture to myself the sequence of events somewhat as follows:--

First, a protostracan ancestor, which, like Peripatus, possessed appendages on every segment into which coelomic diverticula passed, forming a system of coxal glands; such glands, being derived from the segmental organs of the Chætopoda, discharged originally to the exterior by separate openings on each segment. It is, however, possible, and I think probable, that a fusion of these separate ducts had already taken place in the protostracan stage, so that there was only one external opening for the whole of these metasomatic coxal glands, just as there is only one external opening for the corresponding prosomatic coxal glands of Limulus. Then, by the ventral growth of pleural body-folds, such appendages became enclosed and useless, and the coxal glands of the post-branchial segments, with their segmental or pronephric duct, were all that remained as evidence of such appendages. This dwindling of the metasomatic appendages was accompanied by the getting-rid of free appendages generally, in the manner already set forth, with the result that a smooth fish-like body-surface was formed; then the necessity of increasing mobility brought about elongation by the addition of segments between those last formed and the cloacal region. Each of such new-formed segments was appendageless, so that its segmental organ was not a coxal gland, but entirely somatic in position, and formed, therefore, a mesonephric tubule, not a pronephric one. Such glands could no longer excrete to the exterior, owing to the enclosing shell of the pleural folds; but the pronephric duct was there, already formed, and so these nephric tubules opened into that, instead of, as in the case of the branchial slits, forcing their way through the pleural walls when the atrium became closed.

{418}THE MEANING OF THE DUCTLESS GLANDS.

If it is a right conception that the excretory organs of the protostracan group, which gave origin to the vertebrates as well as to the crustaceans and arachnids, were of the nature of coxal glands, then it follows that such coxal glands must have existed originally on every segment, because they themselves were derived from the segmental organs of the annelids; it is therefore worth while making an attempt to trace the fate of such segmental organs in the vertebrate as well as in the crustacean and arachnid.

Such an attempt is possible, it seems to me, because there exists throughout the animal kingdom striking evidence that excretory organs which no longer excrete to the exterior do not disappear, but still perform excretory functions of a different character. Their cells still take up effete or injurious substances, and instead of excreting to the exterior, excrete into the blood, forming either ductless glands of special character, or glands of the nature of lymphatic glands.

The problem presented to us is as follows:--

The excretory organs of both arthropods and vertebrates arose from those of annelids, and were therefore originally present in every segment of the body. In most arthropods and vertebrates they are present only in certain regions; in the former case, as the coxal glands of the prosomatic or head-region; in the latter, as the nephric glands of the metasomatic or trunk-region, and, in the case of Amphioxus, of the mesosomatic or branchial region.

In the original arthropod, judging from Peripatus, they were present, as in the annelid, in all the segments of the body, and formed coxal glands. Therefore, in the ancestors of the living Crustacea and Arachnida, coxal glands must have existed in all the segments of the body, and we ought to be able to find the vestiges of them in the mesosomatic or branchial and metasomatic or abdominal regions of the body.

Similarly, in the vertebrates, derived, as has been shown, not from the annelids, but from an arthropod stock, evidence of the previous existence of coxal glands ought to be manifested in the prosomatic or trigeminal region, in the mesosomatic or branchial region, as well as in the metasomatic or post-branchial region.

How does an excretory organ change its character when it ceases {419}to excrete to the exterior? What should we look for in our search after the lost coxal glands?

The answer to these questions is most plainly given in the case of the pronephros, especially in Myxine, where Maas has been able to follow out the whole process of the conversion of nephric tubules into a tissue resembling that of a lymph-gland.

He states, in the first place, that the pronephros possesses a capillary network, which extends over the pronephric duct, while the tubules of the mesonephros possess not only this capillary network, equivalent to the capillaries over the convoluted tubules in the higher vertebrates, but also a true glomerulus, in that the nephric segmental arteriole forms a coil (Knauel), and pushes in the wall of the mesonephric tubule. He describes the pronephros of large adult individuals as consisting of--

1. Tubules with funnels which open into the pericardial coelom.

2. A large capillary network (the glomus) at the distal end.

3. A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel controversy), which Spengel considers to be composed of the altered epithelium of pronephric tubules, while Semon looks on it as an amalgamation of glomeruli.

Maas is entirely on the side of Spengel, and shows that this peculiar tissue is actually formed by modified pronephric tubules, which become more and more lymphatic in character.

He says: "The pronephros consists of a number of nephric tubules, placed separately one behind the other, which were originally segmental in character, each one of which is supplied by a capillary network from a segmental branch of the aorta. The tubules begin with many mouths (dorso-lateral and medial-ventral) in the pericardial cavity; on their other blind end they have lost their original external opening, and there, in the cranial portion of the head-kidney, before they have joined together to form a collecting duct, they, together with the vascular network, are transformed into a peculiar adrenal-like tissue. The most posterior of the segmental capillary nets retain their original character, and are concentrated into the separate capillary mass known as the glomus."

Later on he says: "Further, the separate head-kidney is more and more removed in structure from an excretory organ in the ordinary sense. One cannot, however, speak of it as an organ becoming rudimentary; this is proved not only by the progressive transformation {420}of its internal tissue into a tissue of a very definite character, but also by the cilia in its canals, and the steady increase in the number of its funnels. It appears, therefore, to be the conversion of an excretory organ into an organ for the transference of fluid out of the coelom into a special tissue, i.e. into its blood-sinus; in other words, into an organ which must be classed as belonging to the lymph-system."

In exact correspondence with this transformation of a nephric tubule into a ductless gland of the nature of a lymphatic gland, is the formation of the head-kidney in the Teleostea. Thus, Weldon points out that, though the observations of Balfour left it highly probable that the "lymphatic" tissue described by him was really a result of the transformation of part of the embryonic kidney, he did not investigate the details of its development. This was afterwards done by Emery, with the following results: "In those Teleostea which he has studied, Professor Emery finds that at an early stage the kidney consists entirely of a single pronephric funnel, opening into the pericardium, and connected with the segmental duct, which already opens to the exterior. Behind this funnel, the segmental duct is surrounded by a blastema, derived from the intermediate cell-mass, which afterwards arranges itself more or less completely into a series of solid cords, attaching themselves to the duct. These develop a lumen, and become normal segmental tubules, but it is, if I may be allowed the expression, a matter of chance how much of the blastema becomes so transformed into kidney tubules, and how much is left as the 'lymphatic' tissue of Balfour, this 'lymphatic' tissue remaining either in the pronephros only, or in both pro- and meso-nephros."

If we turn now to the invertebrates, we see also how close a connection exists between lymphatic and phagocytic organs and excretory organs. The chief merit for this discovery is due to Kowalewsky, who, taking a hint from Heidenhain's work on the kidney, in which he showed how easy it was to find out the nature of different parts of the mammalian excretory organ by the injection of different substances, such as a solution of ammoniated carmine, or of indigo-carmine, has injected into a large number of different invertebrates various colouring matters, or litmus, or bacilli, and thus shown the existence, not only of known excretory organs, but also of others, lymphatic or lymphoid in nature, not hitherto suspected.

In all cases he finds that a phagocytic action with respect to solid {421}bodies is a property of the leucocytes, and that these leucocytes which are found in the coelomic spaces of the Annelida, etc., are apparently derived from the epithelium of such spaces. Also by the proliferation of such epithelium in places, e.g. the septal glands of the terrestrial Oligochæta, segmental glandular masses of such tissue are formed which take up the colouring matter, or the bacilli. In the limicolous Oligochæta such septal glands are not found, but at the commencement of the nephridial organ, immediately following upon the funnel, a remarkable modification of the nephridial wall takes place to form a large cellular cavernous mass, the so-called filter, which in Euaxes is full of leucocytes; the cells are only definable by their nuclei, and look like and act in the same way as the free leucocytes outside this nephridial appendage. As G. Schneider points out, the whole arrangement is very like that described by Kowalewsky in the leeches Clepsine and Nephelis, where, also immediately succeeding the funnel of the nephridial organ, a large accessory organ is found, which is part of the nephridium, and is called the nephridial capsule. This is the organ par excellence which takes up the solid carmine-grains and bacilli, and apparently, from Kowalewsky's description, contains leucocytes in large numbers. We see, then, that in such invertebrates, just as in the vertebrate, modifications of the true excretory organ may give rise to phagocytic glands of the nature of lymphatic glands. Further, these researches of Kowalewsky suggest in the very strongest manner that whenever by such means new, hitherto unsuspected glands are discovered, such glands must belong to the excretory system, i.e. must be derived from coelomic epithelium, even when all evidence of any coelom has disappeared. Kowalewsky himself was evidently so impressed with the same feeling that he heads one of his papers "The Excretory Organs of the Pantopoda," although the organs in question had been discovered by him by this method, and appeared as ductless glands with no external opening.

To my mind these observations of Kowalewsky are of exceeding interest, for it is immediately clear that if the segmental organs of the annelids, which must have existed on all the segments of the forefathers of the Crustacea and Arachnida (the Protostraca), have left any sign of their existence in living crustaceans and arachnids, then such indication would most likely take the form of lymphatic glands in the places where the excretory organs ought to have been.

Now, as already pointed out in Peripatus, such segmental organs {422}were formed by the ventral part of the coelom, and dipped originally into each appendage. We know also that each segment of an arachnid embryo possesses a coelomic cavity in its ventral part which extends into the appendage on each side; this cavity afterwards disappears, and is said to leave no trace in the adult of any excretory coxal gland derived from its walls. If, however, it is found that in the very position where such organ ought to have been formed a segmentally arranged ductless gland is situated, the existence of which is shown by its taking up carmine, etc., then it seems to me that in all probability such gland is the modification of the original coxal gland.

This is what Kowalewsky has done. Thus he states that Metschnikoff had fed Mysis with carmine-grains, and found tubules at the base of the thoracic feet coloured red with carmine. He himself used an allied species, Parapodopsis cornutum, and found here also that the carmine was taken up by tubules situated in the basal segments of the feet. In Nebalia, feeding experiments with alizarin blue and carmine stained the antennal glands, and showed the existence of glands at the base of the eight thoracic feet. These glands resemble the foot-glands of Mysis, Parapodopsis, and Palæmon, and lie in the space through which the blood passes from the thoracic feet, i.e. from the gills, to the heart. In Squilla also, in addition to the shell-glands, special glands were discovered on the branchial feet on the path of the blood to the heart. These glands form continuous masses of cells which constitute large compact glands at the base of the branchial feet. Single cells of the same sort are found along the whole course of the branchial venous canal, right up to the pericardium.

These observations show that the Crustacea possess not only true excretory organs in the shape of coxal glands, i.e. antennary glands, shell-glands, etc., in the cephalic region, but also a series of segmental glands situated at the base of the appendages, especially of the respiratory appendages: a system, that is to say, of coxal glands which have lost their excretory function, through having lost their external opening, but have not in consequence disappeared, but still remain in situ, and still retain an important excretory function, having become lymphatic glands containing leucocytes. Such glands are especially found in the branchial appendages, and are called branchial glands by Cuénot, who describes them for all Decapoda.

Further, it is significant that the same method reveals the {423}existence in Pantopoda of a double set of glands of similar character, one set in the basal segments of the appendage, and the other in the adjacent part of the body.

In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ situated along the whole length of the nerve-cord in the abdominal region takes up carmine grains and bacilli; an organ which in Androctonus does not form one continuous gland, but a number of separate, apparently irregularly grouped, glandular bodies.

In addition to this median lymphatic gland, Kowalewsky has discovered in the scorpion a pair of lateral glands, to which he gives the name of lymphoid glands, which communicate with the thoracic body-cavity (i.e. the pseudocoele), are phagocytic, and, according to him, give origin to leucocytes by the proliferation of their lining cells, thus, as he remarks, reminding us of the nephridial capsules of Clepsine. These glands are so closely related in position to the coxal glands on each side that he has often thought that the lumen of the gland communicated with that of the coxal gland; he, however, has persuaded himself that there is no true communication between the two glands. Neither of these organs appears to be segmental, and until we know how they are developed it is not possible to say whether they represent fused segmental organs or not.

The evidence, then, is very strong that in the Crustacea and Arachnida the original segmental excretory organs do not disappear, but remain as ductless glands, of the nature of lymphatic glands, which supply leucocytes to the system.

Further, the evidence shows that the nephric organs, or parts of the coelom in close connection with these organs, may be transformed into ductless glands, which do not necessarily contain free leucocytes as do lymph-glands, but yet are of such great importance as excretory organs that their removal profoundly modifies the condition of the animal. Such a gland is the so-called adrenal or suprarenal body, disease of which is a feature of Addison's disease; a gland which forms and presumably passes into the blood a substance of remarkable power in causing contraction of blood-vessels, a substance which has lately been prepared in crystalline form by Jokichi Takamine, and called by him "adrenalin"; a gland, therefore, of very distinctly peculiar properties, which cannot be regarded as rudimentary, but is of vital importance for the due maintenance of the healthy state.

In the Elasmobranchs two separate glandular organs have been {424}called suprarenal; a segmental series of paired organs, each of which possesses a branch from the aorta and a sympathetic ganglion, and an unpaired series in close connection with the kidneys, to which Balfour gave the name of interrenal glands. Of these two sets of glands, Swale Vincent has shown that the extract of the interrenals has no marked physiological effect, in this respect resembling the extract of the cortical part of the mammalian gland, while the extract of the paired segmental organs of the Elasmobranch produces the same remarkable rise of blood-pressure as the extract of the medullary portion of the mammalian gland.

The development also of these two sets of glands is asserted to be different. Balfour considered that the suprarenals were derived from sympathetic ganglion-cells, but left the origin of the interrenals doubtful. Weldon showed that the cortical part of the suprarenals in the lizard was derived from the wall of the glomerulus of a number of mesonephric tubules. In Pristiurus, he stated that the mesoblastic rudiment described by Balfour as giving origin to the interrenals is derived from a diverticulum of each segmental tubule, close to the narrowing of its funnel-shaped opening into the body-cavity. With respect to the paired suprarenals he was unable to speak positively, but doubted whether they were derived entirely from sympathetic ganglia.

Weldon sums up the results of his observations by saying: "That all vertebrates except Amphioxus have a portion of the kidney modified for some unknown purpose not connected with excretion; that in Cyclostomes the pronephros alone is so modified, in Teleostei the pro- and part of the meso-nephros; while in the Elasmobranchs and the higher vertebrates the mesonephros alone gives rise to this organ, which has also in these forms acquired a secondary connection with certain of the sympathetic ganglia."

Since Weldon's paper, a large amount of literature on the origin of the adrenals has appeared, a summary of which, up to 1891, is given by Hans Rabl in his paper, and a further summary by Aichel in his paper published in 1900. The result of the investigations up to this latter paper may be summed up by saying that the adrenals, using this term to include all these organs of whatever kind, are in all cases, partly at all events, derived from some part of the walls of either the mesonephric or pronephric excretory organs, but that in addition a separate origin from the sympathetic nervous system must {425}be ascribed to the medullary part of the organ and to the separate paired organs in the Elasmobranchs, which are equivalent to the medullary part in other cases.

The evidence, then, of the transformation of the known vertebrate excretory organs--the pronephros and the mesonephros--leads to the conclusion that in our search for the missing coxal glands of the meso- and pro-somatic regions, we must look for either lymphatic glands, or ductless glands of distinct importance to the body. I have already considered the question in the prosomatic region, and have given my reasons why the pituitary gland must be looked upon as the descendant of the arthropod coxal gland. In this case also the resulting ductless gland is still of functional importance, for disease of it is associated with acromegaly. If, as is possible, it is homologous with the Ascidian hypophysial gland, then it is confirmatory evidence that this latter is said by Julin to be an altered nephridial organ.

Finally, I come to the mesosomatic or branchial region; and here, strikingly enough, we find a perfectly segmental glandular organ of mysterious origin--the thymus gland--segmental with the branchiæ, not necessarily with the myotomes, belonging, therefore, to the appendicular system; and since the branchiæ represent, according to my theory, the basal part of the appendage, such segmental glands would be in the position of coxal glands. Here, then, in the thymus may be the missing mesosomatic coxal glands.

What, then, is the thymus?

The answer to this question has been given recently by Beard, who strongly confirms Kölliker's original view that the thymus is a gland for the manufacture of leucocytes, and that such leucocytes are directly derived from the epithelial cells of the thymus. Kölliker also further pointed out that the blood of the embryo is for a certain period destitute of leucocytes. Beard confirms this last statement, and says that up to a certain stage (varying from 10 to 16 mm. in length of the embryo) the embryos of Raja batis have no leucocytes in the blood or elsewhere. Up to this period the thymus-placode is well formed, and the first leucocytes can be seen to be formed in it from its epithelial cells; then such formation takes place with great rapidity, and soon an enormous discharge of leucocytes occurs from the thymus into the tissue-spaces and blood. He therefore concludes that all lymphoid tissues in the body arise originally from the thymus gland, i.e. from leucocytes discharged from the thymus.

{426}The segmental branchial glands, known by the name of thymus, are, according to this view, the original lymphatic glands of the vertebrate; and it is to be noted that, in fishes and in Amphibia, lymphatic glands, such as we know them in the higher mammals, do not exist; they are characteristic of the higher stages of vertebrate evolution. In the lower vertebrates, the only glandular masses apart from the cell-lining of the body-cavity itself, which give rise to leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified post-branchial segmental glands, known as the head-kidney in Teleostea, etc.

The importance ascribed by Beard to the thymus in the formation of leucocytes in the lowest vertebrates would be considerably reduced in value if the branchial region of Ammocoetes possessed neither thymus glands nor anything equivalent to them. Such, however, is not the case. Schaffer has shown that in the young Ammocoetes masses of lymphatic glandular tissue are found segmentally arranged in the neighbourhood of each gill-slit--tissue which soon becomes converted into a swarming mass of leucocytes, and shows by its staining, etc., how different it is from a blood-space. The presence of this thymus leucocyte-forming tissue, as described by Schaffer, is confirmed by Beard, and I myself have seen the same thing in my youngest specimen of Ammocoetes.

Further, the very methods by which Kowalewsky has brought to light the segmental lymph-glands of the branchial region of the Crustacea, etc., are the same as those by which Weiss discovered the branchial nephric glands in Amphioxus--excretory organs which Boveri considers to represent the pronephros of the Craniota. In this supposition Boveri is right, in so far that both pronephros and the tubules in Amphioxus belong to the same system of excretory organs; but I entirely agree with van Wijhe that the region in Amphioxus is wrong. The tubules in Amphioxus ought to be represented in the branchial region of the Craniota, not in the post-branchial region; van Wijhe therefore suggests that further researches may homologize them with the thymus gland in the Craniota, not with the pronephros. This suggestion of van Wijhe appears to me a remarkably good one, especially in view of the position of the thymus glands in Ammocoetes and the nephric branchial glands in Amphioxus. If, as I have pointed out, the atrial cavity of Amphioxus has been closed in Ammocoetes by the apposition of {427}the pleural fold with the branchial body-surface, then the remains of the position of the atrial chamber must exist in Ammocoetes as that extraordinary space between the somatic muscles and the branchial basket-work filled with blood-spaces and modified muco-cartilage. It is in this very space, close against the gill-slits, that the thymus glands of Ammocoetes are found, in the very place where the nephric tubules of Amphioxus would be found if its atrial cavity were closed completely. Instead, therefore, of considering with Boveri that the branchial nephric tubules of Amphioxus still exist in the Craniota as the pronephros, and that the atrial chamber has narrowed down to the pronephric duct, I would agree with van Wijhe that the pronephros is post-branchial, and suggest that by the complete closure of the atrial space in the branchial region the branchial nephric tubules have lost all external opening, and consequently, as in all other cases, have changed into lymphatic tissue and become the segmental thymus glands.

As van Wijhe himself remarks, the time is hardly ripe for making any positive statement about the relationship between the thymus gland and branchial excretory organs. There is at present not sufficient consensus of opinion to enable us to speak with any certainty on the subject, yet there is so much suggestiveness in the various statements of different authors as to make it worth while to consider the question briefly.

On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and parathymus, are all stated to be derivatives of the epithelium lining the gill-slits, and Maurer would draw a distinction between the organs derived from the dorsal side of the gill-cleft and those derived from the ventral side--the former being thymus, the latter forming the epithelial cell-nests, i.e. parathyroids. The thymus in Ammocoetes, according to Schaffer, lies both ventral and dorsal to the gill-cleft; Maurer thinks that only the dorsal part corresponds to the thymus, the ventral part corresponding to the parathyroids, etc. Structurally, the thymus, parathyroids, and the epithelial cell-nests are remarkably similar, so that the evidence appears to point to the conclusion that, in the neighbourhood of the gill-slits, segmentally arranged organs of a lymphatic character are situated, which give origin to the thymus, parathyroids, tonsils, etc. Now, among these organs, i.e. among those ventrally situated, Maurer places the carotid gland, so that, if he is right, the origin of the carotid gland {428}might be expected to help in the elucidation of the origin of the thymus.

The origin of the carotid gland has been investigated recently by Kohn, who finds that it is associated with the sympathetic nervous system in the same way as the suprarenals. He desires, in fact, to make a separate category for such nerve-glands, or paraganglia, as he calls them, and considers them all to be derivatives of the sympathetic nervous system, and to have nothing to do with excretory organs. The carotid gland is, according to him, the foremost of the suprarenal masses in the Elasmobranchs, viz. the so-called axillary heart.

In my opinion, nests of sympathetic ganglion-cells necessarily mean the supply of efferent fibres to some organ, for all such ganglia are efferent, and also, if they are found in the organ, would have been brought into it by way of the blood-vessels supplying the organ, so that Aichel's statement of the origin of the suprarenals in the Elasmobranchs seems to me much more probable than a derivation from nerve-cells. If, then, it prove that Aichel is right as to the origin of the suprarenals, and Kohn is right in classifying the carotid gland with the suprarenals, then Maurer's statements would bring the parathyroids, thymus, etc., into line with the adrenals, and suggest that they represent the segmented glandular excretory organs of the branchial region, into which, just as in the interrenals of Elasmobranchs, or the cortical part of the adrenals of the higher vertebrates, there has been no invasion of sympathetic ganglion-cells.

Wheeler makes a most suggestive remark in his paper on Petromyzon: he thinks he has obtained evidence of serial homologues of the pronephric tubules in the branchial region of Ammocoetes, but has not been able up to the present to follow them out. If what he thinks to be serial homologues of the pronephric tubules in the branchial region should prove to be the origin of the thymus glands of Schaffer, then van Wijhe's suggestion that the thymus represents the excretory organs of the branchial region would gain enormously in probability. Until some such further investigation has been undertaken, I can only say that it seems to me most likely that the thymus, etc., represent the lymphatic branchial glands of the Crustacea, and therefore represent the missing coxal glands of the branchial region.

This, however, is not all, for the appendages of the mesosomatic region, as I have shown, do not all bear branchiæ; the foremost or {429}opercular appendage carries the thyroid gland. Again, the basal part of the appendage is all that is left; the thyroid gland is in position a coxal gland. It ought, therefore, to represent the coxal gland of this appendage, just as the thymus, tonsils, etc., represent the coxal glands of the rest of the mesosomatic appendages. In the thyroid gland we again see a ductless gland of immense importance to the economy, not a useless organ, but one, like the other modified coxal glands, whose removal involves far-reaching vital consequences. Such a gland, on my theory, was in the arthropod a part of the external genital ducts which opened on the basal joint of the operculum. What, then, is the opinion of morphologists as to the meaning of these external genital ducts?

In a note to Gulland's paper on the coxal glands of Limulus, Lankester states that the conversion of an externally-opening tubular gland (coxal gland) into a ductless gland is the same kind of thing as the history of the development of the suprarenal from a modified portion of mesonephros, as given by Weldon. Further, that in other arthropods with glands of a tubular character opening to the exterior at the base of the appendages, we also have coxal nephridia, such as the shell-glands of the Entomostraca, green glands of Crustacea (antennary coxal gland); and further on he writes: "When once the notion is admitted that ducts opening at the base of limbs in the Arthropoda are possibly and even probably modified nephridia, we immediately conceive the hypothesis that the genital ducts of the Arthropoda are modified nephridia."

So, also, Korschelt and Heider, in their general summing up on the Arthropoda, say: "In Peripatus, where the nephridia appear, as in the Annelida, in all the trunk-segments, a considerable portion of the primitive segments is directly utilized for the formation of the nephridia. In the other groups, the whole question of the rise of the organs known as nephridia is still undecided, but it may be mentioned as very probable that the salivary and anal glands of Peripatus, the antennal and shell-glands of the Crustacea, the coxal glands of Limulus and the Arachnida, as well as the efferent genital ducts, are derived from nephridia, and in any case are mesodermal in origin."

The necessary corollary to this exceedingly probable argument is that glandular structures such as the uterine glands of the scorpion already described, which are found in connection with these terminal {430}genital ducts, may be classed as modified nephridial glands, and that therefore the thyroid gland of Ammocoetes, which, on the theory of this book, arose in connection with the opercular genital ducts of the palæostracan ancestor, represents the coxal glands of this fused pair of appendages. Such a gland, although its function in connection with the genital organs had long disappeared, still, in virtue of its original excretory function, persisted, and even in the higher vertebrates, after it had lost all semblance of its former structure and become a ductless gland of an apparently rudimentary nature, still, by its excretory function, demonstrates its vital importance even to the highest vertebrate.

By this simple explanation we see how these hitherto mysterious ductless glands, pituitary, thymus, tonsils, thyroid, are all accounted for, are all members of a common stock--coxal glands--which originally, as in Peripatus, excreted at the base of the prosomatic and mesosomatic appendages, and are still retained because of the importance of their excretory function, although ductless owing to the modification of their original appendages.

Finally, there is yet another organ in the vertebrate which follows the same law of the conversion of an excretory organ into a lymphatic organ when its connection with the exterior is obliterated, and that is the vertebrate body-cavity itself. According to the scheme here put forth, the body-cavity of the vertebrate arose by the fusion of a ventral prolongation of the original nephrocoele on each side; prolongations which accompanied the formation of the new ventral midgut, and by their fusion formed originally a pair of cavities along the whole length of the abdomen, being separated from each other by the ventral mesentery of the gut. Subsequently, by the ventral fusion of these two cavities, the body-cavity of the adult vertebrate was formed.

This is simply a statement of the known method of formation of the body-cavity in the embryo, and its phylogenetic explanation is that the body-cavity of the vertebrate must be looked upon as a ventral prolongation of the original ancestral body-cavity. Embryology clearly teaches that the original body-cavity or somite was confined to the region of the notochord and central nervous system, and there, just as in Peripatus, was divisible into a dorsal part, giving origin to the myocoele, and a ventral part, forming the nephrocoele. From this original nephrocoele are formed the pronephric excretory organs, the mesonephric excretory organs, and the body-cavity.

{431}That the vertebrate body-cavity was originally a nephrocoele is generally accepted, and its excretory function is shown by the fact that it communicates with the exterior in all the lower vertebrates, either through abdominal pores or by way of nephridial funnels. Bles has shown how largely these two methods of communicating with the exterior mutually exclude each other. In the higher vertebrates both channels become closed, except in the case of the Fallopian tubes, and thus, so to speak, the body-cavity becomes a ductless gland, still, however, with an excretory function, but now, as in all other cases, forming a part of the lymphatic rather than of the true excretory system.

SUMMARY.

The consideration of the formation of the vertebrate cranial region, as set forth in previous chapters, indicates that the ancestor of the vertebrates was not an arachnid purely or a crustacean purely, but possessed partly crustacean and partly arachnid characters. In order to express this conclusion, I have used the term Protostraca, invented by Korschelt and Heider, to indicate a primitive arthropod group, from which both arachnids and crustaceans may be supposed to have arisen, and have therefore stated that the vertebrate did not arise directly from the annelids, but from the Protostraca. Such an origin signifies that the origin of the excretory organs of the vertebrate must not be looked for in the segmental organs of the annelid, but rather in such modified annelid organs as would naturally exist in a primitive arthropod group. The nature of such organs may be inferred, owing to the fortunate circumstance that so primitive an arthropod as Peripatus still exists, and we may conclude that the protostracan ancestor possessed in every segment a pair of appendages and a pair of coelomic cavities, which extended into the base of these appendages. The ventral portion of each of these coelomic cavities separated off from the dorsal and formed a nephrocoele, giving origin to a segmental excretory organ, which, seeing that its end-vesicle was in the base of the appendage, and seeing also the nature of the known arachnid and crustacean excretory organs, may fitly be termed a coxal gland. This, then, is the working hypothesis to explain the difficulties connected with the origin of the pronephros and mesonephros--that the original segmental organs were coxal glands, and therefore indicated the presence of appendages. This hypothesis leads to the following conclusions:--

1. The coxal glands belonging to the post-branchial appendages of the invertebrate ancestor are represented by the pronephric tubules, and existed over the whole metasomatic region.

2. Such glands discharged into a common duct--the pronephric duct--which opened into the cloacal region, either in the protostracan stage, when the metasomatic appendages were still in existence, just as the coxal glands of the prosomatic region in Limulus discharge into a common duct, or else the pronephric duct was formed when the appendages were obliterated.

{432}3. The metasomatic appendages disappeared owing to their enclosure by pleural folds, which, meeting in the mid-ventral line, not only caused the obliteration of the appendages, and gave a smooth fish-like body-surface to the animal, but also caused the formation of an atrial cavity.

4. Into these pleural folds the dorsal longitudinal muscles of the body extended, and ultimately reached to the ventral surface, thus forming the somatic muscles of the vertebrate body.

5. When the pleural folds had met in the mid-ventral line the animal had become a vertebrate, and was dependent for its locomotion on the movements of these somatic muscles, and not on the movements of appendages. Consequently, elongation of the trunk-region took place, for the purpose of increasing mobility, by the formation of new metameres.

6. Each of such metameres possessed its own segmental excretory organ, formed in the same way as the previous pronephric organs, but, as there were no appendages in these new-formed segments, the excretory organs took on the characters of a mesonephros, not a pronephros, and opened into the pronephric duct, because the direct way to the exterior was blocked by the enveloping pleural folds.

7. The group of annelids from which the protostracan ancestor of the vertebrates arose was the highest annelidan group, viz. the Polychæta, as shown by the nature of the excretory organs in Amphioxus.

8. The coxal glands of the protostracan ancestor existed on all the segments, and were, therefore, divisible into three groups, prosomatic, mesosomatic, and metasomatic; these three groups of coxal glands still exist in the vertebrate as ductless glands.

9. The prosomatic coxal glands form the pituitary body.

10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids, tonsils, etc.

11. The metasomatic coxal glands form the adrenals.

12. The procoelom of the vertebrate is the procoelom of the protostracan ancestor, which splits into a dorsal part, the myocoele, and a ventral part, the nephrocoele. This latter part not only forms the pronephros and mesonephros, but also by a ventral extension gives origin to the walls of the vertebrate body-cavity or metacoele.

13. This ventral extension of the original nephrocoele at first excreted to the exterior, through abdominal pores, or through peritoneal funnels. When such paths to the exterior became closed, it also became a ductless gland, belonging to the lymphatic system.

{433}CHAPTER XIII

THE NOTOCHORD AND ALIMENTARY CANAL

Relationship between notochord and gut.--Position of unsegmented tube of notochord.--Origin of notochord from a median groove.--Its function as an accessory digestive tube.--Formation of notochordal tissue in invertebrates from closed portions of the digestive tube.--Digestive power of the skin of Ammocoetes.--Formation of new gut in Ammocoetes at transformation.--Innervation of the vertebrate gut.--The three outflows of efferent nerves belonging to the organic system.--The original close contiguity of the respiratory chamber to the cloaca.--The elongation of the gut.--Conclusion.

In the previous chapters all the important organs of the arthropod have been found in the vertebrate in their appropriate place, of similar structure, and innervated from corresponding parts of the central nervous system. Such comparison is possible only as long as the ventral and dorsal surfaces of the vertebrate correspond with the respective surfaces of the arthropod, and no reversal is assumed. This method of comparative anatomy is the surest and most certain guide to the relationship between two animals, and when the facts obtained by the anatomical method are so strikingly confirmatory of the palæontological evidence, the combined evidence becomes so strong as to amount almost to a certainty that vertebrates did arise from arthropods in the manner mapped out in previous chapters, and not from a hypothetical group of animals, such as is postulated in the theory of their origin from forms like Balanoglossus.

The latter theory derives the alimentary canal of the vertebrate from that of the invertebrate, and finds in the latter the commencement of the notochord. In the comparison which I have made the alimentary canal of the invertebrate ancestor has become the tube of the central nervous system of the vertebrate, and there is no sign of a notochord whatever. All the organs of the arthropod have already been allocated; where the notochord is situated in the {434}vertebrate there is nothing but a gap in the invertebrate, but the position of that gap can be settled with great accuracy from the previous comparison of organs in the two groups. So, also, the alimentary canal of the vertebrate is from the very nature of the case a new organ, yet, as has been shown in Chapter V., the comparison of the respiratory organs in the two groups gives a strong suggestion of the manner in which such a canal was formed.

THE ORIGIN OF THE NOTOCHORD.

The time has now come to endeavour to frame a plausible theory of the method of formation of the notochord and the new alimentary canal, and thus to complete the diagram on p. 413. The comparative method is no longer available, for these structures are both unrepresented as such in the arthropod; any suggested explanation, therefore, must be more tentative, and cannot give the same feeling of certainty as is the case with all the organs already considered. Our only chance of finding out the past history of the notochord lies in the embryological method, in the hope that, according to the 'law of recapitulation,' the ancestral history may be repeated in the ontogeny with sufficient clearness to enable some conclusion to be drawn.

At the outset, one point comes out clearly--the close relationship between the notochord and the vertebrate gut; they are both derived from the same layer, both parts of the same structure. On this point all embryologists are agreed; it is expressed in such statements as, "the notochord, as well as the alimentary canal, is formed from hypoblast"; "the notochord arises as a thickening in the dorsal wall of the alimentary canal." The two structures are so closely connected together that they must be considered together. If we can conjecture the origin of the one, we may be sure that we have the clue to the origin of the other. The two together form the one new organ which distinguishes the vertebrate from the arthropod, the only thing left which requires explanation for the completion of this strange history.

What, then, is the notochord? What are its characteristics? In the highest vertebrates it is conspicuous only in the embryo; with the development of the axial skeleton it is more and more squeezed out of existence, until in the adult it is no longer visible. By the 'law of recapitulation' this developmental history implies that, as we descend the vertebrate phylum, the notochord ought to be more and {435}more conspicuous, more and more permanent during the life of the animal. Such is, indeed, found to be the case, until at last, in the lowest vertebrates, such as the lamprey, and in forms like Amphioxus, the notochord persists throughout the life of the animal as a large important axial supporting rod.

This rod has a number of striking characteristics which distinguish it from all other structures, and are the only means of guessing its probable origin. Its position in the body is always the same in all vertebrates and is very significant, for it lies just ventrally to the central nervous system, along nearly the whole length of the animal, not quite the whole length, for it invariably terminates close to the place where the infundibulum comes to the surface of the brain; it is, in fact, always confined to the infra-infundibular and spinal cord part of the central nervous system. Interpreting this into the language of the arthropod, it means that a rod was formed just ventrally to the nervous system, which extended the whole length of the infraoesophageal and ventral chain of ganglia, and terminated at the orifice of the mouth. Moreover, this rod was unsegmented, for the notochord is devoid of segmentation.

At the anterior end the rod tapers to a point, as in Fig. 166. In its middle part it is very large and conspicuous, cylindrical in shape; its interior is filled with a peculiar vacuolated tissue, different to any other known vertebrate tissue, which has therefore received the name of notochordal tissue. Outside this is a thick sheath formed of many layers, of which the external one gives the staining reactions of elastin, and is called the external elastic layer. Between this sheath and the notochordal tissue a thin layer of lining cells, of normal appearance, is conspicuous in Ammocoetes. These cells secrete the layers of the sheath, and have originally, by proliferation, given rise to the notochordal tissue. In the notochord of Ammocoetes there is no sign of either nerves, blood-vessels, or muscles.

The centre of the notochord presents the appearance of a slight slit, as though it had originated from a tube, and that is the opinion now generally held, for its mode of formation in the embryo is as that of a tube formed from an open groove, as will be explained immediately.

We may, then, conceive of the notochord as originally a tube lying in the mid-line just ventrally to the central nervous system, and extending from the original mouth to the end of the body. Translate this into the language of the arthropod and it denotes a tube on the {436}mid-ventral surface of the body, which extended from mouth to anus. Such a tube might be formed from the mid-ventral surface as follows:--

In Fig. 163, A, the lining of the ventral surface between two appendages is represented flat, in B is shown how the formation of a solid rod may arise from the bulging of that ventral surface, and in C how a groove on that surface may lead to the formation of a tube between the two appendages. The difference between a notochordal rod formed as in B from that in C would be shown in the sheath, for in B the sheath would be formed from the cuticle of the lining cells, and in C from the basement membrane. The structure of the sheath is in accordance with the embryological evidence that the notochord is formed as a tube from a groove, as in C, and not as a solid rod as in B, for it possesses a well-marked elastin layer, and elastin has never yet been found as a constituent of any cuticular secretion, but invariably in connection with basement-membranes.

The position, then, of the notochord and its method of formation suggests that the mid-ventral surface of the arthropod ancestor of the vertebrate formed a deep groove between the bases of all the prosomatic, mesosomatic, and metasomatic appendages, which was subsequently converted into a tube extending along the whole of the body between mouth and anus, and finally, by the proliferation of its lining cells and their conversion into notochordal tissue, became the notochordal rod of the vertebrate.

As already frequently stated, Apus and Branchipus are the two living arthropods which most nearly resemble the extinct trilobites. The beautiful specimens of Triarthrus (Fig. 165) found by Beecher give an idea of the under surface of the trilobite such as has never been obtained before, and demonstrate how closely the condition of things found in Apus (Fig. 164) was similar to that occurring in the trilobites. In both cases the mid-ventral surface of the animal formed a deep groove which extended the whole length of the {437}animal; on each side of this groove in Apus are closely set the gnatho-bases of the appendages, in such a manner that the groove can be easily converted into a canal by the movements of these bases--a canal which, owing to the great number of the appendages and their closeness to each other, can be completely and efficiently closed.

All those who have seen Apus in the living state assert that this canal so formed is actually used by the animal for feeding purposes. By the movements of the gnatho-bases food is passed up from the hind end of the animal along the whole length of this ventral canal to the mouth, where it is taken in and swallowed. In this way Apus has been seen to swallow its own eggs.

In the trilobites there is a similar deep channel formed by the mid-ventral surface, similar gnatho-bases, and closely set appendages, and the membrane of this ventral groove was extremely thin.

Here, then, in the very group of animals which were the progenitors of the presumed palæostracan ancestor of the vertebrate--a group which is characterized by its extensive prevalence and its {438}enormous variety of form during the great trilobite era--the formation of a mid-ventral canal out of this deep ventral groove is seen to be not only easy to imagine, but most probable, provided that a necessity arose for such a conversion.

For what purpose might such a tube have been formed? I would suggest that it might have acted as an accessory food-channel, which was of sufficient value at the time to give some advantage in the struggle for existence to those members of the group who were thus able to supplement their intake of food, but at the same time was so inefficient that it was quickly superseded by the new alimentary canal, and thus losing its temporary function, became solid, and was utilized to form an axial supporting rod.

There is a very considerable amount of evidence in favour of the view that the notochord was originally a digestive tube; in fact, as far as I know, this conclusion is universally accepted. The evidence is based essentially upon its development and upon its structure. It is formed in the vertebrate from the same layer as the alimentary canal, i.e. the hypoblast, and in Amphioxus it commences as a groove in the dorsal wall of the future alimentary canal; this groove then closes to form the tube of the notochord, and separates from the alimentary canal. Embryologically, then, the notochord is looked upon as a tube formed directly from the alimentary canal.

As regards its structure, its tissue is, as already stated, something sui generis. Notochordal tissue has no resemblance to bone or cartilage, or any of the usual supporting tissues. Such a tissue is not, however, entirely confined to the notochord of the vertebrates, but tissue closely resembling it has been found not only in Amphioxus and the Tunicata, but in certain other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in Cephalodiscus, and in Actinotrocha. In all these latter cases, such a tissue is invariably found in disused portions of the alimentary canal; a diverticulum of the alimentary canal becomes closed, vacuolation of its lining cells takes place, and a tissue resembling notochordal tissue is formed.

Owing to the notochord being invariably so striking and mysterious a feature of the lowest vertebrates, the term vertebrate, which is inappropriate in the members of the group which do not yet possess vertebræ, has been largely superseded by the term chordate, with the result of attributing an undue preponderance to this tissue in any system of classification. Hence, wherever any animal has been found {439}with a tissue resembling that of the notochord, enthusiasts have immediately jumped to the conclusion that a relationship must exist between it and the chordate animals; and, accordingly, they have classified such animals as follows: Amphioxus belongs to the group Cephalochorda because the notochord projects beyond the central nervous system; the Tunicata are called Urochorda because it is confined to the tail; the Enteropneusta, Hemichorda, because this tissue is confined to a small diverticulum of the gut, and, finally, Diplochorda has been suggested for Actinotrocha and Phoronis because two separate portions of the gut are transformed in this way.

This exaggerated importance given to any tissue resembling in structure that of the notochord is believed in by many of those who profess to be our teachers on this subject, the very men who can deliberately shut their eyes to the plain reading of the story of the pineal eyes, and say, "In our opinion this pineal organ was not an eye at all."

The only legitimate inference to be drawn from the similarity of structure between the notochord and these degenerated gut-diverticula, is that the structure of the notochord may have arisen in the same way, and that therefore the notochord may once have functioned as a gut. With cessation of its function its cells became vacuolated, as in these other cases, and its lumen became filled with notochordal tissue. This evidence strongly confirms the suggestion that the notochord was once a digestive tube, but by no means signifies that such tissue, wherever found, indicates the presence of a notochord.

In order to resemble a notochord, this tissue must possess not only a definite structure but a definite position, and this position is a remarkably striking and suggestive one. The notochordal tube is unsegmented, although the vertebrate is markedly segmented. But in all segmented animals the only unsegmented tube which extends the whole length of the body, from mouth to anus, is invariably the gut. In the vertebrate there are three such tubes: (1) the gut itself, (2) the central canal of the nervous system, and (3) the notochordal tube.

The first is the present gut, the second the gut of the invertebrate ancestor, and the third the tube in question.

These three unsegmented tubes, extending along the whole length {440}of the segmented animal, constitute the great peculiarity of the vertebrate group; it is not the unsegmented notochord alone which requires explanation, but the presence of three such tubes in the same animal. Any one of them might be the unsegmented gut of the segmented animal. The most ventral tube is the actual gut of the present vertebrate; the most dorsal--the neural canal--was, according to my view, the original gut of the invertebrate ancestor; the middle one--the notochordal tube--was, in all probability, also once a gut, formed at the time when the exigencies of the situation made it difficult for food to pass along the original gut.

Nc., neural canal with its infundibular termination; Nch., notochord; Al., alimentary canal with its anterior diverticulum; Hy., hypophysial or nasal tube; Or., oral chamber closed by septum.]

Yet another circumstance in favour of this suggestion is the very striking position of the anterior termination of the notochord. It terminates at the point of convergence of three structures:--

(1) The tube of the hypophysis or nasal tube.

(2) The infundibulum or old mouth-termination.

(3) The notochordal tube.

To these may be added, according to Kupffer, in the embryonic stage, the anterior diverticulum of the gut (Fig. 166).

This is a very significant point. Here originally, in the invertebrate stage, the olfactory passage opened into the old mouth and oesophagus. Here, finally, in the completed vertebrate the same olfactory passage opens into the new pharynx. In the stage between the two it may well have opened into an intermediate gut, the notochordal tube, its separation from which would leave the end of the {441}notochord blind, just as it had already left the end of the infundibulum blind.

The whole evidence points to the derivation of the notochord from a ventral groove on the surface of the animal, which closed to form a tube capable of acting as an accessory gut at the critical period before the new gut was fully formed. The essentials of a gut tube are absorption and digestion of food; is it likely that a tube formed as I have suggested would be efficient for such purposes?

As far as absorption is concerned, no difficulty would arise. The gut of the arthropod is lined with a thin layer of chitin, which is traversed, like all other chitinous surfaces, by fine canaliculi. Through these canaliculi, absorption of fluid material takes place, from the gut to the body. Similar canaliculi occur in the chitin covering the animal externally, so that, if such external surface formed a tube, and food in the right condition for absorption passed along it, absorption could easily take place through the chitinous surface. The evidence of Apus proves that food does pass along such a tube in the open condition, and in the trilobites the chitinous surface lining a similar groove was apparently very thin, a condition still more favourable to such an absorption process.

At first sight the second essential of a gut-tube--the power of digestion--appears to present an insuperable difficulty to this method of forming an accessory gut-tube, for it necessitates the formation of a secretion capable of digesting proteid material by the external cells of the body, whereas until recently it was supposed that such a function was confined to cells belonging to the so-called hypoblastic layer. Experiments were made now years ago of turning a Hydra inside out so that its internal layer should become external, and vice versâ, and they were said to have been successful. Such an animal could go on living and absorbing and digesting food, although its epiblastic surface was now its digestive internal surface. More recent observations have shown that these experiments were fallacious. At night-time, when the observer was not looking, the hydra reinverted itself, so that again its original digestive surface was inside and it lived and prospered as before.

Another piece of evidence of somewhat similar kind, which has not as yet been discredited, is seen in the Tunicata. In many of these, new individuals are formed from the parent by a process of budding, and it has been proved that frequently the gut of the new {442}individual thus budded off arises not from the gut or hypoblastic layer of the parent, but from the surface or epiblastic layer. Such gut so formed possesses as efficient digestive powers as the gut of the parent.

The most remarkable evidence of all has been afforded by Miss Alcock's experiments. She examined the different tissues of Ammocoetes for the express purpose of finding out their power of digesting fibrin, with the result that the most active cells were those of the liver. Next in activity came the extract of the lining cells of the respiratory chamber and of the skin. The intestine itself when freed from the liver-secretion had very little digestive power; extracts of muscle, nervous system, and thyroid gland had no power whatever, but the extract of the skin-cells possessed a powerful digesting action.

Furthermore, it is not necessary to make an extract of the skin in order to obtain this digestive fluid, for under the influence of chloroform the skin of Ammocoetes secretes copiously, and this fluid thus secreted was found to possess strong digestive powers. So, also, Miss Alcock has demonstrated the power of digesting fibrin in a similar secretion of the epithelial cells lining the carapace of the crayfish. In both cases a very plausible reason for the presence of a digestive ferment in a skin-secretion is found in the necessity of preventing the growth of parasites, fungoid, or otherwise, especially in those parts where the animal cannot keep itself clean by 'preening.' Thus in a crayfish, in which the oesophageal commissures had been cut, fungus was found to grow on the ventral side, but not on the dorsal carapace. The animal was accustomed to keep its ventral surface clean by preening; owing to the paralysis it could not do so, and consequently the fungus grew there. In the lamprey I found that wherever there was a removal of the surface-epithelium, from whatever cause, that spot was immediately covered with a fungoid growth, although in the intact lamprey the skin was invariably smooth and clean.

I imagine, then, that this digestive power of the skin arose as a protective mechanism against parasitic attacks; it is self-evident how a tube formed of such material must ab initio act as a digestive tube.

In yet another respect this skin secretion of Ammocoetes is most instructive. The surface of Ammocoetes is absolutely smooth, no scales {443}of any kind exist; this smoothness is due to the presence of a very well-defined cuticular layer secreted by the underlying epithelial cells. This cuticle is very much thicker than is usually found in vertebrates, and, strangely enough, has been thought to contain chitin. Whether it really contains chitin or not I am unable to say, but it certainly resembles a chitinous layer in one respect; it is perforated by innumerable very fine tubes or canaliculi, along which, by appropriate staining, it is easy to see the secretion of the underlying cell pass to the exterior (Fig. 140). This marked digestive power of the skin of Ammocoetes, together with the easy passage of the secretion through the thin cuticular layer, renders it almost certain that a tube formed from the deep ventral groove of the trilobite would, from the very first, act as a digestive as well as an absorbent tube; in other words, the notochord as soon as formed was able to act as an accessory digestive tube.

This suggested origin of the notochord from a groove along the mid-ventral surface of the body not only indicates a starting-point from a markedly segmented portion of the body, but also points to its formation at a stage previous to the formation of the operculum by the fusion of the two foremost mesosomatic appendages--indicates therefore its formation at a stage more nearly allied to the trilobite than to the sea-scorpion. The chance of ever finding any direct evidence of such a chordate trilobite stage appears to me exceedingly improbable, and I greatly fear that this conception of the mode of formation of the notochord can never be put to direct proof, but must always remain guesswork.

On the other hand, evidence of a kind in favour of its origin from a segmented part of the body does exist, and that evidence has this special value, that it is found only in that most primitive animal, Amphioxus.

This evidence is as follows:--

At fairly regular intervals, the sheath of the notochord is interrupted on each side of the mid-dorsal line by a series of holes, which penetrate the whole thickness of the sheath. This dorsal part is pressed closely against the spinal cord, and through these holes fibres appear to pass from the spinal cord to the interior of the notochord. So greatly do these fibres present the appearance of ventral roots to the notochord, that Miss Platt looks upon them as paired motor roots to the notochord, or at all events as once having been such motor {444}roots. Lwoff and Rolph both describe a direct communication between the spinal cord and the notochord by means of fibres passing through these holes, without however looking upon this connection as a nervous one. Joseph alone asserts that no absolute connection exists, for the internal elastic layer of the notochord, according to him, is not interrupted at these holes, and forms, therefore, a barrier between the fibres from the spinal cord and those from the interior of the notochord. Still, whatever is the ultimate verdict as to these fibres, the suggestive fact remains of the spaces in the notochordal sheath and of the corresponding projecting root-like fibres from the spinal cord. The whole appearance gives the impression of some former connection, or rather series of connections, between the spinal cord and the notochord, such as would have occurred if nerves had once passed into the notochord. On the other hand, such nerves were not arranged segmentally with the myotomes, for, according to Joseph, in the middle of the animal ten to twelve such holes occur in one body-segment. In Apus the appendages are more numerous than the body-segments, so that it is not necessary for a segmental arrangement to coincide with that of the body-segments.

THE ORIGIN OF THE ALIMENTARY CANAL.

In close connection with the notochord is the alimentary canal. Any explanation of the one must be of assistance in explaining the other.

According to the prevalent embryological teaching, the body is formed of three layers, epiblast, hypoblast, and mesoblast, and the gastræa theory of the origin of all Metazoa implies of necessity that the formation of every individual commences with the formation of the gut. For this reason the alimentary canal must in every case be regarded as the earliest formed organ, however late in the development it may attain its finished appearance. Hence the notochord is spoken of as developed from the mid-dorsal wall of the alimentary canal. It is possible to look at the question the other way round, and suppose that the organ whose development is finished first is older than the one still in process of making. In this case it would be more right to say a ventral extension of the tissue, which gives rise to the notochord, takes place and forms the alimentary canal. It is, to my mind, perfectly possible, and indeed probable, that {445}the formation of the vertebrate alimentary canal was a repetition of the same process which had already led to the formation of the notochordal tube. The formation of the anterior part of the alimentary canal in Ammocoetes at the time of transformation strongly suggests the marked similarity of the two processes.

Of all the startling surprises which occur at transformation, this formation of a new anterior gut is the most startling. From the oral chamber of Petromyzon two tubes start: the one leads into the gill-chambers, is known as the bronchus, and is entirely concerned with respiration; the other leads without a break from the mouth to the anus, has no connection with respiration, and is the alimentary canal of the animal. Any one looking at Petromyzon would say that its alimentary canal was absolutely non-respiratory in character. Before transformation, this kind of alimentary canal commences at the end of the respiratory chamber; from here to the anus it is of the same character as in Petromyzon, but in Ammocoetes the non-respiratory anterior part simply does not exist: the whole anterior chamber is both respiratory and affords passage to food. This part of the alimentary canal of the adult is formed anew. We see, then, here the formation of a part of the alimentary canal taking place, not in an embryo full of yolk, but in a free-living, independent, grown-up larval form in which all yolk has long since disappeared: a condition absolutely unique in the vertebrate kingdom, but one which more than any other may be expected to give a clue to the method of formation of a vertebrate gut.

The formation of this new gut can be easily followed at transformation, and was originally described by Schneider. His statement has been confirmed by Nestler, and its absolute truth has been demonstrated to me again and again by Miss Alcock, in her specimens illustrative of the transformation process. First, in the mid-dorsal line of the respiratory chamber a distinct groove is formed, the edges of which come together and form a solid rod. This solid rod blocks the opening of the respiratory chamber into the mid-gut, so that during this period of the transformation no food can pass out of the pharyngeal chamber. A lumen then begins to appear in this solid rod at the posterior end, which steadily advances mouthwards until it opens into the oral chamber and thus forms an open tube connecting the mouth with the gut.

Here, then, is the foundation of a new gut on very similar lines {446}to that of the notochord, by the conversion of a groove into a tube. Still more suggestive is it to find that the tube so formed has no appearance whatever of segmentation; it is as unsegmented as the rest of the gut, although, as is seen in Fig. 62, the dorsal wall of the respiratory chamber from which it arose is as markedly segmented as any part of the animal. Here under our very eyes, in the course of a few days or weeks, an object-lesson in the process of the manufacture of an alimentary canal is carried out and completed, and the teaching of that lesson is that a gut-tube may be formed in the same way as the notochordal tube, by the conversion of a grooved surface into a canal, and that gut-tube so formed, like the notochord, loses all sign of segmentation, even although the original grooved surface was markedly segmented.

The suggestion then is, that the new gut may have been formed by a repetition of the same process which had already given origin to the notochord.

Such a method of formation is not, in my opinion, opposed to the evidence given by embryology, but in accordance with it; the discussion of this point will come best in the next chapter, which treats of the embryological evidence as a whole, and will therefore be left till then.

THE EVIDENCE GIVEN BY THE INNERVATION OF THE VERTEBRATE ALIMENTARY CANAL.

Throughout this investigation the one fixed landmark to which all other comparisons must be referred, is the central nervous system, and the innervation of every organ has given the clue to the meaning of that organ. So also it must be with the new alimentary canal; by its innervation we ought to obtain some insight into the manner of its origination. In any organ the nerves which are specially of value in determining its innervation, are of necessity the efferent or motor nerves, for the limits of their distribution in the organ are much more easily determined than those of the afferent or sensory nerves. The question therefore of primary importance in endeavouring to determine the nature of the origin of the alimentary canal from its innervation is the determination of the efferent supply to the musculature of its walls.

Already in previous chapters a commencement has been made in {447}this direction; thus the musculature of the oral chamber has been derived directly from the musculature of the prosomatic appendages; the muscles which move the eyes from the prosomatic and mesosomatic dorso-ventral somatic muscles; the longitudinal body-muscles from the dorsal longitudinal somatic muscles of the arthropod; the muscles of respiration from the dorso-ventral muscles of the mesosomatic appendages.

In all these cases we have been dealing with striated musculature and consequently with only the motor nerves of the muscle; but the gut posterior to the pharyngeal or respiratory chamber contains unstriped instead of striped muscle, and is innervated by two sets of nerves, those which cause contraction and are motor, and those which cause relaxation and are inhibitory. It is by no means certain that these two sets of nerves possess equal value from a morphological point of view. The meaning of an inhibitory nerve is at present difficult to understand, and in this instance, is rendered still more doubtful owing to the presence of Auerbach's plexus along the whole length of the intestine--an elaborate system of nerve-cells and nerve-fibres situated between the layers of longitudinal and circular muscles surrounding the gut-walls, which has been shown by the recent experiments of Magnus, to constitute a special enteric nervous system.

One of the strangest facts known about the system of inhibitory nerves is their marked tendency to leave the central nervous system at a different level to the corresponding motor nerves, as is well known in the case of the heart, where the inhibitory nerve--the vagus--arises from the medulla oblongata, while the motor nerve--the augmentor or accelerator--leaves the spinal cord in the upper thoracic region. It is very difficult to obtain any idea of the origin of such a peculiarity; I know of only one suggestive fact, which concerns the innervation of the muscles which open and close the chela of the crayfish, lobster, etc. These muscles are antagonistic to each other, and both possess inhibitory as well as motor nerves. The central nervous system arrangements are of such a character that the contraction of the one muscle is accompanied by the inhibition of its opposer, and the nerves which inhibit the contraction of the one, leave the central nervous system with the nerves which cause the other to contract. Thus the inhibitory and motor nerves of either the abductor (opener) or adductor (closer) muscles of the crayfish claw do not leave the central nervous system together, but in separate nerves.

{448}If now for some cause the one set of muscles either disappeared, or were so altered as no longer to present any appearance of antagonism, then there would be left a single set of muscles, the inhibitory and motor nerves of which would leave the central nervous system at different levels, and the older such systems might be, the greater would be the modification in the shape and arrangements of parts in the animal, so that the two sets of fibres might ultimately arise from very different levels.

As mentioned in the introductory chapter, the whole of this investigation into the origin of vertebrates arose from my work on the system of efferent nerves which innervate the vascular and visceral systems. One of the main points of that investigation was the proof that such nerves did not leave the central nervous system uniformly along the whole length of it, but in three great outflows, cranial, thoracico-lumbar, and sacral; there being two marked gaps separating the three outflows, caused by the interpolation of the plexuses for the innervation of the anterior and posterior limbs respectively. All these nerves are characterized by the presence of ganglion-cells in their course to the periphery, they are, therefore, distinguished from ordinary motor nerves to striated muscle in that their impulses pass through a ganglion-cell before they reach the muscle.

The ganglia of the large middle thoracico-lumbar outflow constitute the ganglia of the sympathetic system.

The functions of the nerves constituting these three outflows are very different, as I pointed out in my original papers. Since then a large amount of further information has been obtained by various observers, especially Langley and Anderson, which enable the following statements to be made:--

All the nerves which cause contraction of the unstriped muscles of the skin, whether pilomotor or not, all the nerves which cause secretion of sweat glands wherever situated, all the nerves which cause contraction or augmentation of the action of muscles belonging to the vascular system, all the nerves which are motor to the muscles belonging to all organs derived from the Wolffian and Müllerian ducts, e.g. the uterus, ureters, urethra, arise from the thoracico-lumbar outflow, never from the cranial or sacral outflows. It is essentially an efferent skin-system.

On the other hand, the latter two sets of nerves are concerned {449}with the supply of motor nerves to the alimentary canal; they form essentially an efferent gut-system in contradistinction to the sympathetic or skin-system.

A marked distinction exists between these cranial and sacral nerves. The vagus never supplies the large intestine, the sacral nerves never supply the small intestine. Associated with the large intestine is the bladder, the whole system arising from the original cloacal region; the vagus never supplies the bladder, its motor nerves belong to the sacral outflow. The motor nerves to the ureters, to the urethra, and to the trigonal portion of the bladder between the ureters and the urethra, do not arise from the sacral outflow, but from the thoracico-lumbar. These muscles belong really to the muscles in connection with the Müllerian and Wolffian ducts and skin, not to the cloacal region.

The motor innervation then of the alimentary canal reveals this striking and suggestive state of affairs. The motor innervation of the whole of the small intestine arises from the cranial region, and is immediately followed by an innervation from the sacral region for the whole of the muscles of the cloaca. It thus indicates a head-region and a tail-region in close contiguity, the whole of the spinal cord region between these two extremes being apparently unrepresented. Not, however, quite unrepresented, for Elliott has shown recently that the ileo-colic valve at the junction of the small and large intestine is in reality an ileo-colic sphincter muscle, and that this muscle receives its motor nerves neither from the vagus nor from the sacral nerves, but from the thoracico-lumbar outflow or sympathetic system. This may mean one of two things, either that a band of fibres belonging to the skin-system has been added to the gut-musculature, for the purpose of forming a sphincter at this spot, or that the region between the vagus territory and the cloaca is represented by this small band of muscle. The second explanation seems to me the more probable of the two. Between the mesosomatic region represented by the vagus, and the cloacal region, there existed a small metasomatic region, represented by the pronephros, with its segmental duct, as already discussed in Chapter XII. That part of the new alimentary canal which belonged to this region is the short piece indicated by the ileo-colic sphincter, and innervated, therefore, from the same region as the organs derived from the segmental duct.

Such innervation seems to me to suggest that originally the {450}vertebrate consisted, as far as its gut was concerned, of a prosomatic and mesosomatic (branchial) region, close behind which came the cloaca and anus. Between the two there was a short metasomatic region (possibly pronephric), so that the respiratory chamber did not open directly into the cloaca.

Such an interpretation is, I think, borne out by the study of the most ancient forms of fish. In Bothriolepis, according to Patten, and in Drepanaspis, according to Traquair, the cloacal region and anus follow immediately upon the posterior end of the head-shield, i.e. immediately after that region which presumably contained the branchiæ. Similarly, on the invertebrate side, all those forms which resembled Limulus must have possessed a very short region between the branchial and cloacal parts of the body. The original cloacal part of the vertebrate gut may well have been the original cloaca of the arthropod, into which its intestine emptied itself, especially when we see the tendency of the scorpion group of animals to form an accessory cloacal pouch known as the stercoral pouch or pocket.

Again, it is striking to see how, in certain of the scorpion group, e.g. Thelyphonus and Phrynus, there is a caudal massing of the central nerve-cells as well as a cephalic massing, so that their central nervous system is composed of a cephalic and caudal brain. These two brains are connected together by commissures extending the whole length of the body, in which I have been unable to find any sign of ganglion-cells. What this caudal brain innervates I do not know; it is, I think, a matter worth further investigation, especially as there are many indications in the vertebrate that the lumbo-sacral region of the cord possesses higher functions than the thoracic region.

The method of formation of the alimentary canal as indicated by its innervation is as follows:--

In front an oral chamber, formed, as already pointed out, by the modification of the prosomatic appendages, followed by a respiratory chamber, the muscles and branchiæ of which were the muscles and branchiæ of the mesosomatic appendages. This mesosomatic, or branchial, part was in close contiguity to the cloaca and anus, being separated from it only by a short tube formed in the metasomatic or pronephric region.

I imagine that this connection was originally in the form of an {451}open groove, as already explained for both notochord and the anterior part of the gut itself in Ammocoetes; an open groove formed from the mid-ventral surface of the body, on each side of which were the remnants of the pronephric appendages. By the closure of this groove ventrally, and the growing round of the pleural folds, as already suggested, the remains of the pronephric appendages are indicated by the segmental duct and the form of the vertebrate body is attained.

Even in the branchial region the same kind of thing must, I think, have occurred. The grooved ventral surface became a tube, on each side of which were lying in regular order the in-sunk branchial appendages, the whole being subsequently covered by the pleural folds to form an atrial chamber. A tube thus formed from the grooved ventral surface would carry with it to the new ventral surface the longitudinal venous sinuses, and thus form, in the way already suggested, the heart and ventral aorta. Posterior to the heart in the pronephric region, the same process would give rise to the sub-intestinal vein.

The evidence of comparative anatomy bears out most conclusively the suggestion that in the original vertebrate the gut was mainly a respiratory chamber. In man and all mammals the oral chamber opens into a small pharynx, followed by the oesophagus, stomach and small intestine. Of this whole length, a very small part is taken up by the pharynx, in which, in the embryo, the branchial arches are found, showing that this represents the original respiratory part of the gut. In the ordinary fish this branchial part is much more conspicuous, occupies a large proportion of the gut, and in the lowest fishes, such as Ammocoetes and Amphioxus, the branchial region extends over a large portion of the animal, while the intestine proper is a straight tube, the length of which is insignificant in comparison with its length in the higher vertebrates.

Such a tube was able to act as a digestive tube, owing, as already pointed out, to the digestive powers of the skin-epithelium, and I imagine at first the respiratory chamber, seeing that it composed very nearly the whole of the gut, was at the same time the main digestive chamber; even in Ammocoetes its digestive power is superior to that of the intestine itself.

Just posterior to the branchial part a diverticulum of the gut was formed at an early stage, as seen in Amphioxus, and provided the {452}commencement of the liver. This simple liver-diverticulum became the tubular liver of Ammocoetes, and formed, curiously enough, not a glandular organ of the same character as the liver of the higher vertebrates, but a hepato-pancreas, like the so-called liver of the arthropods, which also is a special diverticulum of the gut, or rather the main true gut of the animal. In both cases the liver is the chief agent in digestion, for in Ammocoetes the liver-extract is very much more powerful in the digestion of proteids than the extract of any other organ tried by Miss Alcock. Subsequently in the vertebrate the gastric and pancreatic glands arise and relieve the liver of the burden of proteid digestion.

It is, to my mind, somewhat significant that the liver on its first formation in the vertebrate should have arisen as a digestive organ of the same character as the so-called liver in the arthropods; whether it originally belonged to any separate segment is in our present state of knowledge difficult to say.

CONCLUSION.

In conclusion, I will endeavour to illustrate crudely the way in which, on my theory, the notochord and vertebrate gut may have been formed, the agencies at work being in the main two, viz. the dwindling of appendages as mere organs of locomotion, and the conversion of a ventral groove into a tube.

I imagine that, among the Protostraca, forms were found somewhat resembling trilobites with markedly polychætan affinities; which, like Apus, possessed a deep ventral groove from one end of the body to the other, and also pleural fringes, as in many trilobites. This might be called the Trilobite stage (Fig. 167, A).

This groove became converted into a tube and so gave rise to the notochord, while the appendages were still free and the pleuræ had not met to form a new ventral surface. This might be called the Chordate Trilobite stage (Fig. 167, B).

Then, passing from the protostracan to the palæostracan stage, the oral and respiratory chambers were formed, not communicating with each other, in the manner described in previous chapters, a ventral groove in the metasomatic region being the only connection between respiratory chamber and cloaca. This might be called the Chordate Palæostracan stage (Fig. 167, C).

{453}[Illustration: FIG. 167.--A, DIAGRAM OF SECTION THROUGH A TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE THE SUGGESTED FORMATION OF THE NOTOCHORD FROM A VENTRAL GROOVE; C, DIAGRAM TO ILLUSTRATE THE SUGGESTED FORMATION OF THE POST-BRANCHIAL GUT BY THE CONTINUATION OF THE SAME PROCESS OF VENTRAL GROOVE-FORMATION, COMBINED WITH OBLITERATION OF APPENDAGES AND GROWTH OF PLEURAL FOLDS; D, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE WITH THE OBLITERATION OF THE ATRIAL CAVITY AND THE CONVERSION OF THE VENTRAL GROOVE INTO THE CLOSED ALIMENTARY CANAL.

Al., alimentary canal; N., nervous system; My., myotome; Pl., pleuron; App., appendage; Neph., nephrocoele; Met., metacoele; Sd., segmental duct; Mes., mesonephros; At., atrial chamber; Nc., notochord; H., heart; F., fat body; Ng., notochordal groove. (These diagrams are intended to complete the diagrams on p. 413, which, as stated there, were purposely left incomplete.)]

{454}Finally, with the conversion of this groove into a tube, the opening of the oral into the respiratory chamber, and the formation of an atrium by the ventralwards growth of the pleural folds, the formation of a Vertebrate was completed (Fig. 167, D).

In my own mind I picture to myself an animal which possessed eurypterid and trilobite characters combined, in which a notochordal tube had been formed in the way suggested, and a respiratory chamber which communicated with the cloaca by means of a grooved channel along the mid-ventral line of the metasomatic portion of the body. On each side of this channel were the remains of the metasomatic appendages (pronephric). The whole was enveloped in the pleural folds, which probably at this time did not yet meet in the middle line to form a new ventral surface. This respiratory chamber, owing to the digestive power of the epidermis, assisted in the process of alimentation to such an extent as to supersede the temporary notochordal tube, with the effect of bringing about the conversion of the metasomatic groove into a closed canal, and so the formation of an alimentary tube continuous with the respiratory chamber. The amalgamation of the pleural folds ventrally completed the process, and so formed an animal resembling the Cephalaspidæ, Ammocoetes, or Amphioxus.

I have endeavoured in this chapter to make some suggestions upon the origin of the notochord and of the vertebrate gut in accordance with my theory of the origin of vertebrates. I feel, however, strongly that these suggestions are much more speculative than those put forward in the previous chapters, and of necessity cannot give the same feeling of soundness as those based directly upon comparative anatomy and histology. Still, the fact remains that the origin of the notochord is at present absolutely unknown, and that my speculation that it may have originated as an accessory digestive tube is at all events in accordance with the most widely spread opinion that it arises in close connection with an alimentary canal.

{455}CHAPTER XIV

THE PRINCIPLES OF EMBRYOLOGY

The law of recapitulation.--Vindication of this law by the theory advanced in this book.--The germ-layer theory.--Its present position.--A physiological not a morphological conception.--New fundamental law required.--Composition of adult body.--Neuro-epithelial syncytium and free-living cells.--Meaning of the blastula.--Derivation of the Metazoa from the Protozoa. Importance of the central nervous system for Ontogeny as well as for Phylogeny.--Derivation of free-living cells from germ-cells.--Meaning of coelom.--Formation of neural canal.--Gastrula of Amphioxus and of Lucifer.--Summary.

In a discussion upon this theory of mine, which took place at Cambridge on November 25 and December 2, 1895, it was said that such a theory was absolutely and definitely put out of court, because it contravened the principles of embryology, was opposed, therefore, to our surest guide in such matters; and the law was laid down with great assurance that no claim for genetic relationship between two groups of animals can be allowed which is based upon topographical and structural coincidences revealed by the study of the anatomy of two adult animals, however numerous and striking they may be, if there are fundamental differences in the embryology of the members of these two groups.

According to my theory the old gut of the arthropod still exists in the vertebrate as the tubular lining of the central nervous system, and the vertebrate has formed a new gut. According to the principles of embryology as held up to the present, in all animals above the Protozoa, the different structures of the body arise from three definite embryonic layers, the epiblast, mesoblast, and hypoblast, and in all cases the gut arises from the hypoblastic layer. In the vertebrate the gut also arises from the hypoblast, while the neural canal is epiblastic. My theory, then, makes the impossible assertion that what was hypoblast in the arthropod has become epiblast in the vertebrate, and what was epiblast in the arthropod has become hypoblast in the vertebrate. Such a conception is supposed to be so {456}absolutely impossible that it only requires to be stated to be dismissed as an absurdity.

Against this opinion I claim boldly that my theory is not only not contrary to the principles of embryology, but is mainly based upon the teachings of embryology. I wish here not to be misunderstood. The great value of the study of embryology for questions of the sequence of the evolution of animals is to be found in what is known as the Law of Recapitulation, which asserts that every animal gives some indication in the stages of its individual development of its ancestral history. Naturally enough it cannot pass through all the stages of its past history with equal clearness, for what has taken millions of years to be evolved has to be compressed into an evolution lasting only a few months or weeks, or even less.

When in the highest vertebrate a vestigial organ, such as the pineal gland, can be traced back without leaving the vertebrate kingdom to a distinct median eye, such as is found in the lamprey, that rudimentary organ is evidence of an organ which was functional in the earliest vertebrates or their immediate ancestors. So it is generally with well defined vestigial organs found in the adult animal; they always indicate an organ which was functional in the near ancestor.

Passing from the adult to the embryo we still find the same law. Here, also, vestigial organs are met with, which may leave no trace in the adult, but indicate organs which were functional in the near ancestor. Thus, but for embryology, we should have no certainty that the air-breathing vertebrates had been derived from water-breathing fishes; the indication is not given by any close resemblance between the formation of the embryos in their earliest stages, but by the formation of vestigial gill-arches even in the embryos of the highest mammal.

For all questions of evolution the presence of vestigial organs in the embryo is the important consideration, for they give an indication of near ancestry; the early formation of the embryo concerns a much more remote ancestral period, all vestigial organs of which may well have been lost and obscured by coenogenetic changes. Let us, then, consider the two things--the vestigial organs and the early formation of the embryo--separately, and see how far my opponents are justified in their statement that my theory contravenes the principles of embryology.

{457}First, I will take the teachings of vestigial organs and the arrangement of organs found in the vertebrate embryo. Here it is impossible to say that my theory is contrary to the teaching of embryology, for as the previous chapters have shown again and again, the argument is based very largely upon the facts of embryology. In the first place, the comparison which I have chiefly made is a comparison between the larval form of a very low vertebrate and the arthropod group, a comparison which exists only for the larval form, and not for the adult. The whole theory, then, is based upon a developmental stage of the vertebrate, and not upon the anatomy of the adult.

Throughout the whole history it seems to me perfectly marvellous how completely the law of recapitulation is vindicated by my theory of the origin of the vertebrate. The theory asserts that the clue to the origin of vertebrates is to be found in the tubular nature of the central nervous system of the vertebrate; in that the vertebrate central nervous system is in reality formed of two things: (1) a central nervous system of the arthropod type, and (2) an epithelial tube in the position of the alimentary canal of the arthropod.

Is it possible for embryology to recapitulate such a phylogenetic history more clearly than is here the case? In order to avoid all possibility of our mistaking the clue, the nerve-tube in the embryo always opens into the anus at its posterior end, while in the larval Amphioxus it is actually still open to the exterior at the anterior end. The separateness of the tube from the nervous system at its first origin is shown especially well in the frog, where, as Assheton has pointed out, owing to the pigment in the cells of the external layer of epithelium, a pigmented tube is formed, on the outside of which the nervous tissue is lying, and step by step the gradual intermingling of the nerve-cells and the pigmented lining cells can be followed out.

Consider the shape of the nerve-tube when first formed in the vertebrate. At the cephalic end a simple bulged-out tube with two simple anterior diverticula, which passes into a narrow straight spinal tube; from this large cephalic bulging a narrow diverticulum, the infundibulum, passes to the ventral surface of the forming brain. This tube is the embryological expression of the simple dilated cephalic stomach, with its ventral oesophagus and two anterior diverticula, which opens into the straight intestine of the arthropod. Nay, more, by its very shape, and the invariable presence of two anterior {458}diverticula, it points not only to an arthropod ancestry, but to a descent from a particular group of primitive arthropods. Then comes the formation of the cerebral vesicles, and the formation of the optic cup, telling us as plainly as can be how the invasion of nervous material over this simple cephalic stomach and its diverticula has altered the shape of the original tube, and more and more enclosed it with nervous elements.

So, too, in the spinal cord region. When the tube is first formed, it is a large tube, the latero-ventral part of which presents two marked bulgings; connecting these two bulgings is the anterior commissure. These two lateral bulgings, with their transverse commissure, represent, with marked fidelity, the ventral ganglion-masses of the arthropod with their transverse commissure, and occupy the same position with respect to the spinal tube, as the ganglion-masses do with respect to the intestine in the arthropod. Then the further development shows how, by the subsequent growth of the nervous material, the calibre of the tube is diminished in size, and the spinal cord is formed.

Again, I say, is it possible to conceive that embryology should indicate the nature of the origin of the vertebrate nervous system more clearly than it does?

It is the same with all the other organs. Take, for instance, the skeletal tissues. The study of the vertebrate embryo asserts that the cartilaginous skeleton arose as simple branchial bars and a simple cranio-facial skeleton, and also that the parenchymatous variety of cartilage represents the embryonic form. Word for word, the early embryonic stage of the vertebrate skeleton closely resembles the stage reached in the arthropod, as shown by Limulus, and again records, unmistakably, the past history of the vertebrate.

So, too, with the whole of the prosomatic region; the situation of the old mouth, the manner in which the nose of the cephalaspidian fishes arose from the palæostracan, are all shown with vivid clearness by Kupffer's investigations of the early stage of Ammocoetes, while at the same time the closure of the oral cavity by the septum shows how the oral chamber was originally bounded by the operculum. Nay, further, the very formation of this chamber embryologically was brought about by the forward growth of the lower lip, just as it must have been if the chilaria grew forward to form the metastoma.

So, too, the study of the embryo teaches that the branchiæ arise as {459}ingrowths, that the heart arises as two longitudinal veins, just as the theory supposes from the facts provided by Limulus and the scorpions. No indication of the origin of the thyroid gland is given by the study of its structure in any adult vertebrate, but in the larval form of the lamprey there is still preserved for us a most graphic record of its past history.

The close comparisons which it is possible to make between the eye-muscles of the vertebrate and the recti muscles of the scorpion group on the one hand, and between the pituitary and coxal glands on the other, are based upon, or at all events are strikingly confirmed by, the study of the coelomic cavities and the origin of these muscles in the two groups. In fact the embryological evidence of the double segmentation in the head and the whole nature of the cranial segments is one of the main foundation-stones on which the whole of my theory rests.

So it is throughout. Turn to the excretory organs--it is not the kidney of the adult animal which leads direct to the excretory organs of the primitive arthropod, but the early embryonic origin of that kidney.

So far from having put forward a theory which runs counter to the principles of embryology, I claim to have vindicated the great Law of Recapitulation which is the foundation-stone of embryological principles. My theory is largely based upon embryological facts, and its strength consists in the manner in which it links together into one harmonious whole, the facts of Embryology, Palæontology, Anatomy, and Physiology. Why, then, is it possible to assert that my theory disregards the principles of embryology, when, as we have seen, embryology is proclaiming as loudly as possible how the vertebrate arose? In my opinion, it is because the embryologists have to a large extent gone wrong in their fundamental principles, and have attached more weight to these faulty fundamental principles than to the obvious facts which, looked at thoughtfully, could not have failed to suggest a doubt as to the correctness of these 'principles.'

The current laws of embryology upon which such weight is laid are based on the homology of the germinal layers in all Metazoa, and state that in all cases after segmentation is finished a blastula is formed, from which there arises a gastrula, formed of an internal layer, the hypoblast, and an external layer, the epiblast; subsequently {460}between these arises a third layer, the mesoblast. These layers are strictly morphological conceptions, and are stated to be homologous in all cases, so that the hypoblast of one animal must be homologous to the hypoblast of another. In order, therefore, to compare two adult animals for the purpose of finding kinship between them, it is necessary to find whether parts such as the gut, which in both cases have the same function, arise from the same germinal layer in the embryo. We can, in fact, have no certainty of kinship, even although the two animals are built up as far as the adult state is concerned on a remarkably similar plan, unless we can study their respective embryos and find out what parts arise from the hypoblast and what from the epiblast. The homology of the germinal layers constitutes in all cases of disputed relationship the court of final appeal. A new gut, therefore, in any animal can only be formed from hypoblast, and any theory, such as that advocated in this book, which deals with the formation of a new gut, and does not form that gut from pre-existing hypoblast, must of necessity be wrong and needs no further consideration.

Such is the result of current conceptions--conceptions which to be valid must be based upon an absolutely clear morphological definition of the formation of the germinal layers, a definition not based on their subsequent history and function, but determined solely by the uniformity of the manner of their origin.

What, then, is a germinal layer? How can we identify it when it first arises? What is the morphological criterion by which hypoblast can be distinguished from epiblast, or mesoblast from either?

This is the question put by Braem, in an admirable series of articles in the Biologisches Centralblatt, and is one that must be answered by every worker who bases his views of the process of evolution upon embryological investigation. As Braem points out, the germinal layers are definable either from a morphological or physiological standpoint. In the one case they must arise throughout on the same plan, and whatever be their fate in the adult, they must form at an early stage structures strictly homologous in all animals. In the other case the criterion is based on function, and the hypoblast, for instance, is that layer which is found afterwards to form the definitive alimentary canal. There is no longer any morphological homology; such layers are analogous; they may be, but are not necessarily, homologous. Braem gives a sketch of the history of the views held on {461}the germinal layers, and shows how they were originally a purely physiological conception, and how gradually such conception changed into a morphological one, with the result that what had up to that time been looked upon as analogous structures became strictly homologous and of fundamental importance in deciding the position of any animal in the whole animal series.

This change of opinion was especially due to the lively imagination of Haeckel, who taught that the germinal layers of all Metazoa must be strictly homologous, because they were all derived from a common ancestral stock, represented by a hypothetical animal to which he gave the name Gastræa; an animal which was formed by the simple invagination of a part of the blastula, thus giving rise to the original hypoblast and epiblast, and he taught that throughout the animal kingdom the germinal layers were formed by such an invagination of a part of the blastula to form a simple gastrula. If further investigation had borne out Haeckel's idea, if therefore the hypoblast was in all cases formed as the invagination of a part of a single-layered blastula, then indeed the dogma of the homology of the germinal layers would be on so firm a foundation that no speculation which ran counter to it could be expected to receive acceptance; but that is just what has not taken place. The formation of the gastrula by simple invagination of the single-layered blastula is the exception, not the rule, and, as pointed out by Braem, is significantly absent in the earliest Metazoa; in those very places where, on the Gastræa theory, it ought to be most conspicuous.

Braem discusses the question most ably, and shows again and again that in every case the true criterion upon which it is decided whether certain cells are hypoblastic or not is not morphological but physiological. The decision does not rest upon the answer to the question, Are these cells in reality the invaginated cells of a single-celled blastula? but to the question, Do these cells ultimately form the definitive alimentary canal? The decision is always based on the function of the cells, not on their morphological position. Not only in Braem's paper, but elsewhere, we see that in recent years the physiological criterion is becoming more and more accepted by morphologists. Thus Graham Kerr, in his paper on the development of Lepidosiren, says: "It seems to me quite impossible to define a layer as hypoblastic except by asking one or other of the two questions: (1) Does it form the lining of an archenteric cavity? and (2) {462}Does it become a certain part of the definitive epithelial lining of the gut?"

The appearance of Braem's paper was followed by a criticism from the pen of Samassa, who agrees largely with Braem, but thinks that he presses the physiological argument too far. He considers that morphological laws must exist for the individual development as well as for the phylogenetic, and finishes his article with the following sentence, a sentence in which it appears to me he expresses what is fast becoming the prevailing view: "Mit dem Satz, den man mitunter lesen kann: 'es muss doch auch für die Ontogenie allgemeine Gesetze geben' kann leicht Missbrauch getrieben werden; diese allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand und bis zu ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber wohl heute schon sagen, die Keimblätterlehre gehört zu diesen allgemeinen Gesetzen nicht."

I conclude, then, that we ought to go back to a time previous to that of Haeckel and ask ourselves seriously the question, When we lay stress on the germinal layers and speak of this or that organ arising from this or that germinal layer, are we thereby adding anything to the knowledge that we already possess from the study of the anatomy and physiology of the adult body? If by hypoblast we only mean the internal surface or alimentary canal and its glands, etc., and by epiblast we mean the external surface or skin and its glands, etc., while mesoblast indicates the middle structures between the other two, then I fail to see what advantages we obtain by using Greek terms to express in the embryo what we express in English in the adult.

The evidence given by Braem, and it could be strengthened considerably, is conclusive against the morphological importance of the theory of the germinal layers, and transfers the fundamental importance of the early embryonic formation, from that of a three-layered embryo to that of a single-layered embryo--the blastula--from which, in various ways, the adult animal has arisen.

The derivation of both arthropod and vertebrate from such a single-layered animal is perfectly conceivable, even though the gut of the latter is not homologous with the gut of the former. We have seen that the teachings of embryology, as far as its later stages are concerned, afford one of the main supports upon which this theory rests. What, therefore, is required to complete the story is the way {463}in which these later stages arise from the blastula stage; here, as in all cases, the ontogenetic laws must be in harmony with the phylogenetic; of the latter the most important is the steady development of the central nervous system for the upward progress of the animal race. The study of comparative anatomy indicates the central nervous system, not the gut, as the keystone of the edifice. So, also, it must be with ontogeny; here also the central factor in the formation of the adult from the blastula ought to be the formation of the central nervous system, not that of the gut.

Such, it appears to me, is the case, as may be seen from the following considerations.

The study of the development of any animal can be treated in two ways: either we can trace back from the adult to the very beginning in the ovum, or we can trace forward from the fertilized egg to the adult. Both methods ought to lead to the same result; the difference is, that in the first case we are passing from the more known to the less known, and are expressing the unknown in terms of the known. In the second case we are passing from the less known to the more known, and are expressing the known in speculative terms, invented to explain the unknown. What has just been said with respect to the germinal layers means that, however much we may study the embryo and try to express the adult in terms of it, we finally come back to the first way of looking at the question, and, starting with the adult, trace the continuity of function back to the first formation of cells having a separate function.

Let us, then, apply this throughout, and see what are the logical results of tracing back the various organs and tissues from the adult to the embryo.

The adult body is built up of different kinds of tissues, which fall naturally, from the standpoint of physiology, into groups. Such groups are, in the first place--

1. All those tissues which are connected with the central nervous system, including in that group the nervous system itself.

2. All those tissues which have no connection with the nervous system.

In the second group the physiologist places all germinal cells, all blood- and lymph-corpuscles, all plasma-cells and connective tissue and its derivatives--in fact, all free-living cells, whether in a free state or in a quiescent, so to speak encysted, condition, such as is {464}found in connective tissue. In the first group the physiologist recognizes that the central nervous system is connected with all muscular tissues, whether striped or unstriped, somatic or splanchnic, and that such connection is of an intimate character. Further, all epithelial cells, either of the outer or inner surfaces, whether forming special sense-organs and glands, such as the digestive and sweat-glands, or not, are connected with the nervous system. Besides these structures, there is another set of organs as to which we cannot speak definitely at present, which must be considered separately, viz. all the cells, together with their derived organs, which line the body-spaces. Whatever may be the ultimate decision as to this group of cells, it must fall into one or other of the two main groups.

The members of these two groups are so interwoven with one another that either, if taken alone, would still give the form of the body, so that, in a certain sense, we can speak of the body as formed of two syncytia, separate from each other, but interlaced, of which the one forms a continuous whole by means of cells connected together by a fluid medium or by solid threads formed in such fluid medium, while the other does not form a syncytium in the sense that any cell of one kind may be connected with any cell of another kind, but a syncytium of which all the different elements are connected together only through the medium of the nervous system.

If we choose to speak of the body as made up of two syncytia in this way, we must at the same time recognize the fundamental difference in character between them. In the one case the elements are connected together only by what may be called non-living material; there is no direct metabolic activity caused by the action of one cell over a more distant cell in consequence of such connection, it is not a true syncytium; in the second case there is a living connection, the metabolism of one part is directly influenced by the activity of another, and the whole utility of the system depends upon such functional connection.

The tissues composing this second syncytium may be spoken of as the master-tissues of the body, and we may express this conception of the building up of the body of the higher Metazoa by saying that it is composed of a syncytial host formed of the master-tissues, which contains within its meshes a system of free-living cells, none of which have any connection with the nervous system. This syncytial {465}host is in the adult composed of a number of double elements, a nerve-cell element, and an epithelial element, such as muscle-cell, gland-cell, etc., connected together by nerves; and if such connection is always present as we pass from the adult to the embryo, if there is no period when, for example, the neural element exists alone free from the muscle-cell, no period when the two can be seen to come together and join, then it follows that when the single-layered blastula stage is reached, muscle-cell and nerve-cell must have fused together to form a neuro-muscular cell. Similarly with all the other neuro-epithelial organs; however far apart their two components may be in the adult, they must come together and fuse in the embryo to form a neuro-epithelial element.

The close connection between muscle and nerve which has always been recognized by physiologists, together with the origin of muscle from a myo-epithelial cell in Hydra and other Coelenterata, led the older physiologists to accept thoroughly Hensen's views of the neuro-epithelial origin of all tissues connected with the central nervous system. Of late years this conception has been largely given up owing to the statement of His that the nervous system arises from a number of neuroblasts, which are entirely separate cells, and have at first no connection with muscle-cells or any peripheral epithelial cells, but subsequently, by the outgrowing of an axial fibre, find their way to the muscle, etc., and connect with it. I do not think that His' statement by itself would have induced any physiologist to give up the conception of the intimate connection of muscle and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought into prominence the neurone theory, i.e. that each element of the central nervous system is an independent element, without real connection with any other element and capable of influencing other cells by contact only. These two statements, emanating as they did from embryological and anatomical studies respectively, have done much to put into the background Hensen's conceptions of the syncytial nature of the motor, neural, and sensory elements, which make up the master-tissues of the body, and have led to the view that all the elements of the body are alike, in so far as they are formed of separate cells each leading an independent existence, without any real intimate connection with each other.

The further progress of investigation is, it seems to me, bringing us back to the older conception, for not only has the neuroblast theory {466}proved very difficult for physiologists to accept, but also Graham Kerr, in his latest papers on the development of Lepidosiren, has shown that there is continuity between the nerve-cell and the muscle-cell from the very first separation of the two sets of elements; in fact, Hensen is right and His wrong in their respective interpretation of the earliest stages of the connection between muscle and nerve. So also, it seems to me, the intimate connection between the metabolism of the gland-cell, as seen in the submaxillary gland, and the integrity of its nervous connection implies that the connection between nerve-cell and gland-cell is of the same order as that between nerve-cell and muscle-cell. Graham Kerr also states in his paper that from the very commencement there is, he believes, continuity between nerve-cell and epithelial cell, but so far he has not obtained sufficiently clear evidence to enable him to speak positively on this point.

Further, according to the researches of Anderson, the cells of the superior cervical ganglion in a new-born animal will continue to grow healthily as long as they remain connected with the periphery, even though entirely separated from the central nervous system by section of the cervical sympathetic nerve, and conversely, when separated from the periphery, will atrophy, even though still connected with the central nervous system. So, also, on the sensory side, Anderson has shown that the ganglion-cells of the posterior root-ganglion will grow and remain healthy after separation of the posterior roots in a new-born animal, but will atrophy if the peripheral nerve is cut, even though they are still in connection with the central nervous system. Further, although section of a posterior root in the new-born animal does not affect the development of the nerve-cells in the spinal ganglion, and of the nerve-fibres connecting the posterior root-ganglion with the periphery, it does hinder the development of that part of the posterior root connected with the spinal ganglion.

These experiments of Anderson are of enormous importance, and force us, it seems to me, to the same conclusion as that to which he has already arrived. His words are (p. 511): "I suggest, therefore, that the section of peripheral nerves checked the development of motor and sensory neurones, not because it blocked the passage of efferent impulses in the first case and the reception of stimuli from the periphery in the second, but for the same reason in both cases, {467}viz. that the lesion disturbed the chemico-physical equilibrium of an anatomically continuous (neuro-muscular or neuro-epithelial) chain of cells, by separating the non-nervous from the nervous, and that the changes occurring in denervated muscle, which I shall describe later (and possibly those in denervated skin), are in part due to the reciprocal chemico-physical disturbance effected in these tissues by their separation from the nervous tissues; also that the section of the posterior roots checked the development of those portions of them still attached to the spinal ganglia, because the chemico-physical equilibrium in those processes is maintained not only by the spinal ganglion-cells, but also by the intra-spinal cells with which these processes are anatomically continuous."

What is seen so strikingly in the new-born animal can be seen also in the adult, and in Anderson's paper references are given to the papers of Lugaro and others which lead to the same conclusion.

These experiments seem to me distinctly to prove that the connection between the elements of the peripheral organ and the proximate neurone is more than one of contact.

We can, however, go further than this, for, apart from the observations of Apathy, there is direct physiological evidence that the vitality of other neurones besides the terminal neurones is dependent upon their connection with the peripheral organ, even though their only connection with the periphery is by way of the terminal neurone. Thus, as is seen from Anderson's experiments, section of the cervical sympathetic nerve in a very young animal causes atrophy of many of the cells in the corresponding intermedio-lateral tract, cells which I supposed gave origin to all the vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking experiment given by Anderson is the effect of the removal of the periphery upon the medullation of those efferent fibres which arise from these same spinal cells, for, as he has shown, section of the nerves from the superior cervical ganglion to the periphery in a very young animal delays the medullation in the fibres of the cervical sympathetic--that is, in preganglionic fibres which are not directly connected with the periphery but with the terminal neurones in the superior cervical ganglion. So also on the afferent side a sufficiently extensive removal of sensory field will cause atrophy of the cells of Clarke's column, so that, just as in the case of the primary neurones, {468}the secondary neurones show by their degenerative changes the importance of their connection with the peripheral organs.

In this way I can conceive the formation of a series of both efferent and afferent relays in the nervous system by proliferation of the original neural moiety of the neuro-epithelial elements, every one of which is dependent upon its connection with the peripheral epithelial elements for its due vitality, the whole system being a scheme for co-ordination of a larger and larger number of peripheral elements. Thus the cells of the vasomotor centre are in connection with the whole system of segmental vaso-constrictor centres in the lateral horns of the thoracic region of the cord, so that to cause atrophy of these cells a very extensive removal of the vascular system would be required. Each of the segmental centres in the cord supplies a number of sympathetic segments, the connection with all of which would have to be cut in order to ensure complete removal of the connection of each of its cells with the periphery, and finally each of the cells in the sympathetic ganglia supplies a number of peripheral elements, all of which must be removed to ensure complete severance.

Thus, if we take any arbitrary number, such as 4, to represent the number of peripheral organ-elements with which each terminal neurone is connected, and suppose that each neurone has proliferated into sets of 4, then a cell of the third order, such as a cell of the vasomotor centre, would require the removal of 64 peripheral elements to cause its complete separation from the periphery, one of the second order (a cell of the thoracic lateral horn) 16 elements, one of the first order (a cell of a sympathetic ganglion) 4 elements.

Such intimate inter-relationship between the neurones, both afferent and efferent, and their corresponding peripheral organs does not imply that all nerve-cells are necessarily as closely dependent upon some connection with the periphery, for just as the proliferation of epithelial or muscle-cells forms an epithelial or muscular sheet, the elements of which are so loosely, if at all, connected together that their metabolism is in no way dependent upon such connection, so also a similar proliferation of the neural elements may form connections between nerve-cell and nerve-cell of a similarly loose nature.

It is this kind of proliferation which, in my opinion, would bind together the separate relays of efferent and afferent neurones, and {469}so give origin to reflex actions at different levels. Such neurones would not be in the direct chain of either the afferent or efferent neurones, and so not directly connected with the periphery, and could therefore be removed without affecting the vitality of either the efferent or afferent chain of neurones. In other words, the vitality of the cells on the efferent side ought not to be dependent on the integrity of the reflex arc. With regard to the development of the anterior roots, Anderson has shown that this is the case, for section of all the posterior roots conveying afferent impulses from the lower limb in a new-born animal does not hinder the normal development of the anterior roots supplying that limb. Also Mott, who originally thought that section of all the posterior roots to a limb caused atrophy of the corresponding anterior roots, has now come to the same conclusion as other observers, and can find no degeneration on the efferent side due to removal of afferent impulses.

Again, the process of regeneration after section of a nerve is not in favour of the neuroblast theory. There is no evidence that the cut end of a nerve can grow down and attach itself to a muscular or epithelial element without the assistance of a nerve tube down which to grow. When the cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the medullary sheath, not to the neurilemma; the connective tissue elements remain alive and form a tube into which the growing axon finds its way, and so is conducted to the end-plate or end-organ of the peripheral structure.

Possibly, as suggested by Mott and Halliburton, the products of degeneration of the axis-cylinder and medullary sheath stimulate these connective tissue sheath-cells into active proliferation, and so bring about the great multiplication of cells arranged as cell-chains, which are so often erroneously spoken of as forming the young nerves. These sheath-cells are then supposed to re-form and secrete a pabulum which is important for the process of regeneration of the down-growing axis-cylinder and medullary sheath. Without such pabulum regeneration does not take place, as is seen in the central nervous system, where the sheath of Schwann is absent.

Again, it is becoming more and more doubtful whether the peripheral terminations of nerves are ever really free. As far as efferent nerves are concerned the nervous element may entirely {470}predominate over the muscular or glandular, as in the formation of the electric organs of the Torpedo and Malapterurus, but still the final effect is produced by the alteration of the muscle or gland-cell. On the afferent side especially free nerve-terminations are largely recognized, or, as in Barker's book, nerves are spoken of as arising in connective tissue. Thus the numerous kinds of special sense-organs, such as Pacinian bodies, tendon-organs, genital corpuscles, etc., are all referred to by Barker under the heading of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these organs has been known for a long time in the shape of Grandry's corpuscles or the tactile corpuscles in the duck's bill, where it has been proved that the nerve terminates in special large tactile cells derived from the surface-epithelium.

So also with all the others, further investigation tends to put them all in the same category, all special sensory organs originating from a localized patch of surface-epithelium. Thus Anderson has shown me in his specimens how the young Pacinian body is composed of rows of epithelial cells, into each of which a twig from the nerve passes. He has also shown me how, in the case of the tendon-organ, each nerve-fibre passes towards the attachment of the tendon and then bends back to supply the tendon-organ, thus indicating, as he suggests, how the nest of epithelial cells has wandered inwards from the surface to form the tendon-organ. Again, Meissner's corpuscles and Herbst's corpuscles are evidently referable to the same class as those of Grandry and Pacini.

Yet another instance of the same kind is to be found in the chromatophores of the frog and other animals which are under the influence of the central nervous system and yet have been supposed by various observers to be pigmented connective tissue cells. The most recent work of Leo Loeb and others has conclusively shown that such cells are invariably derived from the surface-epithelium.

Finally, in fishes we find the special sense-organs of the lateral line and other accessory sensory organs, all of which are indisputably formed from modified surface epithelial cells.

The whole of this evidence seems to me directly against Barker's classification of sensory nerve-beginnings in mesoblastic tissues; in none of these cases are we really dealing with free nervous tissue alone, the starting point is always a neuro-epithelial couple.

We may then, I would suggest, look upon the adult as formed of {471}a neural syncytium, which we may call the host, which carries with it in its meshes a number of free cells not connected with the nervous system. If, then, we confine our attention to the host and trace back this neural syncytium to its beginnings in the embryo, we see that, from the very nature of the neuro-epithelial couple, each epithelial moiety must approach nearer and nearer to its neural moiety, until at last it merges with it; the original neuro-epithelial cell results, and we must obtain, as far as the host is concerned, a single-layered blastula as the origin of all Metazoa. It follows, further, that there must always be continuity of growth in the formation of the host, i.e. in the formation of the neuro-epithelial syncytium; that therefore cells which have been previously free cannot settle down and take part in its formation, as, for instance, in the case of the formation of any part of the gut-epithelium or of muscle-cells from free-living cells.

Further, since the neural moiety is the one element common to all the different factors which constitute the host, it follows that the convergence of each epithelial moiety to the neural moiety, as we pass from the adult to the embryo, is a convergence of all outlying parts to the neural moiety, i.e. to the central nervous system, if there is a concentrated nervous system. Conversely, in the commencing embryo the place from which the spreading out of cells takes place, i.e. from which growth proceeds, must be the position of the central nervous system, if the nervous system is concentrated. If the nervous system is diffuse, and forms a general sub-epithelial layer, then the growth of the embryo would take place over the whole surface of the blastula.

Turning now to the consideration of the second group of tissues, those that are not connected with the central nervous system, we find that they include among them such special cells as the germinal cells, free cells of markedly phagocytic nature, and cells which were originally free and phagocytic, but have settled down to form a supporting framework of connective tissue, and are known as plasma-cells. In the embryo we find also in many cases free cells in the yolk, forming more or less of a layer, which function as phagocytes and prepare the pabulum for the fixed cells of the growing embryo; these cells are known by the name of vitellophags, and in meroblastic vertebrate eggs form somewhat of a layer known by the name of periblast. Such cells must be included in the second group, and, {472}indeed, have been said again and again to give origin to the free-living blood-corpuscles of the adult. In other cases they are said to disintegrate after their work is done.

In the adult the free-living lymphocytes and hæmocytes reproduce themselves from already existing free-living cells, but as we pass back to the embryo there comes a time, comparatively late in the history of the embryo, when such free-living cells are not found in the fluids of the body, and they are said to arise from the proliferation and setting free of cells which form a lining epithelium. Such formation of leucocytes has been especially described in connection with the lining epithelium of the coelomic cavities, as stated in Chapter XII., so that anatomists look upon the origin of these free cells as being largely from the coelomic epithelium, or mesothelium, as Minot calls it.

Then, again, the free cells which form the germinal cells can be traced back to a germinal epithelium, which also is part of the coelom. Thus the suggestion arises that in the embryo a cellular lining is formed to a coelomic cavity (mesothelium) composed of cells which have no communication with the nervous system, and are capable of a separate existence as free individuals, either in the form of germinal cells or of lymphocytes, hæmocytes, and plasma-cells, so that these latter free cells may be considered as living an independent existence in the body, and ministering to it in the same sense as the germ-cells live an independent existence in the body. Again, the function of this mesothelium apart from the germ-cell is essentially excretory and phagocytic. It is the cells of the excretory organs as well as the lymphocytes which pick up carmine-grains when injected. It is the cells of the modified excretory organs, as mentioned in