Part C Is Modified to Form an Optic Sense-Plate, Then an Invagination

occurring between A and B will throw the retinal sense-cells with the optic nerve further from the surface, and the layers B and A between the retina and the source of light will be available for the formation of the dioptric apparatus. The lens is now formed from the cuticular surface of A, and the {69}hypodermal cells of A elongate to form the layer known by the name of corneagen, or vitreogen, the cells of B remaining small and forming the pre-retinal layer of cells. The large optic nerve end-cells of the retinal layer, C, take up the position shown in the figure, and their cuticular surface becomes modified to form rods of varying shape called rhabdites, which are attached to the retinal cells. Frequently the rhabdites of neighbouring cells form definite groups, each group being called a rhabdome. Whatever shape they take it is invariably found that these little rods (bacilli), or rhabdites, are modifications of the cuticular surface of the cells which form the retinal layer. Also, as must necessarily be the case from the method of formation, the optic nerve arises from the nuclear end of the retinal cells, never from the bacillary end. As in the case first mentioned, so in this case, the light strikes direct upon the bacillary end of the retinal cells; such eyes, therefore, are eyes with an upright retina.

It may happen that the part invaginated is the optic sense-plate itself, as would be the case if in the former figure, instead of C, the part B was modified to form a sense-plate. This will give rise to an eye of a character different from the former (Fig. 29). The optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as is seen, the light passes through the retinal layer to the {70}tapetum. The cuticular surface of the retinal cells forming the rods or bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal layer.

Such an eye, in contradistinction to the former type, is an eye with an inverted retina; but still the same law holds as in the former case--the optic nerve-fibres enter at the nuclear ends of the cells, and the rods are formed from the cuticular surface.

In these eyes the pigmented tapetal layer is believed to act as a looking-glass; the dioptric apparatus throws the image on to its shiny surface, from whence it is reflected directly on to the rods, which are in close contact with the tapetum. A similar process has been suggested in the case of the mammalian lateral eye, with its inverted retina. Johnson describes the post-retinal pigmented layer as being frequently coloured and shiny, and imagines that it reflects the image directly back on to the rods.

The arrow shows the direction of the source of light in this as in the preceding figure. In both figures the cuticular rhabdites are represented by thick black lines.]

Thus we see that eyes can be placed in different categories, e.g. those with an upright retina and those with an inverted retina; also, according to the presence or absence of a tapetum, eyes have been grouped as tapetal or non-tapetal. All the eyes considered so far are called simple eyes, or ocelli; and a number of ocelli may be {71}contiguous though separate, as in the lateral eyes of the scorpion. They may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a very large number of cases, retain each its own dioptric apparatus, and therefore the external appearance of the compound eye represents not a single lens, but a large number of facets, as is seen in the eyes of insects. Owing to these differences, eyes have been divided into simple and compound, and into facetted and non-facetted.

Yet another complication occurs in the formation of eyes, which is, perhaps, the most important of all: the retinal portion of the eye, instead of consisting of simple retinal cells, with their accompanying rhabdites, may include within itself a portion of the central nervous system.

The rationale of such a formation is as follows: The external covering of the body is formed by a layer of external epithelial cells--the ectodermal cell-layer--and an underlying neural layer, of which the latter gives origin to the central nervous system. As development proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm the sensory nerves of the skin. That part of the neural layer which underlies the optic plate forms the optic ganglion, and when the central nervous system leaves the surface to take up its deeper position, the strand of nerve-fibres known as the optic nerve, is left connecting it with the retinal cells as seen in Figs. 28, 29. It may, however, happen that part of the optic ganglion remains at the surface, in close connection with the retinal end-cells, when the rest of the central nervous system sinks inwards. The retina of such an eye is composed of the combined optic ganglion and retinal end-cells; the strand of nerve-fibres which is left as the connection between it and the rest of the brain, which is also called the optic nerve, is not a true peripheral nerve, as in the first case, but rather a tract of fibres connecting two parts of the brain, of which one has remained at the periphery. Such a retina, in contradistinction to the first kind, may be called a compound retina.

The optic ganglion, as seen in eyes with a simple retina, consists of a cortical layer of small, round nerve-cells, and an internal medulla of fine nerve-fibres, which form a thick network known as 'Punctsubstanz,' or in modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from other parts of the brain connect them with the optic ganglion.

{72}At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous system is considered to be composed of a number of separate nerve-units, called neurones, each neurone consisting of a nerve-cell with its various processes; one of these--the neuraxon--constitutes the nerve-fibre belonging to that nerve-cell, the other processes--the dendrites--establish communication with other neurones. The place where these processes come together is called a synapse, and the tangle of fine fibres formed at a number of synapses forms the 'neuropil.'

ABCD, as in Fig. 28. Op. g. I. and Op. g. II., two optic ganglia which combine to form the retinal ganglion, Rt. g.]

When, therefore, a compound retina is formed by the amalgamation of the ectodermal part--the retinal cells proper--with the neurodermic part--to which the name 'retinal ganglion' may be given,--such a retina consists of neuropil substance and nerve-cells, as well as the retinal end-cells. In all such compound retinas, the retinal ganglion is not single, but two optic ganglia at least are included in it, so that there are two sets of nerve-cells and two synapses are always formed; one between the retinal end-cells and the neurones of the first optic ganglion, which may be called the ganglion of the retina, the other between the first and second ganglia, which, seeing that the neuraxons of its cells form the optic nerve, may be called the ganglion of the optic nerve. The 'neuropil' formed by these synapses forms the molecular layers of the compound retina, and the cells themselves form the nuclear layers. Thus an upright compound retina, formed in the same way as the upright simple retina, would be illustrated by Fig. 30.

{73}Further, in precisely the same way as in the case of the simple retina, such a compound retina may be upright or inverted. Thus, in the lateral eyes of crustaceans and insects, a compound retina of this kind is formed, which is upright; while in the vertebrates the compound retina of the lateral eyes is inverted.

The compound retina of vertebrates is usually described as composed of a series of layers, which may be analyzed into their several components as follows:--

Layer of rods and cones } External nuclear layer } retina proper } Ectodermic part External molecular layer }} Internal nuclear layer } ganglion of retina } Internal molecular layer }} } retinal } neurodermic Optic nerve-cell layer } ganglion of optic nerve} ganglion} part Layer of optic nerve } fibres

The difference between the development of these two types of eye--those with a simple retina and those with a compound retina--has led, in the most natural manner, to the conception that the retina is developed, in the higher animals, sometimes from the cells of the peripheral epidermis, sometimes from the tissue of the brain--two modes of development termed by Balfour 'peripheral' and 'cerebral.' An historical survey of the question shows most conclusively that all investigators are agreed in ascribing the origin of the simple retina to the peripheral method of development, the retina being formed from the hypodermal cells by a process of invagination, while the cerebral type of development has been described only in the development of the compound retina. The natural conclusion from this fact is that, in watching the development of the compound retina, it is more difficult to differentiate the layers formed from the epidermal retinal cells and those formed from the epidermal optic ganglion-cells, than in the case of the simple retina, where the latter cells withdraw entirely from the surface. This is the conclusion to which Patten has come, and, indeed, judging from the text-book of Korschelt and Heider, it is the generally received opinion of the day that, as far as the Appendiculata are concerned, the retina, in the true sense--the retinal end-cells, with their cuticular rods,--is formed, in all cases, from the peripheral cells of the hypodermal layer, the cuticular rods being modifications of the general cuticular surface of the body. The apparent cerebral development of the crustacean {74}retina, as quoted from Bobretsky by Balfour, is therefore in reality the development of the retinal ganglion, and not of the retina proper.

There is, I imagine, a universal belief that the natural mode of origin of a sense-organ, such as the eye, must always have been from the cells forming the external surface of the animal, and that direct origin from the central nervous system is a priori most improbable. It is, therefore, a matter of satisfaction to find that the evidence for the latter origin has universally broken down, with the single exception of the eyes of vertebrates and their degenerated allies; a fact which points strongly to the probability that a reconsideration of the evidence upon which the present teaching of the origin of the vertebrate eye is based will show that here, too, a confusion has arisen between that part formed from the epidermal surface and that from the optic ganglion.

THE MEDIAN OR PINEAL EYES.

Undoubtedly, in recent times, the most important clue to the ancestry of vertebrates has been given by the discovery that the so-called pineal gland in the vertebrate brain is all that remains of a pair of median or pineal eyes, the existence of which is manifest in the earliest vertebrates; so that the vertebrate, when it first arose, possessed a pair of median eyes as well as a pair of lateral eyes. The ancestor of the vertebrate, therefore, must also have possessed a pair of median eyes as well as a pair of lateral eyes.

Very instructive, indeed, is the evidence with regard to these median eyes, for one of the great characteristics of the ancient palæostracan forms is the invariable presence of a pair of median eyes as well as a pair of lateral eyes. In the living representative of such forms--Limulus--the pair of median eyes (Fig. 5) is well shown, and it is significant that here, according to Lankester and Bourne, these eyes are already in a condition of degeneration; so also in many of the Palæostraca (Fig. 7) the lateral eyes are the large, well-developed eyes, while the median eyes resemble those of Limulus in their insignificance.

We see, then, that in the dominant arthropod race at the time when the fishes first appeared, the type of eyes consisted of a pair of well-developed lateral eyes and a pair of insignificant, partially degenerated, median eyes. Further, according to all palæontologists, {75}in the best-preserved head-shields of the most ancient fishes, especially well seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis, Keraspis, a pair of large, prominent lateral eyes existed, between which, in the mid-line, are seen a pair of small, insignificant median eyes.

The evidence of the rocks, therefore, proves that the pair of median eyes which were originally the principal eyes (Hauptaugen), had already, in the dominant arthropod group been supplanted by a pair of lateral eyes, and had, in consequence, become small and insignificant, at the time when vertebrates first appeared. This dwindling process thus initiated in the arthropod itself has steadily continued ever since through the whole development of the vertebrates, with the result that, in the highest vertebrates, these median or pineal eyes have become converted into the pineal gland with its 'brain-sand.'

In the earliest vertebrate these median eyes may have been functional; they certainly were more conspicuous than in later forms. Alone among living vertebrates the right median eye of Ammocoetes is so perfect and the skin covering it so transparent that I have always felt doubtful whether it may not be of use to the animal, especially when one takes into consideration the undeveloped state of the lateral eyes in this animal, hidden as they are under the skin. Thus the one living vertebrate which is comparable with these extinct fishes is the one in which one of the pineal eyes is most well defined, most nearly functional.

Before passing to the consideration of the structure of the median eyes of Ammocoetes, it is advisable to see whether these median eyes in other animals, such as arachnids and crustaceans, belong to any particular type of eyes, for then assuredly the median eyes of Ammocoetes ought to belong to the same type if they are derived from them.

In the specialized crustacean, as in the specialized vertebrate, the median eyes have disappeared, at all events in the adult, but still exist in the primitive forms, such as Branchipus, which resemble the trilobites in some respects. On the other hand, the median eyes have persisted, and are well developed in the arachnids, both scorpions and spiders possessing a well-developed pair. The characteristics of the median eyes must then be especially sought for in the arachnid group.

Both scorpions and spiders possess many eyes, of which two are {76}always separate and median in position, while the others form lateral groups; all these eyes possess a simple retina and a simple corneal lens. Grenacher was the first to point out that in the spiders two very distinct types of eye are found. In the one the retina is upright; in the other the retina is inverted, and the eye possesses a tapetal layer. The distribution of these two types is most suggestive, for the inverted retina is always found in the lateral eyes, never in the two median eyes; these always possess a simple upright retina.

In the crustaceans, the lateral eyes differ also from the median eyes, but not in the same way as in the arachnids; for here both types of eye possess an upright retina, but the retina of the lateral eyes is compound, while that of the median eyes is simple. In other words, the median eyes are in all cases eyes with a simple upright retina and a simple cuticular lens, while the retina of the lateral eyes is compound or may be inverted, according as the animal in question possesses crustacean or arachnid affinities. The lateral eye of the vertebrate, possessing, as it does, an inverted compound retina, indicates that the vertebrate arose from a stock which was neither arachnid nor crustacean, but gave rise to both groups--in fact, was a member of the great palæostracan group. What, then, is the nature of the median eyes in the vertebrate?

THE MEDIAN EYES OF AMMOCOETES.

The evidence of Ammocoetes is so conclusive that I, for one, cannot conceive how it is possible for any zoologist to doubt whether the parietal organ, as they insist on calling it, had ever been an eye, or rather a pair of eyes.

Anyone who examines the head of the larval lamprey will see on the dorsal side, in the median line, first, a somewhat circular orifice--the unpaired nasal opening; and then, tailwards to this, a well-marked circular spot, where the skin is distinctly more transparent than elsewhere. This spot coincides in position with the underlying dorsal pineal eye, which shines out conspicuously owing to the glistening whiteness of its pigment. Upon opening the brain-case the appearance as in Fig. 20 is seen, and the mass of the right ganglion habenulæ (G.H.R.), as it has been called, stands out conspicuously as well as the right or dorsal pineal eye (Pn.). Both eye and ganglion appear at first sight to be one-sided, but further examination shows that a left ganglion habenulæ is present, though much smaller than on {77}the right side. In connection with this is another eye-like organ--the left or ventral pineal eye,--much more aborted, much less like an eye than the dorsal one; so also there are two bundles of peculiar fibres called Meynert's bundles, which connect this region with the infra-infundibular region of the brain; of these, the right Meynert's bundle is much larger than the left.

l.m., upper lip muscles; m.c., muco-cartilage; n., nose; na.c., nasal cartilage; pn., right pineal eye and nerve; g.h.r., right ganglion habenulæ; s.m., somatic muscles; cr., membranous wall of cranium; ch., choroid plexus; gl., glandular substance and pigment filling up brain-case.]

{78}[Illustration: FIG. 32.--EYE OF ACILIUS LARVA, WITH ITS OPTIC GANGLION.

On the right side the nerve end-cells have been drawn free from pigment.]

On the left side the eye is drawn as it appeared in the section. On the right side I have removed the pigment from the nerve end-cells, and drawn the eye as, in my opinion, it would appear if it were functional.]

This difference between right and left indicates a greater degeneration on the left side, and points distinctly to a close relationship between the nerve-masses known as ganglia habenulæ and the median eyes. In my opinion this ganglion is, in part, at all events, the optic ganglion of the median eye on each side. It is built up on the same type as the optic ganglia of invertebrate simple eyes, with a cortex of small round cells and a medulla of fine nerve-fibres. Into this ganglion, on the right side, there passes a very well-defined nerve--the nerve of the dorsal eye. The eye itself with its nerve, pn., and its optic ganglion, g.h.r., is beautifully shown by means of a horizontal section through the head of Ammocoetes (Fig. 31). Originally, as described by Scott, the eye stood vertically {79}above its optic ganglion, and presented an appearance remarkably like Fig. 32, which represents one of the simple eyes and optic ganglia of a larva of Acilius as described by Patten; then, with the forward growth of the upper lip, the right pineal eye was dragged forward and its nerve pulled horizontally over the ganglion habenulæ. For this reason the eye, nerve, and ganglion are better shown in a nearly horizontal than in a transverse section.

The optic nerve belonging to this eye is most evident and clearly shown in Fig. 31, and in the series of consecutive sections which follow upon this section; no doubt can arise as to the structure in question having been the nerve of the eye, even though, as is possible, it does not contain any functional nerve-fibres.

pn.2, left or ventral pineal eye; pn.1, last remnant of right, or dorsal pineal eye; g.h.r., right ganglion habenulæ; g.h.l.1, g.h.l.3, parts of left ganglion habenulæ; pi., fold of pia mater which separates the left ganglion habenulæ from the left pineal eye; f., strands of nerve-fibres connecting the left eye with its ganglion, g.h.l.3; V3, third ventricle; V.aq., ventricle of aquæduct.]

The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in appearance, being much less evidently an eye. Fig. 34 is one of the same series of horizontal sections as Fig. 31, pn.1 being the last remnant of the right, or dorsal, eye, while pn.2 shows the left, or ventral, eye with its connection with the left ganglion habenulæ.

{80}In a series of sections I have followed the nerve of the right pineal eye to its destination, as described in my paper in the Quarterly Journal of Microscopical Science, and have found that it enters into the ganglion habenulæ just as the nerve to any simple eye enters into its optic ganglion. This nerve, as I have shown, is a very distinct, well-defined nerve, with no admixture of ganglion-cells or of connective tissue, very different indeed to the connection between the left pineal eye and its optic ganglion. Here there is no defined nerve at all; but the cells of the ganglion habenulæ stretch right up to the remains of the eye itself. Seeing, then, that both the eye and ganglion on this side have reached a much further grade of degeneration than on the right side, it may be fairly concluded that the original condition of these two median eyes is more nearly represented by the right eye, with its well-defined nerve and optic ganglion, than by the left eye, or by the eyes in lizards and other animals which do not show so well-defined a nerve as is possessed by Ammocoetes. Quite recently Dendy has examined the two median eyes in the New Zealand lamprey Geotria australis. In this species the second eye is much better defined than in the European lamprey, and its connection with the ganglion habenulæ is more nerve-like. In neither eye, however, is the nerve so clean cut and isolated as is the nerve of the dorsal, or right, eye in the Ammocoetes stage of Petromyzon Planeri; in both, cells resembling those of the cortex of the ganglion habenulæ and connective tissues are mixed up with the nerve-fibres which pass from each eye to its respective optic ganglion.

THE RIGHT PINEAL EYE OF AMMOCOETES.

The optic fibres of the right median eye of Ammocoetes are connected with a well-defined retina, the limits of which are defined by the white pigment so characteristic of this eye. This pigment is apparently calcium phosphate, which still remains as the 'brain-sand' of the human pineal gland. The cells, which are hidden by this pigment, were described by me in 1890 as the retinal end-cells with large nuclei. In 1893, Studniçka examined them more closely, and concluded that the retinal cells are of two kinds: the one, nerve end-cells, the sensory cells proper; the other, pigmented epithelial cells, which surround the sense-cells. The sense-cells may contain some of the white pigment, but not so much as the other cells. Similarly, in the {81}median eyes of Limulus, Lankester and Bourne find it difficult to determine how far the retinal end-cells contain pigment and how far that pigment really is in the cells surrounding these nerve end-cells.

The interior of the eye presents the appearance of a cavity in shape like a cornucopia, the stalk of which terminates at the place where the nerve enters. This cavity is not empty, but the posterior part of it is filled with the termination of the nerve end-cells of the retina, as pointed out by me and confirmed by Studniçka. These terminations are free from pigment, and contain strikingly translucent bodies, which I have described in my paper in the Quarterly Journal, and called rhabdites, for they present the same appearance and are situated in the same position as are many of the rhabdites on the terminations of the retinal end-cells of arthropod eyes. Studniçka has also seen these appearances, and figures them in his second paper on the nerve end-cells of the pineal eye of Ammocoetes.

Up to this point the following conclusions may be drawn:--

1. Ammocoetes possesses a pair of median eyes, just as was the case with the most ancient fishes, and with the members of the contemporary palæostracan group.

2. The retina of one of these eyes is well-defined and upright, not inverted, and therefore in this respect agrees with that of all median eyes.

3. The presence of nerve end-cells, with pigment either in them or in cells around them, to the unpigmented ends of which translucent bodies resembling rhabdites are attached, is another proof that this retina agrees with that of the median eyes of arthropods.

4. The simple nature of the nerve with its termination in an optic ganglion closely resembling in structure an arthropod optic ganglion, together with Studniçka's statement that the nerve end-cells pass directly into the nerve, points directly to the conclusion that this retina is a simple, not a compound, retina, and that it therefore in this respect also agrees with the retina of all median eyes.

With respect to this last conclusion, neither I myself nor Studniçka have been able to see any definite groups of cells between the nerve end-cells and the optic nerve such as a compound retina necessitates.

{82}On the other hand, Dendy describes in the New Zealand lamprey, Geotria australis, a cavity where the nerve enters into the eye, which he calls the atrium. This cavity is distinct from the main cavity of the eye, and is separated from it by a mass of cells similar in appearance to those of the cortex of the ganglion habenulæ. In these two eyes then, groups of cells, resembling in appearance those belonging to an optic ganglion, exist in the eyes themselves. This atrium is evidently that part of the central cavity which I have called the handle of the cornucopia in the European lamprey, and the very fact that it is separated from the rest of the central cavity is evidence that we are dealing here with a later stage in the history of the pineal eyes than in the case of the Ammocoetes of Petromyzon Planeri. Taking also into consideration the continuity of the mass of small ganglion-cells which surround this atrium with the cells of the ganglion habenulæ by means of the similar cells scattered along the course of the nerve, and also bearing in mind the fact already stated that in the more degenerate left eye of Ammocoetes the cells of the ganglion habenulæ extend right up to the eye itself, it seems more likely than not that these cells do not represent the original optic ganglion of a compound retina, but rather the subsequent invasion, by way of the pineal nerve, of ganglion-cells belonging to a portion of the brain. In the last chapter it has been suggested that the presence of the trochlear or fourth cranial nerve has given rise to the formation of the cerebellum by a similar spreading.

There is certainly no appearance in the least resembling a compound retina such as is seen in the vertebrate or crustacean lateral eye. In the median eyes of scorpions and of Limulus, cells are found within the capsule of the eye among the nerve-fibres and the nerve end-cells. These are especially numerous in the median eyes of Limulus, as described by Lankester and Bourne, and are called by them intrusive connective tissue cells. The meaning of these cells is not, to my mind, yet settled. It is sufficient for my purpose to point out that the presence of cells interneural in position among the nerve end-cells of the retina of the median eyes of Ammocoetes is more probable than not, on the assumption that the retina of these eyes is built up on the same plan as that of the median eyes in Limulus and the scorpions.

It is further to be borne in mind that these specimens of Geotria worked at by Dendy were in the 'Velasia' stage of the New Zealand {83}lamprey, and correspond, therefore, more nearly to the Petromyzon than to the Ammocoetes stage of the European lamprey.

THE DIOPTRIC APPARATUS.

Besides the retina, all eyes possess a dioptric apparatus. What is the evidence as to its nature in these vertebrate median eyes? Lankester and Bourne have divided the eyes of scorpions and Limulus into two kinds, monostichous and diplostichous. In the first the retinal cells are supposed to give rise to not only rhabdites but also the cuticular chitinous lens, so that the eye is one-layered; in the second the lens is formed by a well-marked hypodermal layer, in front of the retina, composed of elongated cells, so that these eyes are two-layered or diplostichous. The lateral eyes, according to them, are all monostichous, but the median eyes are diplostichous.

l., chitinous lens; c., corneagen; pr., pre-retinal layer; rh., rhabdites; ret., retinal end-cells.]

This distinction is not considered valid by other observers. Thus, {84}as already indicated, Patten looks on all these eyes as three-layered, and states that in all cases a corneagen or vitreogen layer exists, which gives origin to the lens. For my own part I agree with Patten, but we are not concerned here with the lateral eyes. It is sufficient to note that all observers are agreed that the median eyes are characterized by this well-marked cell-layer, the so-called vitreous or corneagen layer of cells.

l., chitinous lens; c., corneagen; pr., pre-retinal layer; rh., rhabdites; ret., retinal end-cells.]

This layer (c., Fig. 35) is composed of much-elongated cells of the hypodermal layer, in each of which the large nucleus is always situated towards the base of the cell. The space between it and the retina contains, according to Patten the cells of the pre-retinal layer (pr.). These may be so few and insignificant as to give the impression that the vitreous layer is immediately adjacent to the retina (ret.).

Let us turn now to the right pineal eye of Ammocoetes (Fig. 37) and see what its further structure is. The anterior part of the eye is free from pigment, and is composed, as is seen in hæmatoxylin or carmine specimens, of an inner layer of nuclei which are frequently arranged in a wavy line. From this nucleated layer, strands of tissue, free from nuclei, pass to the anterior edge of the eye.

In the horizontal longitudinal sections it is seen that these strands are confined to the middle of the eye. On each side of them the nuclear layer reaches the periphery, so that if we consider these strands to represent long cylindrical cells, as described by Beard, then the anterior wall may be described as consisting of long cylindrical cells, which are flanked on either side by shorter cells of a similar kind. The nuclei at the base of these cylindrical cells are not all alike. We see, in the first place, large nuclei resembling the large nuclei belonging to the nerve end-cells; these are the nuclei of {85}the long cylindrical cells. We see also smaller nuclei in among these larger ones, which look like nuclei of intrusive connective tissue, or may perhaps form a distinct layer of cells, situated between the cells of the anterior wall and the terminations of the nerve end-cells already referred to.

These elongated cells are in exactly the same position and present the same appearance as the cells of the corneagen layer of any median eye. Like the latter they are free from pigment and never show with osmic staining any sign of the presence of translucent rhabdite-like bodies, such as are seen in the termination of the retinal cells, and like the latter their nuclei are at the base. The resemblance between this layer and the corneagen cells of any median eye is absolute. Between it and the terminations of the retinal cells there exists some ill-defined material certainly containing cells which may well correspond to Patten's pre-retinal layer of cells.

Retina, corneagen, nerve, optic ganglion, all are there, all in their right position, all of the right structure, what more is needed to complete the picture?

In order to complete the dioptric apparatus a lens is necessary. Where, then, is the lens in these pineal eyes? In all the arachnid eyes, whether median or lateral, the lens is a single corneal lens composed of the external cuticle, which is thickened over the corneagen cells. This thickened cuticle is composed of chitin, and is not cellular, but is dead material formed out of the living underlying corneagen cells. Such a lens is in marked contrast to the lens of the lateral vertebrate eye, which is formed by living cells themselves. This {86}thickening of the cuticular layer to form a lens could only exist as long as that layer is absolutely external, so that the light strikes immediately upon it; for, if from any cause the eye became situated internally, the place of such a lens must be filled by the structures situated between it and the surface, and the thickened cuticle would no longer be formed.

In all vertebrates these pineal eyes are separated from the external surface by a greater or less thickness of tissues; in the case of Ammocoetes, as is seen in Fig. 31, the eye lies within the membranous cranial wall, and is attached closely to it. The position, then, of the cuticular, or corneal lens, as it is often called, on the supposition that this is a median eye of the arachnid type, is taken by the membranous cranium, and, as I have described in my paper in the Quarterly Journal, on carefully lifting the eye in the fresh condition from the cranial wall, it can be seen under a dissecting microscope that the cranial wall often forms at this spot a lens-like bulging, which fits the shallow concavity of the surface of the eye, and requires some little force to separate it from the eye.

As will appear in a subsequent chapter, this cranial wall has been formed by the growth, laterally and dorsally, of a skeletal structure known by the name of the plastron. The last part of it to be completed would be that part in the mid-dorsal line, where apparently, in consequence of the insinking of the degenerating eyes, a dermal and subdermal layer already intervened between the source of light and the eyes themselves.

When the membranous cranium was completed in the mid-dorsal region, it was situated here as elsewhere just internally to the subdermal layer, and therefore enclosed the pineal eyes. This, to my mind, is the reason why the pineal eyes, which, in all other respects, conform to the type of the median eyes of an arachnid-like animal, do not possess a cuticular lens. Other observers have attempted to make a lens out of the elongated cells of the anterior wall of the eye (my corneagen layer), but without success.

Studniçka, who calls this layer the pellucida, does not look upon it as the lens, but considers, strangely enough, that the translucent appearances at the ends of each nerve end-cell represent a lens for that cell, so that every nerve end-cell has its own lens. Still more strange is it that, holding this view, he should yet consider these knobs {87}to be joined by filaments to the cells in the anterior wall of the eye, a conception fatal to the action of such knobs as lenses.

The discovery that the vertebrate possesses, in addition to the lateral eyes, a pair of median eyes, which are most conspicuous in the lowest living vertebrate, together with the fact that such eyes are built up on the same plan as the median eyes of living crustaceans or arachnids, not only with respect to the eye itself but also to its nerve and optic ganglion, constitutes a fact of the very greatest importance for any theory of the origin of vertebrates; especially in view of the further fact, that similar eyes in the same position are found not only in all the members of the Palæostraca, but also in all those ancient forms (classed as fishes) which lived at that time. At one and the same moment it proves the utter impossibility of reversing dorsal and ventral surfaces, points in the very strongest manner to the origin of the vertebrate from some member or other of the palæostracan group, and insists that the advocates of the origin of vertebrates from the Hemichordata, etc., should give an explanation of the presence of these two median eyes of a more convincing character than that given here.

THE LATERAL EYES.

Turning now to the consideration of the lateral eyes, we see that these eyes in the arachnids often possess an inverted retina, in the crustaceans always an upright retina. In the arachnids they possess a simple retina, while in the crustaceans their retina is compound; so that in the latter case the so-called optic nerve is in reality a tract of fibres connecting together the brain-region with a variable number of optic ganglia, which have been left at the periphery in close contact with the retinal cells, when the brain sunk away from the superficial epithelial covering.

There is, then, this difference between the lateral eyes of crustaceans and arachnids, that the retina of the former is compound, but never inverted, while that of the latter may be inverted, but is always simple.

The retina of the lateral eyes of the vertebrate resembles both of these, for it is compound, as in the crustacean, and inverted as in the arachnid.

It must always be borne in mind that in the palæostracan epoch {88}the dominant race was neither crustacean nor arachnid, but partook of the characters of both; also, as is characteristic of dominance, there was very great variety of form, so that it seems more probable than not that some of these forms may have combined the arachnid and crustacean characteristics to the extent of possessing lateral eyes with an inverted yet compound retina. A certain amount of evidence points in this direction. As already stated, the compound retina which characterizes the vertebrate lateral eyes is characteristic of all facetted eyes, and in the trilobites facetted lateral eyes are commonly found. From this it may be concluded that many of the trilobites possessed eyes with a compound retina. There have, however, been found in certain species, e.g. Harpes vittatus and Harpes ungula, lateral eyes which were not facetted, and are believed by Korschelt and Heider to be of an arachnid nature. They say, "Palæontologists have appropriately described them as ocelli, although, from a zoological point of view, they do not deserve this name, having most probably arisen in a way similar to that conjectured in connection with the lateral eyes of scorpions." If this conjecture is right, then in these forms the retina may have been inverted, but because they belonged to the trilobite group, the retina was most probably compound, so that here we may have had the combination of the arachnid and crustacean characteristics. On the other hand, in some forms of Branchipus, and many of the Gammaridæ, a single corneal lens is found in conjunction with an eye of the crustacean type, so that a non-facetted lateral eye, found in a fossil form, would not necessarily imply the arachnid type of eye with the possibility of an inverted retina. Whatever may be the ultimate decision upon these particular forms, the striking fact remains, that both in the vertebrate and in the arachnid the median eyes possess a simple upright retina, while the lateral eyes possess an inverted retina, and that both in the vertebrate and the crustacean the median eyes possess a simple upright retina, while the lateral eyes possess a compound retina.

The resemblance of the retina of the lateral eyes of vertebrates to that of the lateral eyes of many arthropods, especially crustaceans, has been pointed out by nearly every one who has worked at these invertebrate lateral eyes. The foundation of our knowledge of the compound retina is Berger's well-known paper, the results of which are summed up by him in the following two main conclusions.

{89}1. The optic ganglion of the Arthropoda consists of two parts, of which the one stands in direct inseparable connection with the facetted eye, and together with the layer of retinal rods forms the retina of the facetted eye, while the other part is connected rather with the brain, and is to be considered as an integral part of the brain in the narrower sense of the word.

Br., brain; O.n., optic nerve; n.l.o.g., nuclear layer of ganglion of optic nerve; m.l., molecular layer (Punktsubstanz); n.l.r.g.i. and n.l.r.g.o., inner and outer nuclear layers of the ganglion of the retina; f.br.r., terminal fibre-layer of retina; r., layer of retinal end-cells (indicated only).]

2. In all arthropods examined by him, the retina consists of five layers, as follows:--

(1) The layer of rods and their nuclei. (2) The layer of nerve-bundles. (3) The nuclear layer. (4) The molecular layer. (5) The ganglion cell layer.

Berger passes under review the structure and arrangement of the optic ganglion in a large number of different groups of arthropods, and concludes that in all cases one part of the optic ganglion is always closely attached to the visual end-cells, and this combination he calls the retina. On the other hand, the nerve-fibres which connect the peripheral part of the optic ganglion with the brain, the so-called optic nerve, are by no means homologous in the different groups; for in some cases, as in many of the stalk-eyed crustaceans, the whole optic ganglion is at the periphery, while in others, as in the Diptera, only the retinal ganglion is at the periphery, and the nerve-stalk connects this with the rest of the optic ganglion, the latter being fused with the main brain-mass. In the Diptera, in fact, according to Berger, the optic nerve {90}and retina are most nearly comparable to those of the vertebrate. For this reason I give Berger's picture of the retina of Musca (Fig. 38), in order to show the arrangement there of the retinal layers.

Ant. I. and Ant. II., nerves to 1st and 2nd antennæ. f.br.r., terminal fibre-layer of retina; Op. g. I., first optic ganglion; Op. g. II., second optic ganglion; O.n., optic nerve-fibres forming an optic chiasma.]

In Branchipus and other primitive Crustacea, Berger also finds the same retinal layers, but is unable to distinguish in the brain the rest of the optic ganglion. Judging from Berger's description of Branchipus, and Bellonci's of Sphæroma, it would almost appear as though the cerebral part of the retina in the higher forms originated from two ganglionic enlargements, an external and internal enlargement, as Bellonci calls them. The external ganglion (Op. g. I., Fig. 39) may be called the ganglion of the retina, the cells of which form the nuclear layer of the higher forms, and the internal ganglion (Op. g. II., Fig. 39), from which the optic nerve-fibres to the brain arise, may therefore be called the ganglion of the optic nerve. Bellonci describes how in this latter ganglion cells are found very different to the small ones of the external ganglion or ganglion of the retina. So also in Branchipus, judging from the pictures of Berger, Claus, and from my own observations (cf. Fig. 46, in which the double nature of the retinal ganglion is indicated), the peripheral part of the optic ganglion--i.e. the retinal ganglion--may be spoken {91}of as composed of two ganglia. The external of these is clearly the ganglion of the retina; its cells form the nuclear layer, the striking character of which, and close resemblance to the corresponding layer in vertebrates, is shown by Claus' picture, which I reproduce (Fig. 40). The internal ganglion with which the optic nerve is in connection contains large ganglion cells, which, together with smaller ones, form the ganglionic layer of Berger.

The most recent observations of the structure of the compound retina of the crustacean eye are those of Parker, who, by the use of the methylene blue method, and Golgi's method of staining, has been able to follow out the structure of the compound retina in the arthropod on the same lines as had already been done for the vertebrate. These two methods have led to the conclusion that the arthropod central nervous system and the vertebrate central nervous system are built up in the same manner--viz. by means of a series of ganglia connected together, each ganglion being composed of nerve-cells, nerve-fibres, and a fine reticulated substance called by Leydig in arthropods 'Punktsubstanz,' and known in vertebrates and in invertebrates at the present time as 'neuropil.' A further analysis resolves the whole system into a combination of groups of neurones, the cells and fibres of which form the cells and fibres of the ganglia, while their dendritic connections with the terminations of other neurones, together with the neuroglia-cells form the 'neuropil.' As is natural to expect, that part of the central nervous system which helps to form the compound retina is built up in the same manner as the rest of the central nervous system.

f.br.r., terminal fibre-layer of retina; n.l.r.g., bipolar cells of the ganglion of the retina = inner nuclear layer; m.l., Punktsubstanz = inner molecular layer; b.m., basement membrane formed by neurilemma round central nervous system.]

Thus, according to Parker, the mass of nervous tissue which occupies the central part of the optic stalk in Astacus is composed {92}of four distinct ganglia; the retina is connected with the first of these by means of the retinal fibres, and the optic nerve extends proximally from the fourth ganglion to the brain. Each ganglion consists of ganglion-cells, nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a neuroglial type. By means of the methylene blue method and the Golgi method, it is seen that the retinal end-cells, with their visual rods, are connected with the fibres of the optic nerve by means of a system of neurones, the synapses of which take place in and help to form the 'neuropil' of the various ganglia. Thus, an impulse in passing from the retina to the brain would ordinarily travel over five neurones, beginning with one of the first order and ending with one of the fifth. He makes five neurones although there are only four ganglia, because he reckons the retinal cell with its elongated fibre as a neurone of the first order, such fibre terminating in dendritic processes which form synapses in the 'neuropil' of the first ganglion with the neurones of the second order.

Similarly the neurones of the second order terminate in the 'neuropil' of the second ganglion, and so on, until we reach the neurones of the fifth order, which terminate on the one hand in the 'neuropil' of the fourth ganglion, and on the other pass to the optic lobes of the brain by their long neuraxons--the fibres of the optic nerve.

He compares this arrangement with that of Branchipus, Apus, Estheria, Daphnia, etc., and shows that in the more primitive crustaceans the peripheral optic apparatus was composed, not of four but of two optic ganglia, not, therefore, of five but of three neurones, viz.--

1. The neurone of the first order--i.e. the retinal cell with its fibre terminating in the 'neuropil' of the first optic ganglion (ganglion of the retina).

2. The neurone of the second order, which terminates in the 'neuropil' of the second ganglion (ganglion of the optic nerve).

3. The neurone of the third order, which terminates in the optic lobes of the brain by means of its neuraxons (the optic nerve).

We see, then, that the most recent researches agree with the older ones of Berger, Claus, and Bellonci, in picturing the retina of the primitive crustacean forms as formed of two ganglia only, and not of four, as in the specialized crustacean group the Malacostraca.

{93}The comparison of the arthropod compound retina with that of the vertebrate shows, as one would expect upon the theory of the origin of vertebrates put forward in this book, that the latter retina is built up of two ganglia, as in the more primitive less specialized crustacean forms. The modern description of the vertebrate retina, based upon the Golgi method of staining, is exactly Parker's description of the simpler form of crustacean retina in which the 'neuropil' of the first ganglion is represented by the external molecular layer, and that of the second ganglion by the internal molecular layer; the three sets of neurones being, according to Parker's terminology:--

1. The neurones of the first order--viz. the visual cells--the nuclei of which form the external nuclear layer, and their long attenuated processes form synapses in the external molecular layer with

2. The neurones of the second order, the cells of which form the internal nuclear layer, and their processes form synapses in the internal molecular layer with

3. The neurones of the third order, the cells of which form the ganglionic layer and their neuraxons constitute the fibres of the optic nerve which end in the optic lobes of the brain.

Strictly speaking, of course, the visual cells with their elongated processes have no right to be called neurones: I only use Parker's phraseology in order to show how closely the two retinas agree even to the formation of synapses between the fine drawn-out processes of the visual cells and the neurones of the ganglion of the retina.

THE RETINA OF THE LATERAL EYE OF AMMOCOETES.

As in the case of all other organs, it follows that if we are dealing here with a true genetic relationship, then the lower we go in the vertebrate kingdom the more nearly ought the structure of the retina to approach the arthropod type. It is therefore a matter of intense interest to determine the nature of the retina in Ammocoetes in order to see whether it differs from that of the higher vertebrates, and if so, whether such differences are explicable by reference to the structure of the arthropod eye.

Before describing the structure of this retina it is necessary to clear away a remarkable misconception, shared among others by {94}Balfour, that this eye is an aborted eye, and that it cannot be considered as a primitive type. Thus Balfour says: "Considering the degraded character of the Ammocoete eye, evidence derived from its structure must be received with caution," and later on, "the most interesting cases of partial degeneration are those of Myxine and the Ammocoete. The development of such aborted eyes has as yet been studied only in the Ammocoete, in which it resembles in most important features that of other Vertebrata."

Again and again the aborted character of the eye is stated to be evidence of degeneration in the case of the lamprey. What such a statement means, why the eye is in any way to be considered as aborted, is to me a matter of absolute wonderment: it is true that in the larval form it lies under the skin, but it is equally true that at transformation it comes to the surface, and is most evidently as perfect an eye as could be desired. There is not the slightest sign of any degeneration or abortion, but simply of normal development, which takes a longer time than usual, lasting as it does throughout the life-time of the larval form.

Kohl, who has especially studied degenerated vertebrate eyes, discusses with considerable fulness the question of the Ammocoetes eye, and concludes that in aborted eyes a retarded development occurs, and this applies on the whole to Ammocoetes, "but with the important difference that in this case the period of retarded development is not followed by a stoppage, but on the contrary by a period of very highly intensified progressive development during the metamorphosis," with the result that "the adult eye of Petromyzon Planeri does not diverge from the ordinary type."

Referring in his summing up to this retarded development, he says: "Such reminiscences of embryonic conditions are after all present here and there in normally developed organs, and by no means entitle us to speak of abnormal development."

The evidence, then, is quite clear that the eye of Petromyzon, or, indeed, of the full-grown Ammocoetes, is in no sense an abnormal eye, but simply that its development is slow during the ammocoete stage. The retina of Petromyzon was figured and described by Langerhans in 1873. He describes it as composed of the following layers:--

(1) Membrana limitans interna. (2) Thick inner molecular layer. (3) Optic fibre layer. (4) Thick inner nuclear layer. (5) Peculiar double-layered ganglionic layer. (6) External molecular layer. (7) External nuclear layer. (8) Membrana limitans externa. (9) Layer of rods. (10) Pigment-epithelium.

{95}[Illustration: FIG. 41.--RETINA AND OPTIC NERVE OF PETROMYZON. (AFTER MÜLLER AND LANGERHANS.)

On the left side the Müllerian fibres and pigment-epithelium are represented alone. The retina is divided into an epithelial part, C (the layer of visual rod-cells), and a neurodermal or cerebral part which is formed of, A, the ganglion of the optic nerve and, B, the ganglion of the retina. 1, int. limiting membrane; 2, int. molecular layer with its two layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear layer; 5, double row of tangential fulcrum cells; 6, layer of terminal retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of rods; 10, layer of pigment-epithelium. D, axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather too thick.]

He points out especially the peculiarity of layer (2) (2, Fig. 41), the inner molecular, in which two rows of nuclei are arranged with great regularity, the one row closely touching the membrana limitans interna, the other at the inner boundary of the middle third of the {96}molecular layer. Of these two rows of nuclei, he describes the innermost as composed almost entirely of large nuclei belonging to ganglion cells, while the outermost is composed mainly of distinctly smaller nuclei, which in staining and appearance appear to belong not to nerve-cells but to the true reticular tissue of the molecular layer.

He also draws special attention to the remarkable layer (5) (5, Fig. 41), which is not found in the retina of the higher vertebrates, the cells of which, in his opinion, are of the nature of ganglion-cells.

W. Müller, in 1874, gave a most careful description of the eye of Ammocoetes and Petromyzon, and traced the development of the retina; the subsequent paper of Kohl does not add anything new, and his drawings are manifestly diagrams, and do not represent the appearances so accurately as Müller's illustrations. In the accompanying figure (Fig. 41) I reproduce on the right-hand side Müller's picture of the retina of Petromyzon, but have drawn it, as in Langerhans' picture, at the place of entry of the optic nerve.

From his comparison of this retina with a large number of other vertebrate retinas, he comes to the conclusion that the retina of all vertebrates is divisible into

A. An ectodermal (epithelial) part consisting of the layer of the visual cells, and

B. A neurodermal (cerebral) part which forms the rest of the retina.

Further, Müller points out that the neuroderm gives origin throughout the central nervous system to two totally different structures, on the one hand to the true nervous elements, on the other to a system of supporting cells and fibres which cannot be classed as connective tissue, for they do not arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In the retina he recognizes two distinct groups of such supporting structures--(1) a system of radial fibres with well-marked elongated nuclei, which extend between the two limiting layers, and form at their outer ends a membrane-like expansion which was originally the outer limit of the retina, but becomes afterwards co-terminous with the membrana limitans externa, owing to the piercing through it of the external limbs of the rods. This system, which is known by the name of the radial Müllerian fibres (shown on the left-hand side of Fig. 41), has no connection with (2) the spongioblasts and neurospongium, which form a framework of neuroglia, in which the terminations of the {97}optic ganglion and of the retinal ganglion ramify to form the molecular layers.

It is evident from Fig. 41 that the retina of Ammocoetes and Petromyzon differs in a striking manner from the typical vertebrate retina. The epithelial part (C) remains the same--viz. the visual rods, the external limiting membrane, and the external nuclear layer; but the cerebral part, the retinal ganglion (A and B), is remarkably different. It is true, it consists in the main of the small-celled mass known as the inner nuclear layer, and of the reticulated tissue or 'neuropil' known as the inner molecular layer, just as in all other compound retinal eyes; but neither the ganglion cell-layer nor the optic fibre-layer is clearly defined as separate from this molecular layer; on the contrary, it is matter of dispute as to what cells represent the ganglionic layer of higher vertebrates, and the optic fibres do not form a distinct innermost layer, but pass into the inner molecular layer at its junction with the inner nuclear layer. A comparison of this innermost part of the retina (A, Fig. 41), with the corresponding part in Berger's picture of Musca (n.l.o.g., Fig. 38), shows a most striking similarity between the two. In both cases the fibres of the optic nerve (O.n., Fig. 38) which cross at their entrance pass into the 'neuropil' of this part of the retinal ganglion, and are connected probably (though that is not proved in either case) with the cells of the ganglionic layer. In both cases we find two well-marked parallel rows of cells in this part of the retina, of which one, the innermost, is composed in Ammocoetes of large ganglion-cells, and the other mainly of smaller, deeper staining cells apparently supporting in function. Similarly, also, in Branchipus, as I conclude from my own observations as well as from those of Berger and Claus, the ganglionic layer is composed partly of true ganglion-cells and partly of supporting cells arranged in a distinct layer. This part, then, of the retina of Ammocoetes is remarkably like that of a typical arthropod retina, and forms that part of the retinal ganglion which may be called the ganglion of the optic nerve.

Next comes the ganglion of the retina (B, Fig. 41) (Parker's first optic ganglion), the cells of which form the small bipolar granule-cells of the inner nuclear layer; granule-cells arranged in rows just as they are shown in Claus' picture of the same layer in the retina of Branchipus (Fig. 40), just as they are found in the cortical layers of the optic ganglion of the pineal eye (ganglion habenulæ), in the {98}optic lobes and other parts of the Ammocoetes brain, or in the cortical layers of the optic ganglia of all arthropods.

Between this small-celled nuclear layer (4, Fig. 41) and the layer of nuclei of the visual rod cells (7, Fig. 41) (the external nuclear layer), we find in the eye of Ammocoetes and Petromyzon two well-marked rows of cells of a most striking character--viz. the two remarkably regular rows of large epithelial-like cells with large conspicuous nuclei, which give the appearance of two opposing rows of limiting epithelium (5, Fig. 41), already mentioned in connection with the researches of Langerhans and W. Müller. Here, then, is a striking peculiarity of the retina of the lamprey, and according to Müller the obliteration of these two layers can be traced as we pass upwards in the vertebrate kingdom. Among fishes, they are especially well seen in the perch; in the higher vertebrates the whole layer is only a rudiment represented, he thinks, by the simple layer of round cells which lies close against the inner surface of the layer of terminal fibres (Nervenansätze), and is especially evident in birds and reptiles. In man and the higher mammals they are probably represented by the horizontal cells of the outer part of the inner nuclear layer.

Seeing, then, that they are most evident in Ammocoetes, and become less and less marked in the higher vertebrates, it is clear that their origin cannot be sought among the animals higher in the scale than Ammocoetes, but must, therefore, be searched for in the opposite direction.

Müller describes them as forming a very conspicuous landmark in the embryology of the retina, dividing it distinctly into two parts, an outer thinner, and an inner somewhat thicker part, the zone formed by them standing out conspicuously on account of the size and regularity of the cells and their lighter appearance when stained. Thus in his description of the retina of an Ammocoetes 95 mm. in length, he says, "The layer of pale tangentially elongated cells formed a double layer and produced the appearance of a pale, very characteristic zone between the outer and inner parts of the retina."

Let us now turn to the retina of the crustacean and see whether there is any evidence there that the retina is divisible into an outer and inner part, separated by a zone of characteristically pale staining cells with conspicuous nuclei. The most elaborate description of the development of the retina of Astacus is given by Reichenbach, {99}according to whom the earliest sign of the formation of the retina is an ectodermic involution (Augen-einstülpung), which soon closes, so that the retinal area appears as a thickening. In close contiguity to this thickening, the thickening of the optic ganglion arises, so that that part of the optic ganglion which will form the retinal ganglion fuses with the thickened optic plate and forms a single mass of tissue. Later on a fold (Augen-falte) appears in this mass of tissue, in consequence of which it becomes divided into two parts. The lining walls of this fold form a double row of cells, the nuclei of which are most conspicuous because they are larger and lighter in colour than the surrounding nuclei, so that by this fold the retina is divided into an outer and an inner wall, the line of demarcation being conspicuous by reason of these two rows of large, lightly-staining nuclei.

Reichenbach is unable to say that this secondary fold is coincident with the primary involution, and that therefore the junction between the two rows of large pale nuclei is the line of junction between the retinal ganglion and the retina proper, because all sign of the primary involution is lost before the secondary fold appears.

Parker compares the appearances in the lobster with Reichenbach's description in the crayfish, and says that he finds only a thickening, no primary involution; at the same time he expressly states that in the very early stages his material was deficient, and that he had not grounds sufficient to warrant the statement that no involution occurs. He also finds that in the lobster the ganglionic tissue which arises by proliferation is divided into an outer and inner part; the separation is effected by a band of large, lightly-staining nuclei, which, in position and structure, resemble the band figured by Reichenbach. According to Parker, then, the line of separation indicated in the development by Reichenbach's outer and inner walls is not the line of junction between the retina and the retinal ganglion, as Reichenbach was inclined to think, but rather a separation of two rows of large ganglion-cells belonging to the retinal ganglion.

The similarity between these conspicuous layers of lightly-staining cells in Ammocoetes and in crustaceans is remarkably close, and in both cases observers have found the same difficulty in interpreting their meaning. In each case one group of observers looks upon them as ganglion-cells, the other as supporting structures. Thus in the lamprey, Müller considers them to belong to the supporting elements, while Langerhans and Kohl describe them as a double {100}layer of ganglion-cells. In the crustacean, Berger in Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as supporting elements, while Viallanes in Palinurus considers them to be true ganglionic cells.

Whatever the final interpretation of these cells may prove to be, we may, it seems to me, represent an ideal compound retina of the crustacean type by combining the investigations of Berger, Claus, Reichenbach, and Parker in the following figure.

The retina is divided into an epithelial part, C (the layer of retinular cells and rhabdomes), and a neurodermal or cerebral part, which is formed of, A, the ganglion of the optic nerve, and, B, the ganglion of the retina. 1, optic nerve fibres which cross at their entrance into the retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5, layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer of crystalline cones; 9, cornea.]

The comparison of this figure (Fig. 42) with that of the Petromyzon retina (Fig. 41) shows how great is the similarity of the latter with the arthropod type, and how the very points in which it deviates from the recognized vertebrate type are explainable by comparison with that of the arthropod. The most striking difference between the retinas in the two figures is that the layer of terminal nerve fibres (5, Fig. 42), which, after all, are only the elongated terminations of the retinal cells belonging to Parker's neurones of the first order, is very much longer than in Petromyzon or in any vertebrate, for the external molecular layer (6, Fig. 41) (Müller's layer of Nervenansätze) is very short and inconspicuous (in Fig. 41 it is drawn too thick).

Turning from the retina to the fibres of the optic nerve we again find a remarkable resemblance, for in Ammocoetes, as pointed out by {101}Langerhans and carefully figured by Kohl, a crossing of the fibres of the optic nerve occurs as the nerve leaves the retina, just as is so universally the case in all compound retinas. To this crossing Kohl has given the name chiasma nervi optici, in distinction to the cerebral chiasma, which he calls chiasma nervorum opticorum. Further, we find that even this latter chiasma is well represented in the arthropod brain; thus Bellonci in Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a true optic chiasma, the only difference in opinion being, whether the crossing of the optic nerves is complete or not. Especially instructive are Bellonci's figures and description. He describes the brain of Sphæroma as composed of three segments--a superior segment, the cerebrum proper, a middle segment, and an inferior segment; the optic fibres, as is seen in Fig. 39, after crossing, pass direct into the middle segment, in the ganglia of which they terminate. From this segment also arises the nerve to the first antenna of that side--i.e. the olfactory nerve. The optic part, then, of this middle segment is clearly the brain portion of the optic ganglionic apparatus, and may be called the optic lobes, in contradistinction to the peripheral part, which is usually called the optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as already mentioned. These optic lobes are therefore homologous with the optic lobes of the vertebrate brain.

The resemblance throughout is so striking as to force one to the conclusion that the retina of the vertebrate eye is a compound retina, composed of a retina and retinal ganglion of the type found in arthropods. From this it follows that the development of the vertebrate retina ought to show the formation of (1) an optic plate formed from the peripheral epidermis and not from the brain; (2) a part of the brain closely attached to this optic plate forming the retinal ganglion, which remains at the surface when the rest of the optic ganglion withdraws; (3) an optic nerve formed in consequence of this withdrawal, as the connection between the retinal and cerebral parts of the optic ganglion.

This appears to me exactly what the developmental process does show according to Götte's investigations. He asserts that the retina arises from an optic plate, being the optical portion of his 'Sinnes-platte.' At an early stage this is separated by a furrow (Furche) from the general mass of epidermal cells which ultimately form the brain. This separation then vanishes, and the retina and brain-mass {102}become inextricably united into a mass of cells, which are still situated at the surface. By the closure of the cephalic plate and the withdrawal of the brain away from the surface, a retinal mass of cells is left at the surface connected with the tubular central nervous system by the hollow optic diverticulum or primary optic vesicle. If we regard only the retinal and nervous elements, and for the moment pay no attention to the existence of the tube, Götte's observation that the true retina has been formed from the optic plate (Sinnes-platte) to which the retinal portion of the brain (retinal ganglion) has become firmly fixed, and that then the optic nerve has been formed by the withdrawal of the rest of the brain (optic lobes), is word for word applicable to the description of the development of the compound retina of the arthropod eye, as has been already stated.

THE SIGNIFICANCE OF THE OPTIC DIVERTICULA.

The origin of the retina from an optic epidermal plate in vertebrates, as in all other animals, brings the cephalic eyes of all animals into the same category, and leaves the vertebrate eye no longer in an isolated and unnatural position. In one point the retina of the vertebrate eye differs from that of a compound retina of an invertebrate; in the former, a striking supporting tissue exists, known as Müller's fibres, which is absent in the latter. This difference of structure is closely associated with another of the same character as in the central nervous system, viz. the apparent development of the nervous part from a tube. We see, in fact, that the retinal and nervous arrangements of the vertebrate eye are comparable with those of the arthropod eye, in precisely the same way and to the same extent as the nervous matter of the brain of the vertebrate is comparable with the brain of the arthropod. In both cases the nervous matter is, in structure, position, and function, absolutely homologous; in both cases there is found in the vertebrate something extra which is not found in the invertebrate--viz. a hollow tube, the walls of which, in the case of the brain, are utilized as supporting tissues for the nerve structures. The explanation of this difference in the case of the brain is the fundamental idea of my whole theory, namely, that the hollow tube is in reality the cephalic stomach of the invertebrate, around which the nervous brain-matter was originally grouped in precisely the same manner as in the invertebrate. What, then, are the optic diverticula?

{103}"The formation of the eye," as taught by Balfour, "commences with the appearance of a pair of hollow outgrowths from the anterior cerebral vesicle. These outgrowths, known as the optic vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they soon, however, become partially constricted, and form vesicles united to the base of the brain by comparatively narrow, hollow stalks, the rudiments of the optic nerves."

"After the establishment of the optic nerves, there takes place (1) the formation of the lens, and (2) the formation of the optic cup from the walls of the primary optic vesicle."

He then goes on to explain how the formation of the lens forms the optic cup with its double walls from the primary optic vesicle, and says--

"Of its double walls, the inner, or anterior, is formed from the front portion, the outer, or posterior, from the hind portion of the wall of the primary optic vesicle. The inner, or anterior, which very speedily becomes thicker than the other, is converted into the retina; in the outer, or posterior, which remains thin, pigment is eventually deposited, and it ultimately becomes the tesselated pigment-layer of the choroid."

The difficulties in connection with this view of the origin of the eye are exceedingly great, so great as to have caused Balfour to discuss seriously Lankester's suggestion that the eye must have been at one time within the brain, and that the ancestor of the vertebrate was therefore a transparent animal, so that light might get to the eye through the outer covering and the brain-mass; a suggestion, the unsatisfactory nature of which Balfour himself confessed. Is there really evidence of any part of either retina or optic nerve being formed from the epithelial lining of the tube?

This tube is formed as a direct continuation of the tube of the central nervous system, and we can therefore apply to it the same arguments as have been used in the discussion of the meaning of the latter tube. Now, the striking point in the latter case is the fact that the lining membrane of the central canal is in so many parts absolutely free from nervous matter, and so shows, as in the so-called choroid plexuses, its simple, non-nervous epithelial structure. This also we find in the optic diverticulum. Where there is no evidence of any invasion of the tube by nervous elements, there it retains its simple non-nervous character of a tube composed of a single layer of {104}epithelial cells--viz. in that part of the tube which, as Balfour says, remains thin, in which pigment is eventually deposited, and which ultimately becomes the tesselated pigment-layer of the choroid. Nobody has ever suggested that this pigment-layer is nervous matter, or ever was, or ever will be, nervous matter; it is in precisely the same category as the membranous roof of the brain in Ammocoetes, which never was, and never will be, nervous matter. Yet, according to the old embryology both in the case of the eye and the brain, the pigment-layer and the so-called choroid plexuses are a part of the tubular nervous system.

Turning now to the optic nerve, Balfour describes it as derived from the hollow stalk of the optic vesicle. He says--

"At first the optic nerve is equally continuous with both walls of the optic cup, as must of necessity be the case, since the interval which primarily exists between the two walls is continuous with the cavity of the stalk. When the cavity within the optic nerve vanishes, and the fibres of the optic nerve appear, all connection is ruptured between the outer wall of the optic cup and the optic nerve, and the optic nerve simply perforates the outer wall, and becomes continuous with the inner one."

In this description Balfour, because he derived the optic nerve fibres from the epithelial wall of the optic stalk, of necessity supposed that such fibres originally supplied both the outer and inner walls of the optic cup and, therefore, seeing that when the fibres of the optic nerve appear they do not supply the outer wall, he supposes that their original connection with the outer wall is ruptured, because a discontinuity of the epithelial lining takes place coincidently with the appearance of the optic nerve-fibres, and, according to him, the optic nerve simply perforates the outer wall and becomes continuous with the inner one. This last statement is very difficult to understand. I presume he meant that some of the fibres of the optic nerve supplied from the beginning the inner wall of the optic cup, but that others which originally supplied the outer wall were first ruptured, then perforated the outer wall, and finally completed the supply to the inner wall or retina.

This statement of Balfour's is the necessary consequence of his belief, that the epithelial cells of the optic stalk gave rise to the fibres of the optic nerve. If, instead of this, we follow Kölliker and His, who state that the optic nerve-fibres are formed outside the {105}epithelial walls of the optic stalk, and that the cells of the latter form supporting structures for the nerve-fibres, then the position of the optic nerve becomes perfectly simple and satisfactory without any rupturing of its connection with the outer wall and subsequent perforation, for the optic nerve-fibres from their very first appearance pass directly to supply the retina--i.e. the inner wall of the optic cup and nothing else.

They pass, as is well known, without any perforation by way of the choroidal slit to the inner surface of the inner wall (retina) of the optic cup; then, when the choroidal slit becomes closed by the expansion of the optic cup, the optic nerve naturally becomes situated in the centre of the base of the cup and spreads over its inner surface as that surface expands.

A section across the optic cup at an early stage at the junction of the optic stalk and optic cup would be represented by the upper diagram in Fig. 43; at a later stage, when the choroidal slit is closed, by the lower diagram.

The upper diagram represents a stage before the formation of the choroidal slit, the lower one the stage of closure of the choroidal slit. R., retina; O.n., optic nerve; p., pigment epithelium.]

The evident truth of this manner of looking at the origin of the optic nerve is demonstrated by the appearance of the optic nerve in Ammocoetes and Petromyzon. In the latter, although the development is complete, and the eye, and consequently also the optic nerve-fibres, are fully functional, there is still present in the axial core of the nerve a row of epithelial cells (Axenstrang) which are altered so as to form supporting structures, in the same way as a row of epithelial cells in the retina is altered to form the system of supporting cells known by the name of the Müllerian fibres.

The origin of this axial core of cells is perfectly clear, as has been pointed out by W. Müller. He says--

"The development of the optic nerve shows peculiarities in {106}Petromyzon of such a character as to make this animal one of the most valuable objects for deciding the various controversial questions connected with the genesis of its elements. The lumen of the stalk of the primary optic vesicle is obliterated quite early by a proliferation of its lining epithelium. Also the original continuity of this epithelium with that of the pigment-layer is at an early period interrupted at the point of attachment of the optic stalk. This interruption occurs at the time when the fibres of the optic nerve first become visible."

Further on he says--

"The epithelium of the optic stalk develops entirely into supporting cells, which in Petromyzon fill up the original lumen and so form an axial core (Axenstrang) to the nerve-fibres which are formed entirely outside them; the projections of these supporting cells are directed towards the periphery, and so separate the bundles of the optic nerve-fibres. The mesodermal coat of the optic stalk takes no part in this separation; it simply forms the connective tissue sheath of the optic nerve. The development of the optic nerve in the higher vertebrates also obeys the same law, as I am bound to conclude from my own observations."

The evidence, then, of Ammocoetes is very conclusive. Originally a tube composed of a single layer of epithelial cells became expanded at the anterior end to form a bulb. On the outside of this tube or stalk the fibres of the optic nerve make their appearance, arising from the ganglion-cell layer of the retina, and, passing over the surface of the epithelial tube at the choroidal fissure, proceed to the brain by way of the optic chiasma. Owing to the large number of fibres, their crossing at the junction of the stalk with the bulb, and the narrowness at this neck, the obliteration of the lumen of the tube which takes place in the stalk is carried out to a still greater extent at this narrow part. The result of this is that all continuity of the cell-layers of the original tube of the optic stalk with those of both the inner and outer walls of the bulb is interrupted, and all that remains in this spot of the original continuous line of cells which connected the tube of the stalk with that of the bulb are possibly some of the groups of cells which are found scattered among the fibres of the optic nerve at their entrance into the retina. Such separation of the originally continuous elements of the epithelial wall of the optic stalk, which is apparent only at this neck of the nerve in Petromyzon, takes place {107}along the whole of the optic nerve in the higher vertebrates, so that no continuous axial core of cells exist, but only scattered supporting cells.

If further proof in support of this view be wanted, it is given by the evidence of physiology, which shows that the fibres of the optic nerve are not different from other nerve-fibres of the central nervous system, but that they degenerate when separated from their nerve-cell, and that the nerve-cell of which the optic nerve-fibre is a process is the large ganglion-cell of the ganglionic layer of the retina. The origin of the ganglionic layer of the retina cannot therefore be separated from that of the optic nerve-fibres. If the one is outside the epithelial tube, so is the other, and what holds true of the ganglionic layer must hold good of the rest of the retinal ganglion and, from all that has been said, of the retina itself. We therefore come to the conclusion that the evidence is distinctly in favour of the view, that the retina and optic nerve in the true sense are structures which originally were outside a non-nervous tube, but, just like the central nervous system as a whole, have amalgamated so closely with the elements of this tube as to utilize them for supporting structures. One part of this non-nervous tube, its dorsal wall, like the corresponding part of the brain-tube, still retains its original character, and by the deposition of pigment has been pressed into the service of the eye to form the pigmented epithelial layer.

We can, however, go further than this, for we know definitely in the case of the retina what the fate of the epithelial cells lining this tube has been. They have become the system of supporting structures known as Müllerian fibres.

The epithelial layer of the primary optic vesicle can be traced into direct continuity with the lining epithelium of the brain cavity, as a single layer of epithelial cells in the core of the optic nerve, forming the optic stalk, which, in consequence of close contact, becomes the well-known axial layer of supporting cells. This epithelial layer of the optic stalk then expands to form the optic bulb, the outer or dorsal wall of which still remains as a single layer of epithelium and becomes the layer of pigment epithelium. This layer of epithelium becomes doubled on itself by the approximation of the inner or ventral wall of the optic cup to the outer or dorsal wall in consequence of the presence of the lens, and still remaining a single layer, forms the pars ciliaris retinæ; then suddenly, at the ora {108}serrata, the single epithelial layer vanishes, and the layers of the retina take its place. It has long been known, however, that even throughout the retina this single epithelial layer still continues, being known as the fibres of Müller. This is how the fact is described in the last edition of Foster's "Text-book of Physiology," p. 1308--

"Stretching radially from the inner to the outer limiting membrane in all regions of the retina are certain peculiar-shaped bodies known as the radial fibres of Müller. Each fibre is the outcome of the changes undergone by what was at first a simple columnar epithelial cell. The changes are, in the main, that the columnar form is elongated into that of a more or less prismatic fibre, the edges of which become variously branched, and that while the nucleus is retained the cell substance becomes converted into neuro-keratin. And, indeed, at the ora serrata the fibres of Müller may be seen suddenly to lose their peculiar features and to pass into the ordinary columnar cells which form the pars ciliaris retinæ."

O.n., axial core of cells in optic nerve; p., pigment epithelium; p.c.r., pars ciliaris retinæ; m.f., Müllerian fibres; l., lens.]

It is then absolutely clear that the essential parts of the eye may be considered as composed of two parts--

1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the accompanying Fig. 44, and the left side of Fig. 41.

2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described. In this part supporting elements are found, just as in any other compound retina, of the nature of neuroglia, which are independent of the Müllerian fibres.

{109}Of these two parts we have already seen that the second is to all intents and purposes a compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is continuous with the single-layered epithelial tube of the central nervous system--i.e. with the cephalic part of the gut of the arthropod ancestor--it follows with certainty that the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the wall of which, near the anterior extremity, the compound retina has amalgamated on either side, just as the infra-oesophageal ganglia have amalgamated with the ventral wall of the main gut-tube. In this way, and in this way alone, does the interpretation of the structure of the vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions already arrived at.

The question therefore arises:--Have we any grounds for believing that the ancient forms of primitive crustaceans and primitive arachnids, which were so abundant in the time when the Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the consideration of the vertebrate eye strongly indicates must have been the case?

The beautiful pictures of Blanchard, and his description, show how, on the arachnid side, paired diverticula of the stomach are nearly universal in the group. Thus, although they are not present in the scorpions, still, in the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked characteristic of the stomach-region is the presence of four pairs of coecal diverticula, which spread laterally over the prosomatic region. In the spiders the number of such diverticula increases, and the whole prosomatic region becomes filled up with these tubes. Blanchard considers that they form nutrient tubes for the direct nutrition of the organs in the prosoma, especially the important brain-region of the central nervous system. He points out that these animals are blood-suckers, and that, therefore, their food is already in a suitable form for purposes of nutrition when it is taken in by them, so that, as it were, the anterior part of the gut is transformed into a series of vessels or diverticula conveying blood directly to the important organs in the prosoma, by means of which they obtain nourishment in addition to their own blood-supply.

The universality of such diverticula among the arachnids makes it highly probable that their progenitors did possess an alimentary canal with one or more pairs of anterior diverticula. In the {110}vertebrate, however, the paired diverticula are associated with a compound retina, a combination which does not occur among living arachnids; we must, therefore, examine the crustacean group for the desired combination, and naturally the most likely group to examine is the Phyllopoda, especially such primitive forms as Branchipus and Artemia, for it is universally acknowledged that these forms are the nearest living representatives of the trilobites. If, therefore, it be found that the retina and optic nerve in Artemia is in specially close connection with an anterior diverticulum of the gut on each side, then it is almost certain that such a combination existed also in the trilobites.

The section is through the extreme anterior end of the diverticulum, thus cutting through many of the columnar cells at right angles to their axis. Al., gut diverticulum; rt. gl., retinal ganglion.]

{111}[Illustration: FIG. 46.--THE BRAIN, EYES, AND ANTERIOR TERMINATION OF THE ALIMENTARY CANAL OF ARTEMIA, VIEWED FROM THE DORSAL ASPECT.

Br., brain; l.e., lateral eyes; c.e., median eyes; Al., alimentary canal.]

R., retina; l., lens; O.n., optic nerve fibres; Al., cephalic end of invertebrate alimentary canal; V., cavity of ventricles of brain; Al.d., anterior diverticulum of alimentary canal; op.d., optic diverticulum.]

My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the course of his work he cut serial sections through the whole animal, and, as mentioned in my paper in the Journal of Anatomy and Physiology, he discovered that the retinal ganglion of each lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion (Fig. 45). In this animal there are only two gut-diverticula, which are situated most anteriorly. I have plotted out this series of sections by means of a camera lucida, with the result that the retina appears as a bulging attached ventro-laterally to the extremity of each gut-diverticulum, as is shown in Fig. 46. It is instructive to compare with this figure Scott's picture of the developing eye in Ammocoetes, where he figures the retina as {112}a bulging attached ventrally to the extremity of the narrow tube of the optic diverticulum. In Fig. 47, A, I reproduce this figure of Scott, and by the side of it, Fig. 47, B, I have represented the origin of the vertebrate eye as I believe it to have occurred.

We see, then, this very striking fact, that in the most primitive of the Crustacea, not only are there two anterior diverticula of the gut, but also the retinal ganglion of the lateral eye is in specially close connection with the end of the diverticulum on each side. In fact, we find in the nearest living representative of the trilobites a retina and retinal ganglion and optic nerve, closely resembling that of the vertebrate, in close connection with an epithelial tube which has nothing to do with the organ of sight, but is one of a pair of anterior gut-diverticula. It is impossible to obtain more decisive evidence that the trilobites possessed a pair of gut-diverticula surrounded to a greater or less extent by the retina and optic nerve of each lateral eye.

Such anterior diverticula are commonly found in the lower Crustacea; they are usually known by the name of liver-diverticula, but as they take no part in digestion, and, on the contrary, represent that part of the gut which is most active in absorption, the term liver is not appropriate, and it is therefore better to call them simply the pair of anterior diverticula. Our knowledge of their function in Daphnia is given in a paper by Hardy and M^cDougall, which does not appear to be widely known. Hardy succeeded in feeding Daphnia with yolk of egg in which carmine grains were mixed, and was able in the living animal to watch the whole process of deglutition, digestion, and absorption. The food, which is made into a bolus, is moved down to the middle region of the gut, and there digestion takes place. Then by an antiperistaltic movement the more fluid products of the digestion-process are sent right forward into the two anterior diverticula, where the single layer of columnar cells lining these diverticula absorbs these products, the cells becoming thickly studded with fat-drops after a feed of yolk of egg. The carmine particles, which were driven forward with the proteid- and fat-particles, are not absorbed, but are at intervals driven back by contractions of the anterior diverticula to the middle region of the gut.

These observations prove most clearly that the anterior diverticula have a special nutrient function, being the main channels by which new nutrient material is brought into the body, and, as {113}pointed out by the authors, it is a remarkable exception in the animal kingdom that absorption should occur in that portion of the gut which is anterior to the part in which digestion occurs. In all these animals the two anterior diverticula extend forwards over the brain, and, as we have seen in Artemia, the anterior extremity of each one is so intimately related to a part of the brain--viz. the retinal ganglion--as to form a lining membrane to that mass of nerve-cells. It follows, therefore, that the nutrient fluid absorbed by the cells of this part of the gut-diverticulum must be primarily for the service of the retinal ganglion. In fact, the relations of this anterior portion of the gut to the brain as a whole suggest strongly that the marked absorptive function of this anterior portion of the gut exists in order to supply nutrient material in the first place to the most vital, most important organ in the animal--the brain and its sense-organs. This conclusion is borne out by the fact that in these lower crustaceans the circulation of blood is of a very inefficient character, so that the tissues are mainly dependent for their nutrition on the fluid immediately surrounding them. It stands to reason that the establishment of the anterior portion of the gut as a nutrient tube to the brain would necessitate a closer and closer application of the brain to that tube, so that the process of amalgamation of the brain with the single layer of columnar epithelial cells which constitutes the wall of the gut (which we see in its initial stage in the retinal ganglion of Artemia), would tend rapidly to increase as more and more demands were made upon the brain, until at last both the supra- and infra-oesophageal ganglia, as well as the retinal ganglia and optic nerves, were in such close intimate connection with the ventral wall of the anterior portion of the gut and its diverticula as to form a brain and retina closely resembling that of Ammocoetes.

Such an origin for the lateral eyes of the vertebrate explains in a simple and satisfactory manner why the vertebrate retina is a compound retina, and why both retina and optic nerve have an apparent tubular development.

At the same time one discrepancy still exists which requires consideration--viz. in no arthropod eye possessing a compound retina is the retina inverted. All the known cases of inversion among arthropods occur in eyes, the retina of which is simple, and are all natural consequences of the process of invagination by which {114}the retina is formed. On the other hand, eyes with an inverted compound retina are not entirely unknown among invertebrates, for the eyes of Pecten and of Spondylus possess a retina which is inverted after the vertebrate fashion and still may be spoken of as compound rather than simple. It is clear that an invagination, the effect of which is an inversion of the retinal layer, would lead to the same result, whether the retinal optic nerves were short or long, whether, in fact, a retinal ganglion existed or not. Undoubtedly the presence of the retinal ganglion tends greatly to obscure any process of invagination, so that, as already mentioned, many observers, with Parker, consider the retina of the crustacean lateral eye to be formed by a thickening only, without any invagination, while Reichenbach says an obscure invagination does take place at a very early stage. So in the vertebrate eye most observers speak only of a thickening to form the retina, but Götte's observation points to an invagination of the optic plate at an early stage. So also in the eye of Pecten, Korschelt and Heider consider that the thickening, by which the retina is formed according to Patten, in reality hides an invagination process by means of which, as Bütschli suggests, an optic vesicle is formed in the usual manner. The retina is formed from the anterior wall of this vesicle, and is therefore inverted.

The origin of the inverted retina of the vertebrate eye does not seem to me to present any great difficulty, especially when one takes into consideration the fact that the retina is inverted in the arachnid group, only in the lateral eyes. The inversion is usually regarded as associated with the tubular formation of the vertebrate retina, and it is possible to suppose that the retina became inverted in consequence of the involvement of the eye with the gut-diverticulum. I do not myself think any such explanation is at all probable, because I cannot conceive such a process taking place without a temporary derangement--to say the least of it--of the power of vision, and as I do not believe that evolution was brought about by sudden, startling changes, but by gradual, orderly adaptations, and as I also believe in the paramount importance of the organs of vision for the evolution of all the higher types of the animal kingdom, I must believe that in the evolution from the Arthropod to the Cephalaspid, the lateral eyes remained throughout functional. I therefore, for my own part, would say that the inversion of the {115}retina took place before the complete amalgamation with the gut-diverticulum, that, in fact, among the proto-crustacean, proto-arachnid forms there were some sufficiently arachnid to have an inverted retina, and at the same time sufficiently crustacean to possess a compound retina, and therefore a compound inverted retina after the vertebrate fashion existed in combination with the anterior gut-diverticula. Thus, when the eye and optic nerve sank into and amalgamated with the gut-diverticulum, neither the dioptric apparatus nor the nervous arrangements would suffer any alteration, and the animal throughout the whole process would possess organs of vision as good as before or after the period of transition.

Further, not only the retina but also the dioptric apparatus of the vertebrate eye point to its origin from a type that combined the peculiarities of the arachnids and the crustaceans. In the former it is difficult to speak of a true lens, the function of a lens being undertaken by the cuticular surface of the cells of the corneagen (Mark's 'lentigen'), while in the latter, in addition to the corneal covering, a true lens exists in the shape of the crystalline cones. Further, these crustacean lenses are true lenses in the vertebrate sense, in that they are formed by modified hypodermal cells, and not bulgings of the cuticle, as in the arachnid. We see, in fact, that in the compound crustacean eye an extra layer of hypodermal cells has become inserted between the cornea and the retina to form a lens. So also in the vertebrate eye the lens is formed by an extra layer of the epidermal cells between the cornea and the retina. The fact that the vertebrate eye possesses a single lens, though its retina is composed of a number of ommatidia, while the crustacean eye possesses a lens to each ommatidium, may well be a consequence of the inversion of the vertebrate retina. It is most probable, as Korschelt and Heider have pointed out, that the retina of the arachnid eyes is composed of a number of ommatidia, just as in the crustacean eyes and in the inverted eyes it is probable that the image is focussed on to the pigmented tapetal layer, and thence reflected on to the percipient visual rods. In such a method of vision a single lens is a necessity, and so it must also be if, as I suppose, eyes existed with an inverted compound retina. Owing to the crustacean affinities of such eyes, a lens would be formed and the retina would be compound: owing to the arachnid affinities, the retina would be inverted and the hypodermal cells which formed the lens would be massed {116}together to form a single lens, instead of being collected in groups of four to form a series of crystalline cones.

To sum up: The study of the vertebrate eyes, both median and lateral, leads to most important conclusions as to the origin of the vertebrates, for it shows clearly that whereas, as pointed out in this and subsequent chapters, their ancestors possessed distinct arachnid characteristics, yet that they cannot have been specialized arachnids, such as our present-day forms, but rather they were of a primitive arachnid type, with distinct crustacean characteristics: animals that were both crustacean and arachnid, but not yet specialized in either direction: animals, in fact, of precisely the kind which swarmed in the seas at the time when the vertebrates first made their appearance. In the opinion of the present day, the ancestral forms of the Crustacea, which were directly derived from the Annelida, may be classed as an hypothetical group the Protostraca, the nearest approach to which is a primitive Phyllopod.

"Starting from the Protostraca," say Korschelt and Heider, "according to the present condition of our knowledge, we may, as has been already remarked, assume three great series of development of the Arthropodan stock, by the side of which a number of smaller independent branches have been retained. One of these series leads through the hypothetical primitive Phyllopod to the Crustacea; the second through the Palæostraca (Trilobita, Gigantostraca, Xiphosura) to the Arachnida; the third through forms resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and the Tardigrada must probably be regarded as smaller independent branches of the Arthropodan stock."

To these "three great series of development of the Arthropodan stock" the evidence of Ammocoetes shows that a fourth must be added, which, starting also from the Protostraca, and closely connected with the second, palæostracan branch, leads through the Cephalaspidæ to the great kingdom of the Vertebrata. Such a direct linking of the earliest vertebrates with the Annelida through the Protostraca is of the utmost importance, as will be shown later in the explanation of the origin of the vertebrate coelom and urinary apparatus.

{117}SUMMARY.

The most important discovery of recent years which gives a direct clue to the ancestry of the vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing how perfect one of them still is in Ammocoetes. The vertebrate ancestor, then, possessed two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of the origin of the vertebrate from Palæostracans it is universally admitted that all the Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which are the earliest fishes known, are all said to show the presence of a pair of median eyes as well as of a pair of lateral eyes. This evidence directly suggests that the structure of both the median and lateral vertebrate eyes ought to be very similar to that of the median and lateral arthropod eyes. Such is, indeed, found to be the case.

The retina of the simplest form of eye is formed from a group of the superficial epidermal cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.

In other cases a portion of the optic ganglion remains at the surface, when the brain sinks inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve. Such a retina may be called a compound retina and characterizes the lateral eyes of both crustaceans and vertebrates. Also, owing to the method of formation of the retina by invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed, may be directed towards the source of light or away from it. In the first case the retina may be called upright, in the second inverted.

Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the arachnids, but not of the crustaceans.

The evidence shows that all the invertebrate median eyes possess a simple upright retina, and in structure are remarkably like the right median or pineal eye of Ammocoetes; while the lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a compound inverted retina.

This retina, however, is extraordinarily similar in its structure to the compound crustacean retina, and these similarities are more accentuated in the retina of the lateral eye of Petromyzon than that of the higher vertebrates.

The evidence afforded by the lateral eye of the vertebrate points unmistakably to the conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid characters--belonged, therefore, to a group of animals which gave rise to both the crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which is regarded as the ancestor of both the crustaceans and arachnids. {118}In two respects the retina of the lateral eyes of vertebrates differs from that of all arthropods, for it possesses a special supporting structure, the Müllerian fibres, which do not exist in the latter, and it is developed in connection with a tube, the optic diverticulum, which is connected on each side with the main tube of the central nervous system. These two differences are in reality one and the same, for the Müllerian fibres are the altered lining cells of the optic diverticulum, and this tube has the same significance as the rest of the tube of the nervous system; it is something which has nothing to do with the nervous portion of the retina but has become closely amalgamated with it. The explanation is, word for word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate possessed two anterior diverticula of its alimentary canal which were in close relationship to the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly allied to the trilobites, does possess two anterior diverticula of the gut which are in extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on each side.

The evidence of the optic apparatus of the vertebrate points most remarkably to the derivation of the Vertebrata from the Palæostraca.

{119}CHAPTER III

THE EVIDENCE OF THE SKELETON

The bony and cartilaginous skeleton considered, not the notochord.--Nature of the earliest cartilaginous skeleton.--The mesosomatic skeleton of Ammocoetes; its topographical arrangement, its structure, its origin in muco-cartilage.--The prosomatic skeleton of Ammocoetes; the trabeculæ and parachordals, their structure, their origin in white fibrous tissue.--The mesosomatic skeleton of Limulus compared with that of Ammocoetes; similarity of position, of structure, of origin in muco-cartilage.--The prosomatic skeleton of Limulus; the entosternite or plastron compared with the trabeculæ of Ammocoetes; similarity of position, of structure, of origin in fibrous tissue.--Summary.

The explanation of the two optic diverticula given in the last chapter accounts in the same harmonious manner for every other part of the tube around which the central nervous system of the vertebrate has been grouped. The tube conforms in all respects to the simple epithelial tube which formed the alimentary canal of the ancient type of marine arthropods such as were dominant in the seas when the vertebrates first appeared. The whole evidence so far is so uniform and points so strongly in the direction of the origin of vertebrates from these ancient arthropods, as to make it an imperative duty to proceed further and to compare one by one the other parts of the central nervous system, together with their outgoing nerves in the two groups of animals.

Before proceeding to do this, it is advisable first to consider the question of the origin of the vertebrate skeletal tissues, for this is the second of the great difficulties in the way of deriving vertebrates from arthropods, the one skeleton being an endo-skeleton composed of cartilage and bone, and the other an exo-skeleton composed of chitin. Here is a problem of a totally different kind to that we have just been considering, but of so fundamental a character that it must, if possible, be solved before passing on to the consideration of the cranial nerves and the organs they supply.

{120}Is there any evidence which makes it possible to conceive the method by which the vertebrate skeleton may have arisen from the skeletal tissues of an arthropod? By the vertebrate skeleton I mean the bony and cartilaginous structures which form the backbone, the cranio-facial skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do not include the notochord in these skeletal tissues, because there is not the slightest evidence that the notochord played any part in the formation of these structures; the notochordal tissue is something sui generis, and never gives rise to cartilage or bone. The notochord happens to lie in the middle line of the body and is very conspicuous in the lowest vertebrate; with the development of the backbone the notochord becomes obliterated more and more, until at last it is visible in the higher vertebrates only in the embryo; but that obliteration is the result of the encroachment of the growing bone-masses, not the cause of their growth. Although, then, the notochord may in a sense be spoken of as the original supporting axial rod of the vertebrate, it is so different to the rest of the endo-skeleton, has so little to do with it, that the consideration of its origin is a thing apart, and must be treated by itself without reference to the origin of the cartilaginous and bony skeleton.

THE COMMENCEMENT OF THE BONY SKELETON IN THE VERTEBRATE.

What is the teaching of the vertebrate? What evidence is there as to the origin of the bony skeleton in the vertebrate phylum itself?

The axial bony skeleton of the higher Mammalia consists of two parts, (1) the vertebral column with its attached bony parts, and (2) the cranio-facial skeleton. Of these two parts, the bony tissue of the first arises in the embryo from cartilage, of the second partly from cartilage, partly from membrane.

In strict accordance with their embryonic origin is their phylogenetic origin: as we pass from the higher vertebrates to the lower these structures can be traced back to a cartilaginous and membranous condition, so that, as Parker has shown, the cranio-facial bony skeleton of the higher vertebrates can be derived directly from a non-bony cartilaginous skeleton, such as is seen in Petromyzon and the cartilaginous fishes.

Balfour, in his "Comparative Embryology," states that the {121}primitive cartilaginous cranium is always composed of the following parts:--

1. A pair of cartilaginous plates on each side of the cephalic section of the notochord known as the parachordals (pa.ch., Fig. 49; iv., Fig. 48). These plates, together with the notochord (ch.) enclosed between them, form a floor for the hind and mid-brain.

ch., notochord; iv., parachordals; au., auditory capsule; py., pituitary body; tr., trabecula; ctr., trabecular cornu; pn., pre-nasal cartilage; ppg., palato-pterygoid tract; mn., mandibular arch; th.h., first branchial arch; VII.-XII., cranial nerves.]

ol., olfactory sacs; au., auditory capsule; py., pituitary body; pa.ch., parachordal cartilage; tr., trabecula; inf., infundibulum; pt.s., pituitary space; e., eye.]

2. A pair of bars forming the floor for the fore-brain, known as the trabeculæ (tr). These bars are continued forward from the parachordals. They meet posteriorly and embrace the front end of the notochord, and after separating for some distance bend in again in such a way as to enclose a space--the pituitary space (pt.s.). In {122}front of this space they remain in contact, and generally unite. They extend forward into the nasal region (pn.).

3. The cartilaginous capsules of the sense organs. Of these the auditory (au.) and the olfactory capsules (ol.) unite more or less intimately with the cranial walls; while the optic capsules, forming the usually cartilaginous sclerotics, remain distinct.

The parachordals and notochord form together the basilar plate, which forms the floor for that section of the brain belonging to the primitive postoral part of the head, and its extent corresponds roughly to that of the basioccipital of the adult skull.

The trabeculæ, so far as their mere anatomical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as do the parachordals for the mid- and hind-brain. They differ, however, from the parachordals in one important feature, viz. that except at their hinder end they do not embrace the notochord. The notochord always terminates at the infundibulum, and the trabeculæ always enclose a pituitary space, in which lies the infundibulum (inf.) and the pituitary body (py.).

In the majority of the lower forms the trabeculæ arise quite independently of the parachordals, though the two sets of elements soon unite.

The trabeculæ are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts. Into this space the whole base of the fore-brain primitively projects, but the space itself gradually becomes narrowed until it usually contains only the pituitary body.

The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types, two regions may be distinguished, viz. an alisphenoidal region behind, growing out from what is known as the basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front, growing out from the presphenoidal region of the trabeculæ. These plates form at first a continuous lateral wall of the cranium. The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity.

The basi-cranial cartilaginous skeleton reduces itself always into trabeculæ and parachordals with olfactory and auditory cartilaginous capsules.

{123}In addition, a branchial skeleton exists, which consists of a series of bars known as the branchial bars, so situated as to afford support to the successive branchial pouches. An anterior arch known as the mandibular arch (Fig. 50, Mn.), placed in front of the hyo-mandibular cleft, and a second arch, known as the hyoid arch (Hy.), placed in front of the hyo-branchial cleft, are developed in all types; the succeeding arches are known as the true branchial arches (Br.), and are only fully developed in the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates the first arch has become a supporting skeleton for the mouth (Fig. 51), and in the higher vertebrates in combination with the second or hyoid arch takes part in the formation of the ear-bones.

Tr., trabecula; Mn., mandibular cartilage; Hy., hyoid arch; Br1., first branchial arch; Na., olfactory sac; E., eye; Au., auditory capsule; Hm., hemisphere; C1, C2, C3, cerebral vesicles.]

cr., cranium; Br., branchial arches; Mn. + Hy., mandibular and hyoid arches.]

The true branchial arches persist, to a certain extent, in the Amphibia, and become still more degenerated in the Amniota (reptiles, birds, and mammals) in correlation with the total disappearance of a branchial respiration at all periods of their life. {124}Their remnants become more or less important parts of the hyoid bone, and are employed solely in support of the tongue.

In no single animal is there any evidence that the foremost arch, the mandibular, is a true branchial arch. As low down as the Elasmobranchs it becomes divided into two elements which form respectively the upper and lower jaws; the hyoid arch, on the other hand, although it has altered its form and acquired the secondary function of supporting the mandibular arch, still retains its respiratory function.

The evidence afforded by the mode of formation of the skeletal tissues of vertebrates down to the Elasmobranchs indicates that the primitive cranial skeleton arose from two paired basal cartilages, the parachordals and trabeculæ, to which were attached respectively cartilaginous cases enclosing the organs of hearing and smell. In addition, the branchial portion of the cranial region was provided with cartilaginous bars arranged serially for the support of the branchiæ, with the exception of the foremost, the mandibular bar, which formed supporting tissues for the mouth--the upper and lower jaws.

Just as in past times the spinal nerves and the segments they supplied were supposed to represent the type on which the original vertebrate was built, so also the spinal vertebræ afforded the type of the segmented skeleton, and the anatomists of those days strove hard to resolve the cranio-facial skeleton into a series of modified vertebræ. Owing especially to the labours of Huxley, who showed that the segmentation in the head-region was essentially a segmentation due to the presence of branchial bars, this conception was finally laid to rest and nowadays it is admitted to be hopeless to resolve the cranium into vertebral segments. Still, however, the vertebrate is a segmented animal and its segmented nature is visible in the cranial region, so far as the skeletal tissues are concerned, in the shape of the series of branchial and visceral bars.

To this segmentation the name of 'branchiomeric' has been given, while that due to the presence of vertebræ is called 'mesomeric.'

As we have seen, the internal bony skeleton of the vertebrate commences as a cartilaginous and membranous skeleton. For this reason the preservation of such skeletons is impossible in the fossil form, unless the cartilage has become impregnated with lime salts, so that there is but little hope of ever obtaining traces of such {125}structures in the fossils of the Silurian age either among the vertebrate or invertebrate remains. Fortunately for this investigation there are still living on the earth two representatives of that age; on the invertebrate side Limulus, and on the vertebrate side Ammocoetes.

The Elasmobranchs represent the most primitive of the gnathostomatous vertebrates. Below them come the Agnatha, known as the cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon) and the hag-fishes (Myxine).

The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial skeleton composed of a cartilaginous unsegmented cranium, with the basal trabeculæ and parachordals and a series of branchial and visceral cartilaginous bars forming the so-called branchial basket-work; to these must be added auditory and nasal capsules. In contradistinction to this elaborate cranio-facial skeleton, the spinal vertebral skeleton is represented only by segmentally arranged small pieces of cartilage formed in the connective tissue dissepiments between segmented sheets of body-muscles (myotomes).

na., nasal capsule; au., auditory capsule; nc., notochord.]

But Petromyzon is derived from Ammocoetes by a remarkable process of transformation, and a most important part of that transformation is the formation of new cartilaginous structures. Thus we see that in Ammocoetes there is no sign of a cartilaginous vertebral column; at transformation the rudimentary vertebræ of Petromyzon are formed. In Ammocoetes the brain-case is a simple fibrous membranous covering; at transformation this becomes cartilaginous. In Ammocoetes there are no cartilaginous structures corresponding to the sub-ocular arches; these are all formed at transformation. It follows, that we can trace back the bony skeleton of the vertebrate head to the skeleton of Ammocoetes, and we may therefore conclude {126}that the primitive cartilaginous skeleton of the vertebrate consisted of the following structures (Fig. 53, B), viz. the branchial bars forming a basket-work, the trabeculæ and parachordals, the auditory and nasal capsules--a clear proof that the cranial skeleton is older than the spinal. Of these structures the branchial bars are the only evidently segmented parts.

A, Diagram of cartilaginous skeleton of Limulus. Soft cartilage, entapophysial ligaments, deep black; branchial bars simply hatched; hard cartilage, lateral trabeculæ of entosternite, netted; Ph., position of pharynx.

B, Diagram of cartilaginous skeleton of Ammocoetes. Soft cartilage, sub-chordal cartilaginous bands, deep black; branchial basket-work (first formed part), simply hatched; hard cartilage, cranio-facial skeleton, trabeculæ, parachordals and auditory capsules, netted; Inf., position of tube of infundibulum (old oesophagus).]

THE SOFT CARTILAGE OF THE BRANCHIAL SKELETON OF AMMOCOETES.

The study of Ammocoetes gives yet another clue to the nature of the earliest skeleton, for these two marked groups of cartilage--the branchial and basi-cranial--are characterized by a difference in structure as well as a difference in topographical position. J. Müller was the first to point out that these two sets of cartilages differ in appearance and constitution, and he gave to them the name of yellow and grey cartilage. Parker has described them fully under the terms soft and hard cartilage, terms which Schaffer has also used, and I shall also make use of them here. The whole of the branchial cartilaginous skeleton is composed of soft cartilage, while the basi-cranial skeleton, consisting of trabeculæ, parachordals, and auditory capsule, is composed {127}of hard cartilage, the only soft cartilage in this region being that which forms the nasal capsule, not represented in Fig. 53, B.

These two groups of cartilage arise independently, so that at first the basi-cranial system is quite separate from the branchial, and only late in the history of the animal is a junction effected between the branchial system and the trabeculæ and parachordals, an initial separation which is especially striking when we consider that in this animal all the cartilaginous structures of any one system are continuous: there is no sign of anything in the nature of joints.

Of these two main groups, the branchial cartilages are formed first in the embryo, a fact which suggests that they are the most primitive of the vertebrate cartilages, and that, therefore, the first true formation of cartilage in the invertebrate ancestor may be looked for in the shape of bars supporting the branchial mechanism. The evidence of the origin of the cartilaginous structures in Ammocoetes is given by Shipley in the following words:--

"The branchial bases are the first part of the skeleton to appear. They arise about the 24th day as straight bars of cartilage, lying external and slightly posterior to the branchial vessel.

"The first traces of the basi-cranial skeleton appear on the 30th day as two rods of cartilage--the trabeculæ."

Our attention must, in the first place, be directed to this branchial basket-work of Ammocoetes.

Underlying the skin of Ammocoetes in the branchial region is situated the sheet of longitudinal body-muscles, divided into a series of segments or myotomes, which forms the somatic muscles so characteristic of all fishes. This muscular sheet is depicted on the left-hand side of Fig. 54. It does not extend over the lower lip or over that part in the middle line where the thyroid gland is situated. In these parts a sheet of peculiar tissue known by the name of muco-cartilage lies immediately under the skin, covering over the thyroid gland and lower lip. The somatic muscular sheet with the superjacent skin can be stripped off very easily owing to the vascularity and looseness of the tissue immediately underlying it. When this is done the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. 54. It forms a cage within which the branchiæ and their muscles lie entirely concealed.

This is the great characteristic of this most primitive form of the branchial cartilaginous bars and distinguishes it from the branchial {128}bars of other higher fishes, in that it forms a system of cartilages which lie external to the branchiæ--an extra-branchial system.

This branchial basket-work is simpler in Ammocoetes than in Petromyzon, and its actual starting-point consists of a main transverse bar corresponding to each branchial segment; from this transverse bar the system of longitudinal bars by which the basket-work is formed has sprung. These transverse bars arise from a cartilaginous longitudinal rod, situated close against the notochord on each side. These rods may be called the subchordal cartilaginous bands (Fig. 53), and, according to the observations of Schneider and others, each subchordal band does not form at first a continuous cartilaginous rod, but the cartilage is conspicuous only at the places where the transverse bars arise. In the youngest Ammocoetes examined by Schaffer, he could find no absolute discontinuity of the cartilage except between the first two transverse bars, but he says that the thinning between the transverse bars was so marked as to make it highly probable that at an earlier stage there was discontinuity. The whole system of branchial bars and subchordal rods is at first absolutely disconnected from the cranial system of trabeculæ and parachordals, and only later do the two systems join.

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

These observations on Ammocoetes lead most definitely to the conclusion that the starting-point of the whole cartilaginous skeleton of the vertebrate consisted of a series of transverse cartilaginous bars, for the purpose of supporting branchial segments; these were connected with two axial longitudinal cartilaginous rods, which at first contained cartilage only near the places of junction of the branchial {129}bars. This system may be called the mesosomatic skeleton, as it is entirely confined to the branchial or mesosomatic region.

In addition to this primitive cartilaginous framework, which was formed for the support of the mesosomatic or respiratory segments, but at a slightly later period in the phylogenetic history, a separate cartilaginous system was formed for the support of the prosomatic segments, viz. the trabeculæ and parachordals with the auditory capsules: a system which was at first entirely separated from the mesosomatic, and, as we shall see, is more advanced in structure than the branchial system. Later still, the story is completed at the time of transformation to Petromyzon by the formation of the simple cartilaginous skull and the rudimentary vertebræ, the structure of which is also of a more advanced type.

THE STRUCTURE OF THE SOFT BRANCHIAL CARTILAGE.

Having considered the topographical position of the primitive branchial cartilaginous skeleton, we may now inquire, What was its structure and how was it formed?

In the higher vertebrates various forms of cartilage are described, viz. hyaline, fibro-cartilage, elastic cartilage, and parenchymatous cartilage. Of these, the parenchymatous cartilage is looked upon as the most primitive form, because it preserves without modification the characters of embryonic cartilage.

Embryology, then, would lead to the belief that the earliest form of cartilage in the vertebrate kingdom ought to be of this type, viz. large cells, each of which is enclosed in a simple capsule, so that the capsules of the cells form the whole of the matrix, and thus form a simple homogeneous honeycomb-structure, in the alveoli of which the cartilage-cells lie singly. If, then, the branchial cartilages of Ammocoetes are, as has just been argued, the representatives of the cartilaginous skeleton of the primitive vertebrate, it is reasonable to suppose that they should resemble in structure this embryonic cartilage. Such is undoubtedly the case: all observers who have described the branchial basket-work of Ammocoetes or Petromyzon have been struck with the extremely primitive character of the cartilage, and the last observer (Schaffer) describes it as composed of thin walls of homogeneous material, in which there are no lines of separation, which form a simple honeycomb-structure, in the alveoli {130}of which the separate cells lie singly. These branchial cartilages are each surrounded by a layer of perichondrium, and in Fig. 55, A, I give a picture of a section of a portion of one of the bars.

Hence we see that structurally as well as topographically the branchial bars of Ammocoetes justify their claim to be considered as the origin of the vertebrate cartilaginous framework.

ON THE STRUCTURE OF THE MUCO-CARTILAGE IN AMMOCOETES.

We can, however, go further than this, and ask how this cartilage itself is formed in Ammocoetes? The answer is most definite, most instructive and suggestive, for in all cases this particular kind of cartilage is formed from, or at all events in, a peculiar fibrous tissue, which was called by Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is distinguishable from other connective tissues, not only by its structural peculiarities, but also by its strong affinity for all dyes which differentiate mucoid or chondro-mucoid substances.

This muco-cartilage is thus described by Schneider:--The perichondrium in Ammocoetes is not confined to the true cartilaginous structures, but extends itself in the form of thin plates in definite directions. Between these plates of perichondrium a peculiar tissue (Fig. 56)--the muco-cartilage--exists, consisting of fibrillæ, whose direction is mainly at right angles to the planes of the perichondrial plates, with star-shaped cells in among them, and with the spaces between the fibrillæ filled up with a semi-fluid mass.

{131}From this tissue all the primitive cartilages which resemble the branchial bars are formed, either by the invasion of chondroblasts from the surrounding perichondrium, or by the proliferation and encapsulation of the cells of the muco-cartilage itself.

This very distinctive tissue--the muco-cartilage--is of very great importance in all questions of the origin of the skeletal tissues. In all descriptions of the skeletal tissues it has been practically disregarded until recent years when, besides my own observations, its distribution has been mapped out by Schaffer. Thus Parker, in his well-known description of the skeleton of the marsipobranch fishes, does not even mention its existence. Its importance is shown by its absolute disappearance at transformation and its non-occurrence in any of the higher vertebrates. It is entirely confined to the head-region, and its distribution there is most suggestive, for, as will be described fully later on, it forms a skeleton which both in structure and position resembles very closely the head-shields of cephalaspidian fishes. At the present part of my argument its more immediate interest lies in the method of tracing this tissue. For this purpose I made use of the micro-chemical reaction of thionin, a dye which, as shown by Hoyer, stains all mucin-containing substances a bright purple. Schaffer made use of a corresponding basophil stain, hæmalum. When stained with thionin, the matrix, or ground-substance of the branchial cartilages as well as the matrix or semi-fluid substance in which the fibrils of the muco-cartilaginous cells are embedded take on a deep purple colour, while the fibrous material of the cranial walls and other connective tissue strands, such as the perichondrium, are coloured light blue. Muco-cartilage, then, may be described as a peculiar form of connective tissue which differs from other connective tissue not only in its appearance but in {132}its chemical composition, for unlike white fibrous tissue it contains a large amount of mucin, and this tissue is the forerunner of the earliest cartilaginous vertebrate skeleton, the branchial bars of Ammocoetes.

The conclusions to which we are led by the study of the structure, position, and mode of origin of these primitive cartilages of Ammocoetes may be thus summed up:--

1. The immediate ancestor of the vertebrate must have possessed a peculiar fibrous tissue--the ground-substance of which stained deep purple with thionin--in which cartilage arose.

2. The cartilage so formed was not like hyaline cartilage, but resembled in a striking manner parenchymatous cartilage.

3. This cartilage was situated partly in two axial longitudinal bands, partly as transverse bars, which supported the branchial apparatus.

THE PROSOMATIC OR BASI-CRANIAL SKELETON OF AMMOCOETES.

Before searching for any evidence of a similar tissue in any invertebrate group, it is advisable to consider the other portion of the cartilaginous skeleton of Ammocoetes, which consists of the trabeculæ, parachordals and auditory capsules--the basi-cranial skeleton--and is composed of hard, not soft cartilage.

This basi-cranial skeleton represented in Fig. 53, B, is confined to the region of the notochord, the cranial walls being composed entirely of a white fibrous membrane. It is separated at first entirely from the sub-chordal portion of the branchial basket-work, and is composed of a foremost part, the trabeculæ (Tr.), and of a hindermost part, the parachordals (Pr.ch.), which are characterized by the attachment on each side of the large auditory capsule (Au.). In Ammocoetes the trabecular bars are continuous with the parachordals, the junction being marked by a small lateral projection on each side, which at transformation is seen to play an important part in the formation of the sub-ocular arch. The trabecular bar lies close against the notochord on each side up to its termination; it then bends away from the middle line and curves round until it meets its fellow on the opposite side, thus forming, as it were, the head of a racquet of which the notochord forms the splice in the handle. The strings of the racquet are represented by a thin membrane, in the centre of which the position of the infundibulum (Inf.) of the {133}brain can be clearly seen. In an earlier stage of Ammocoetes the two trabecular horns do not meet, but are separated by connective tissue, which afterwards becomes cartilaginous.

As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points are--the shape of the trabecular portion, diverging as it does around the infundibulum, and the presence on the parachordal portion of the two large auditory capsules.

These two points indicate, on the hypothesis that infundibulum and oesophagus are convertible terms, that two supporting structures of a cartilaginous nature must have existed in the ancestor of the vertebrate, the first of which surrounded the oesophagus, and the second was in connection with its auditory apparatus.

STRUCTURE OF THE HARD CARTILAGES.

The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of the soft, in so far that it consists of large cells with a comparatively small amount of intercellular substance. Schaffer, who has described it lately, considers that it is a nearer approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the usual sense of the term. Its peculiarities and its differences from the soft are especially well seen by its staining reactions. I have myself been particularly struck with the effect of picrocarmine or combined hæmatoxylin and picric acid {134}staining (Fig. 57). In the case of the soft cartilage the capsular substance stains respectively a brilliant red or blue, while that of the hard cartilage is coloured a deep yellow, so that the junction between the parachordals and the branchial cartilages is beautifully marked out. Then, again, with thionin, which gives so marked a reaction in the case of the soft cartilage, the hard cartilage of the auditory capsule is not stained at all, and in the trabeculæ the deep purple colour is confined to the mucoid cement-substance between the capsules, just as Schaffer has stated. The same kinds of reactions have been described by Schaffer: thus by double staining with hæmalum-eosin the hard cartilage stains red, the soft blue; and he points out that even with over-staining by hæmalum the auditory capsule remains colourless, just as I have noticed with thionin. He infers, precisely as I have done from the thionin reaction, that chondro-mucoid, which is so marked a constituent of the soft cartilage and of the muco-cartilage, is absent or present in but slight quantities in the hard cartilage. Similarly, he points out that double staining with tropoeolin-methyl-violet stains the hard cartilage a bright orange colour, and the soft cartilage a violet.

The evidence, then, shows clearly that a marked chemical difference exists between these two cartilages, which may be expressed by saying that the one contains very largely a basophil substance, which we may speak of as belonging to the class of chondro-mucoid substances, while the other contains mainly an oxyphil substance, probably a chondro-gelatine substance.

We may perhaps go further and attribute this difference of composition to a difference of origin; for whereas the soft cartilage is invariably formed in a special tissue, the muco-cartilage, which shows by its reaction how largely it is composed of a mucoid substance, the hard cartilage is certainly, in the case of the cartilage of the cranium where its origin has been clearly made out, formed in the membranous tissue of the cranium of Ammocoetes--i.e. in a tissue which stains light blue with thionin, and contains a gelatinous rather than a mucoid substratum.

The best opportunity of finding out the mode of origin of the hard cartilage is afforded at the time of transformation, when so much of this kind of cartilage is formed anew. Unfortunately, it is very difficult to obtain the early transformation stages, consequently we cannot be said to possess any really exhaustive and {135}definite account of how the new cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a more or less definite account of their formation, and the one striking impression left on the mind of the reader is how their descriptions vary. In one point only are they agreed, and in that I also agree with them, viz. the manner in which the new cranial walls are formed. Schaffer describes the process as the invasion of chondroblasts into the homogeneous fibrous tissue of the cranial walls. Such chondroblasts not only form the cartilaginous framework, but also assimilate the fibrous tissue which they invade, so that finally all that remains of the original fibrous matrix in which the cartilage was formed are these lines of cement-substance between the groups of cartilage cells, which, containing some basophil material, are marked out, as already mentioned (Fig. 57).

We may therefore conclude, from the investigation of Ammocoetes, that the front part of the basi-cranial skeleton arose as two trabecular bars, to which muscles were attached, situated bilaterally with respect to the central nervous system. These bars were composed of tendinous material with a gelatinous rather than a mucoid substratum, in which nests of cartilage-cells were formed, the cartilaginous material formed by these cells being of the hard variety, not staining with thionin, and staining yellow with picro-carmine, etc. By the increase of such nests and the assimilation of the intermediate fibrous material, the original fibro-cartilage was converted into the close-set semi-hyaline cartilage of the trabeculæ and auditory capsules, in which the fibrous material still marks out by its staining-reaction the limits of the cell-clusters.

Such I gather to be Schaffer's conclusions, and they are certainly borne out by my own and Miss Alcock's observations. As far as we have had an opportunity of observing at present, the first process at transformation appears to consist of the invasion of the fibrous tissue of the cranial wall by groups of cells which form nests of cells between the fibrous strands. These nests of cells form round themselves capsular material, and thus form cell-territories of cartilage, which squeeze out and assimilate the surrounding fibrous tissue, until at last all that remains of the original fibrous matrix is the lines of cement-substance which mark out the limits of the various cell-groups.

At present I am inclined to think that both soft and hard cartilage originate in a very similar manner, viz. by the formation of capsular {136}material around the invading chondroblasts, and that the difference in the resulting cartilage is mainly due to the difference in chemical composition of the matrix of the connective tissue which is invaded. Thus the difference may be formulated as follows:--

The hard cartilage is formed by the invasion of chondroblasts into a fibrous tissue, which contains a gelatinous rather than a mucoid substratum, in contradistinction to the soft cartilage which is formed, probably also by the invasion of chondroblasts, in a tissue--the muco-cartilage--which contains a specially mucoid substratum.

Such, then, is the very clearly defined starting-point of the vertebrate skeleton--two distinct formations of different histological and chemical structure,--the one forming a segmented branchial skeleton, the other a non-segmented basi-cranial skeleton.

THE CARTILAGINOUS SKELETON OF LIMULUS.

Among the whole of the invertebrates at present living on the earth, is there any sign of an internal cartilaginous skeleton that will give a direct clue to the origin of the primitive vertebrate skeleton? The answer to this question is most significant: only one animal among all those at present known possesses a cartilaginous skeleton, which is directly comparable with that of Ammocoetes, and here the comparison is very close--only one animal among the thousands of living invertebrate forms, and that animal is the only representative still surviving of the palæostracan group, which was the dominant race when the vertebrate first made its appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous skeleton (Fig. 53, A), made up of the same kind of cartilage as the branchial skeleton of Ammocoetes, confined to the mesosomatic or branchial region, just as in Ammocoetes, forming, as in Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are situated externally to the branchiæ, as in Ammocoetes. In addition this animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the entosternite or plastron situated, with respect to the oesophagus, similarly to the position of the trabeculæ with respect to the infundibulum in Ammocoetes. Moreover, the cartilaginous cells in this tissue differ from those in the branchial region, in precisely the same manner as the hard cartilage differs from the soft in Ammocoetes.

{137}This plastron, it is true, is found in other animals, all of which are members of the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean Apus--a strange primitive form, which is acknowledged to be the nearest representative of the Trilobita still living on the earth. None of these forms, however, possess any sign of an internal cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are all surviving types of the stage of organization which had been reached in the animal world when the vertebrate first appeared.

THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT CARTILAGE.

Searching through the literature of the histology of the cartilaginous tissues in invertebrate animals, to see whether any cartilage had been described similar to that seen in the branchial cartilages of Ammocoetes, and whether such cartilage, if found, arose in a fibrous tissue resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed muster for a drawing of the branchial cartilage of Ammocoetes. This clue I followed out in the manner described in my former paper in the Journal of Anatomy and Physiology, and mapped out the topography of this remarkable tissue.

Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in Fig. 59; and these appendages are very different to the ordinary locomotor appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. 58, consists mainly of a broad, basal part, which carries the gill-book on its under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist, not for locomotor purposes, but because they carry on each segment organs of special importance to the animal (see Chapter XI.). As is seen in Fig. 58, the basal parts of each pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma.

{138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF LIMULUS, TO SHOW THE ANTERIOR (A) AND THE POSTERIOR (B) SURFACES OF A MESOSOMATIC OR BRANCHIAL APPENDAGE.

In each figure the branchial cartilaginous bar, Br.C., has been exposed by dissection on one side. Ent., entapophysis; Ent.l., entapophysial ligament cut across; Br.C., branchial cartilaginous bar, which springs from the entapophysis; H., heart; P., pericardium; Al., alimentary canal; N., nerve cord; L.V.S., longitudinal venous sinus; Dv., dorso-ventral somatic muscle; Vp., veno-pericardial muscle.]

At the base of each of these appendages, where it is attached to the body of the animal, the external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked rod of cartilage (Br.C.), which extends from the body {139}of the animal well into each appendage. This bar of cartilage arises on each side from the corresponding entapophysis (Ent.), which is the name given to a chitinous spur which projects a short distance (Fig. 58, B) into the animal from the dorsal side, for the purpose of giving attachment to various segmental muscles. These entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of entapophyses, the number of segments in that region, in contradistinction to the prosomatic carapace, which gives no indication on its surface of the number of its components.

Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial ligament (Ent.l.) (Fig. 58), and in this tissue cartilage is formed. Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a concentration of such groups always takes place at each entapophysis, forming here a solid mass of cartilage, from which the massive cartilaginous bar of each branchial appendage arises.

Further, not only is this cartilage exactly similar to parenchymatous cartilage, as it occurs in the branchial cartilages of Ammocoetes, but also its matrix stains a brilliant purple with thionin in striking contrast to the exceedingly slight light-blue colour of the surrounding perichondrium. In its chemical composition it shows, as might be expected, that it is a cartilage containing a very large amount of some mucin-body.

THE MUCO-CARTILAGE OF LIMULUS.

The resemblance between this structure and that of the branchial bars of Ammocoetes does not end even here, for, as already mentioned, the cartilage originates in a peculiar connective tissue band, the entapophysial ligament, and this tissue bears the same relation in its chemical reactions to the ordinary connective tissue of Limulus, as muco-cartilage does to the white fibrous tissue of Ammocoetes. The white connective tissue of Limulus, as already stated, resembles that of the vertebrate more than does the connective tissue of any other invertebrate, and, similarly to that of Ammocoetes, does not stain, or gives only a light-blue tinge with thionin. The tissue of {140}the entapophysial ligament, on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a perfectly similar soft cartilage is born from it.

The branchial cartilages and the entapophysial ligaments are coloured blue, the branchiæ red. gl., generative and hepatic glands surrounding the central nervous system and passing into the base of the flabellum (fl.).]

One difference, however, exists between the branchial cartilages of these two animals; the innermost axial layer of the branchial bar of Limulus is very apt to contain a specially hard substance, apparently chalky in nature, so that it breaks up in sections, and gives the appearance of a broken-down spongy mass; if, however, the tissue is first placed in a solution of hydrochloric acid, it then cuts easily, and the whole tissue is seen to be of the same structure throughout, the main difference being that the capsular spaces in the axial region are much larger and much more free from cell-protoplasm than are those of the smaller younger cells near the periphery.

{141}I have attempted in Fig. 53 to represent this close resemblance between the segmented branchial skeleton of Limulus and of Ammocoetes, a resemblance so close as to reach even to minute details, such as the thinning out of the cartilage in the subchordal bands and entapophysial ligaments respectively between the places where the branchial bars come off.

The branchial cartilages and sub-chordal ligaments are coloured blue, the branchiæ red. gl., glandular substance surrounding the central nervous system and passing into the auditory capsule with the auditory nerve (VIII.).]

In Fig. 59 I have shown the prosoma and mesosoma of Limulus, and indicated the nerves to the appendages together with the mesosomatic cartilaginous skeleton.

In Fig. 60 I have drawn a corresponding picture of the prosomatic and mesosomatic region of Ammocoetes with the corresponding nerves {142}and cartilages. In this figure the animal is supposed to be slit open along the ventral mid-line and the central nervous system exposed.

THE PROSOMATIC SKELETON OF LIMULUS, COMPOSED OF HARD CARTILAGE.

The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a support for the base of the brain. This skeleton was composed of hard cartilage, and arose in white fibrous tissue containing gelatin rather than mucin.

Is there, then, any peculiar tissue of a cartilaginous nature in Limulus and its allies, situated in the prosomatic region, which is entirely separate from the branchial cartilaginous skeleton, which acts as a supporting internal framework, and contains a gelatinous rather than a mucoid substratum?

It is a striking fact, common to the whole of the group of animals to which our inquiries, deduced from the consideration of the structure of Ammocoetes, have, in every case, led us in our search for the vertebrate ancestor, that they do possess a remarkable internal semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which gives support to a large number of the muscles of that region; which is entirely independent of the branchial skeleton, and differs markedly in its chemical reactions from that cartilage, in that it contains a gelatinous rather than a mucoid substratum.

In Limulus it is a large, tough, median plate, fibrous in character, in which are situated rows and nests of cartilage-cells. The same structure is seen in the plastron of Hypoctonus, of Thelyphonus, and to a certainty in all the members of the scorpion group. Very different is the behaviour of this tissue to staining from that of the branchial region. No part of the plastron stains purple with thionin; it hardly stains at all, or gives only a very slight blue colour. In its chemical composition there is a marked preponderance of gelatin with only a slight amount of a mucin-body. In some cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules of the cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while the fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from the picric acid.

All the evidence points to the plastron as resembling the basi-cranial skeleton of Ammocoetes in its composition and in the origin {143}of its cells in a white fibrous tissue. What, then, is its topographical position? It is in all cases a median structure lying between the cephalic stomach and the infra-oesophageal portion of the central nervous system, and in all cases it possesses two anterior horns which pass around the oesophagus and the nerve-masses which immediately enclose the oesophagus (Fig. 61, A). These lateral horns, then, which lie laterally and slightly ventral to the central nervous system, and are called by Ray Lankester and Benham the sub-neural portion of the entosternite, are very nearly in exactly the position of the racquet-shaped head of the trabeculæ in Ammocoetes. It is easy to see that, with a more extensive growth of the nervous material dorsally, such lateral horns might be caused to take up a still more ventral position. Now, these two lateral horns of the plastron of Limulus are continued along its whole length so as to form two thickened lateral ridges, which are conspicuous on the flat surface of the rest of this median plate. In other cases, as in the Thelyphonidæ, the plastron consists mainly of these two lateral ridges or trabeculæ, as they might be called, and Schimkéwitsch, who more than any one else has made a comparative study of the entosternite, describes it as composed in these animals of two lateral trabeculæ crossed by three transverse trabeculæ. I myself can confirm his description, and give in Fig. 61, B, the appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia and part of the infra-oesophageal ganglia fill up the space Ph.; stretching over the rest of the infra-oesophageal mass is a transverse trabecula, which is very thin; then comes a space in which is seen the rest of the infra-oesophageal mass, and then the posterior part of the plastron, ventrally to which lies the commencement of the ventral nerve-cord.

Ph., position of pharynx.]

{144}In these forms, in which the central nervous system is more concentrated towards the cephalic end than in Limulus, the whole of the concentrated brain-mass is separated from the gut only by this thin transverse band of tissue. Judging, then, from the entosternite of Thelyphonus, it is not difficult to suppose that a continuation of the same growth of the brain-region of the central nervous system would cause the entosternite to be separated into two lateral trabeculæ, which would then take up the ventro-lateral position of the two trabeculæ of Ammocoetes.

On the other hand, it might be that two lateral trabeculæ, similar to those of Thelyphonus and situated on each side of the central nervous system, were the original form from which, by the addition of transverse fibres running between the gut and nervous system, the entosternite of Thelyphonus and of the scorpions, etc., was formed. From an extensive consideration of the entosternite in different animals, Schimkéwitsch has come to the conclusion that this latter explanation is the true one. He points out that the lateral trabeculæ can be distinguished from the transverse by their structure, being much more cellular and less fibrous, and the cell-cavities more rounded, or, as I should express it, the two lateral trabeculæ are more cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of structure and from embryological investigations, comes to the conclusion that the entosternite was originally composed of two parts--

1. A transverse muscle corresponding to the adductor muscle of the shell of certain crustaceans, such as Nebalia.

2. A pair of longitudinal mesodermic tendons, which may have been formed originally out of a number of segmentally arranged mesodermic tendons, and are crossed by the fibrils of the transverse muscular bundles.

These paired tendons of the entosternite he considers to correspond to the intermuscular tendons, situated lengthways, which are found in the ventral longitudinal muscles of most arthropods.

It is clear from these observations of Schimkéwitsch, that the essential part of the entosternite consists of two lateral trabeculæ, which were originally tendinous in nature and have become of the nature of cartilaginous tissue by the increase of cellular elements in the matrix of the tissue: these two trabeculæ function as supports for the attachment of muscles, which are specially attached at certain places. At these places transverse fibres belonging to some {145}of the muscular attachments cross between the two longitudinal trabeculæ, and so form the transverse trabeculæ.

I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly resembling those of Hypoctonus, and he also states that they are confined to the lateral portions of the entosternite.

From this evidence it is easy to see that that portion of the basi-cranial skeleton known as the trabeculæ may have originated from the formation of cartilage in the plastron or entosternite of a palæostracan animal. Such an hypothesis immediately suggests valuable clues as to the origin of the cranium and of the rest of the basi-cranial skeleton--the parachordals and the auditory capsules. The former would naturally be a dorsal extension of the more membranous portion of the plastron, in which, equally naturally, cartilaginous tissue would subsequently develop; and the reason why it is impossible to reduce the cranium into a series of segments would be self-evident, for even though, as Schimkéwitsch thinks, the plastron may have been originally segmented, it has long lost all sign of segmentation. The latter would be derived from a second entosternite of the same nature as the plastron, but especially connected with the auditory apparatus of the invertebrate ancestor. The following out of these two clues will be the subject of a future chapter.

In our search, then, for a clue to the origin of the skeletal tissues of the vertebrate we see again that we are led directly to the palæostracan stock on the invertebrate side and to the Cyclostomata on that of the vertebrate; for in Limulus, the only living representative of the Palæostraca, and in Limulus alone, we find a skeleton marvellously similar to the earliest vertebrate skeleton--that found in Ammocoetes. Later on I shall give reasons for the belief that the earliest fishes so far found, the Cephalaspidæ, etc., were built up on the same plan as Ammocoetes, so that, in my opinion, in Limulus and in Ammocoetes we actually possess living examples allied to the ancient fauna of the Silurian times.

{146}SUMMARY.

The skeleton considered in this chapter is not the notochord, but that composed of cartilage. The tracing downwards of the vertebrate bony and cartilaginous skeleton to its earliest beginnings leads straight to the skeleton of the larval lamprey (Ammocoetes), in which vertebræ are not yet formed, but the cranial and branchial skeleton is well marked.

The embryological and phylogenetic histories are in complete unison to show that the cranial skeleton is older than the spinal, and this primitive branchial skeleton is also in harmony with the laws of evolution, in that its structure, even in the adult lamprey (Petromyzon), never gets beyond the stage characteristic of embryonic cartilage in the higher vertebrates.

The simplest and most primitive skeleton is that found in Ammocoetes and consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton.

The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the simplest kind--the trabeculæ and the parachordals with their attached auditory capsules, just as the embryology of the higher vertebrates teaches us must be the case. There in the free-living, still-existent Ammocoetes we find the manifest natural outcome of the embryological history in the shape of simple trabeculæ and parachordals, from which the whole complicated basi-cranial skeleton of the higher vertebrates arose.

The mesosomatic skeleton, which is formed before the prosomatic, consisted, in the first instance, of simple branchial bars segmentally arranged, which were connected together by a longitudinal subchordal bar, situated laterally on each side of the notochord. These simple branchial bars later on form the branchial basket-work, which forms an open-work cage within which the branchiæ are situated.

The cartilages which compose these two skeletons respectively are markedly different in chemical constitution, in that the first (hard cartilage) is mainly composed of chondro-gelatin, the second (soft cartilage) of chondro-mucoid material.

The same kind of difference is seen in the two kinds of connective tissue which are the forerunners of these two kinds of cartilage. Thus, the cranial walls in Ammocoetes are formed of white fibrous tissue, an essentially gelatin-containing tissue; at transformation these are invaded by chondro-blasts and the cartilaginous cranium, formed of hard cartilage, results. On the other hand, the forerunner of the branchial soft cartilage is a very striking and peculiar kind of connective tissue loaded with mucoid material, to which the name muco-cartilage has been given.

The enormous interest of this muco-cartilage consists in the fact that it forms very well-defined plates of tissue, entirely confined to the head-region, which are not found in any higher vertebrate, not even in the adult form Petromyzon, for every scrap of the tissue as such disappears at transformation.

It is this evidence of primitive non-vertebrate tissues, which occur in the larval but not in the adult form, which makes Ammocoetes so valuable for the investigation of the origin of vertebrates.

The evidence, then, is extraordinarily clear as to the beginnings of the vertebrate skeletal tissues.

{147}In the invertebrate kingdom true cartilage occurs but scantily. There is a cartilaginous covering of the brain of cephalopods. It is never found in crabs, lobsters, bees, wasps, centipedes, butterflies, flies, or any of the great group of Arthropoda, except, to a slight extent, in some members of the scorpion group, and more fully in one single animal, the King-crab or Limulus: a fact significant of itself, but still more so when the nature of the cartilage and its position in the animal is taken into consideration, for the identity both in structure and position of this internal cartilaginous skeleton with that of Ammocoetes is extraordinarily great.

Here, in Limulus, just as in Ammocoetes, an internal cartilaginous skeleton is found, composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As in Ammocoetes, the latter consists of simple branchial bars, segmentally arranged, which are connected together on each side by a longitudinal ligament containing cartilage--the entapophysial ligament. This cartilage is identical in structure and in chemical composition with the soft cartilage of Ammocoetes, and, as in the latter case, arises in a markedly mucoid connective tissue. The former, as in Ammocoetes, consists of a non-segmental skeleton, the plastron, composed of a white fibrous connective tissue matrix, an essentially gelatin-containing tissue, in which are found nests of cartilage cells of the hard cartilage variety.

This remarkable discovery of the branchial cartilaginous bars of Limulus, together with that of the internal prosomatic plastron, causes the original difficulty of deriving an animal such as the vertebrate from an animal resembling an arthropod to vanish into thin air, for it shows that in the past ages when the vertebrates first appeared on the earth, the dominant arthropod race at that time, the members of which resembled Limulus, had solved the question; for, in addition to their external chitinous covering, they had manufactured an internal cartilaginous skeleton. Not only so, but that skeleton had arrived, both in structure and position, exactly at the stage at which the vertebrate skeleton starts.

What the precise steps are by which chitin-formation gives place to chondrin-formation are not yet fully known, but Schmiedeberg has shown that a substance, glycosamine, is derivable from both these skeletal tissues, and he concludes his observations in the following words: "Thus, by means of glycosamine, the bridge is formed which connects together the chitin of the lower animals with the cartilage of the more highly organized creations."

The evidence of the origin of the cartilaginous skeleton of the vertebrate points directly to the origin of the vertebrate from the Palæostraca, and is of so strong a character that, taken alone, it may almost be considered as proof of such origin.

{148}CHAPTER IV

THE EVIDENCE OF THE RESPIRATORY APPARATUS

Branchiæ considered as internal branchial appendages.--Innervation of branchial segments.--Cranial region older than spinal.--Three-root system of cranial nerves, dorsal, lateral, ventral.--Explanation of van Wijhe's segments.--Lateral mixed root is appendage-nerve of invertebrate.--The branchial chamber of Ammocoetes.--The branchial unit, not a pouch but an appendage.--The origin of the branchial musculature.--The branchial circulation.--The branchial heart of the vertebrate.--Not homologous with the systemic heart of the arthropod.--Its formation from two longitudinal venous sinuses.--Summary.

The respiratory apparatus in all the terrestrial vertebrates is of the same kind--one single pair of lungs. These lungs originate as a diverticulum of the alimentary canal. On the other hand, the aquatic vertebrates breathe by means of a series of branchiæ, or gills, which are arranged segmentally, being supported by the segmental branchial cartilaginous bars, as already mentioned in the last chapter.

The transition from the gill-bearing to the lung-bearing vertebrates is most interesting, for it has been proved that the lungs are formed by the modification of the swim-bladder of fishes; and in a group of fishes, the Dipnoi, or lung-fishes, of which three representatives still exist on the earth, the mode of transition from the fish to the amphibian is plainly visible, for they possess both lungs and gills, and yet are not amphibians, but true fishes. But for the fortunate existence of Ceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa, it would have been impossible from the fossil remains to have asserted that any fish had ever existed which possessed at the same moment of time the two kinds of respiratory organs, although from our knowledge of the development of the amphibian we might have felt sure that such a transitional stage must have existed. Unfortunately, there is at present no likelihood of any corresponding transitional stage being discovered {149}living on the earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should simultaneously exist in a functional condition; still it seems to me that even if Ceratodus, Lepidosiren, and Protopterus had ceased to exist on the earth, yet the facts of comparative anatomy, together with our conception of evolution as portrayed in the theory of natural selection, would have forced us to conclude rightly that the amphibian stage in the evolution of the vertebrate phylum was preceded by fishes which possessed simultaneously lungs and gills.

In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found in Ammocoetes, was shown to correspond in a marvellous manner to the cartilaginous skeleton of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the consideration of the necessary consequences which logically follow upon the supposition that the branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of Ammocoetes.

INTERNAL BRANCHIAL APPENDAGES.

Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes are confined to the branchial region, their homology of necessity implies an homology of the two branchial regions, and leads directly to the conclusion that the branchiæ of the vertebrate were derived from the branchiæ of the arthropod, a conclusion which, according to the generally accepted view of the origin of the respiratory region in the vertebrate, is extremely difficult to accept; for the branchiæ of Limulus and of the Arthropoda in general are part of the mesosomatic appendages, while the branchiæ of vertebrates are derived from the anterior part of the alimentary canal. This conclusion, therefore, implies that the vertebrate has utilized in the formation of the anterior portion of its new alimentary canal the branchial appendages of the palæostracan ancestor.

{150}[Illustration: FIG. 62.--Eurypterus.

The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates. M., metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.]

Let us consider dispassionately whether such a suggestion is a priori so impossible as it at first appears. One of the principles of evolution is that any change which is supposed to have taken place in the process of formation of one animal or group of animals from a lower group must be in harmony with changes which are known to have occurred in that lower group. On the assumption, therefore, that the vertebrate branchiæ represent the branchial portion of the arthropod mesosomatic appendages which have sunk in and so become internal, we ought to find that in members of this very group such inclusion of branchial appendages has taken place. This, indeed, is exactly what we do find, for in all the scorpion tribe, which is acknowledged to be closely related to Limulus, there are no external mesosomatic appendages, but in all cases these appendages have sunk into the body, have disappeared as such, and retained only the vital part of them--the branchiæ. In this way the so-called lung-books of the scorpion are formed, which are in all respects homologous with the branchiæ or gill-books of Limulus. Now, as already mentioned, the lords of creation in the palæostracan times were the sea-scorpions, which, as is seen in Fig. 62, resembled the land-scorpions of the present day in the entire absence of any external appendages on the segments of the mesosomatic region. As they lived in the sea, they must have breathed with gills, and those branchial appendages must have been internal, just as in the land-scorpions of the present time. Indeed, markings have been found on the internal side of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the present time no indication of gill-slits has been found, and we cannot say with certainty how these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion (Palæophonus Hunteri), closely resembling the modern scorpion, has been found, which, as Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it {151}breathed is unknown; it shows no signs of stigmata, such as exist in the scorpion of to-day.

Although we possess as yet no certain knowledge of the position of the gill-openings in these ancient scorpion-like forms, what we can say with certainty--and that is the important fact--is, that at the time when the vertebrates appeared, a very large number of the dominant arthropod race possessed internally-situated branchiæ, which had been directly derived from the branchiæ-bearing appendages of their Limulus-like kinsfolk.

This abolition of the branchiæ-bearing appendages as external organs of locomotion, with the retention of the important branchial portion of the appendage as internal branchiæ, is a very important suggestion in any discussion of the way vertebrates have arisen from arthropods; for, if the same principle is of universal application, it leads directly to the conclusion that whenever an appendage possesses an organ of vital importance to the animal, that organ will remain, even though the appendage as such completely vanishes. Thus, as will be shown later, special sense-organs such as the olfactory remain, though the animal no longer possesses antennæ; the important excretory organs, the coxal glands, and important respiratory organs, the branchiæ, are still present in the vertebrate, although the appendages to which they originally belonged have dwindled away, or, at all events, are no longer recognizable as arthropod appendages.

INNERVATION OF BRANCHIAL SEGMENTS.

Passing from a priori considerations to actual facts, it is advisable to commence with the innervation of the branchial segments; for, seeing that the foundation of the whole of this comparative study of the vertebrate and the arthropod is based upon the similarity of the two central nervous systems, it follows that we must look in the first instance to the innervation of any organ or group of organs in order to find out their relationship in the two groups of animals.

The great characteristic of the vertebrate branchial organs is their segmental arrangement and their innervation by the vagus group of nerves, i.e. by the hindermost group of the cranial segmental nerves. These cranial nerves are divided by Gegenbaur into two great groups--an anterior group, the trigeminal, which supplies the muscles of mastication, and a posterior group, the vagus, which is essentially {152}respiratory in function. Of these two groups, I will consider the latter group first.

In Limulus the great characteristic of the branchial region is its pronounced segmental arrangement, each pair of branchial appendages belonging to a separate segment. This group of segments forms the mesosoma, and these branchial appendages are the mesosomatic appendages. Anterior to them are the segments of the prosoma, which bear the prosomatic or locomotor appendages. The latter are provided at their base with gnathites or masticating apparatus, so that the prosomatic group of nerves, like the trigeminal group in the vertebrate, comprises essentially the nerves subserving the important function of mastication. As already pointed out, the brain-region of the vertebrate is comparable to the supra-oesophageal and infra-oesophageal ganglia of the invertebrate, and it has been shown (p. 54) how, by a process of concentration and cephalization, the foremost region of the infra-oesophageal ganglia becomes the prosomatic region, and is directly comparable to the trigeminal region in the vertebrate; while the hindermost region is formed from the concentration of the mesosomatic ganglia, and is directly comparable to the medulla oblongata, i.e. to the vagus region of the vertebrate brain.

As far, then, as concerns the centres of origin of these two groups of nerves and their exits from the central nervous system, they are markedly homologous in the two groups of animals.

COMPARISON OF THE CRANIAL AND SPINAL SEGMENTAL NERVES.

It has often been held that the arrangements of the vertebrate nervous system differ from those of other segmented animals in one important particular. The characteristic of the vertebrate is the origin of every segmental nerve from two roots, of which one contains the efferent fibres, while the other possesses a sensory ganglion, and contains only afferent fibres. This arrangement, which is found along the whole spinal cord of all vertebrates, is not found in the segmental nerves of the invertebrates; and as it is supposed that the simpler arrangement of the spinal cord was the primitive arrangement from which the vertebrate central nervous system was built up, it is often concluded that the animal from which the vertebrate arose must have possessed a series of nerve-segments, from each of which there arose bilaterally ventral (efferent) and dorsal (afferent) roots.

{153}Now, the striking fact of the vertebrate segmental nerves consists in this, that, as far as their structure and the tissues which they innervate are concerned, the cranial segmental nerves are built up on the same plan as the spinal; but as far as concerns their exit from the central nervous system they are markedly different. A large amount of ingenuity, it is true, has been spent in the endeavour to force the cranial nerves into a series of segmental nerves, which arise in the same way as the spinal by two roots, of which the ventral series ought to be efferent and the dorsal series afferent, but without success. We must, therefore, consider the arrangement of the cranial segmental nerves by itself, separately from that of the spinal nerves, and the problem of the origin of the vertebrate segmental nerves admits of two solutions--either the cranial arrangement has arisen from a modification of the spinal, or the spinal from a simplification of the cranial. The first solution implies that the spinal cord arrangement is older than the cranial, the second that the cranial is the oldest.

In my opinion, the evidence of the greater antiquity of the cranial region is overwhelming.

The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial region, for we see how, quite early in the development, the head is folded off, and the organs in that region thereby completed at a time when the spinal region is only at an early stage of development. We see how the first of the trunk somites is formed just posteriorly to the head region, and then more and more somites are formed by the addition of fresh segments posteriorly to the one first formed. We see how, in Ammocoetes, the first formed parts of the skeleton are the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of more and more spinal segments, organs, such as the heart, which were originally in the head, travel down, and the vagus and lateral-line nerves reach their ultimate destination. Again, we see that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the only shifting being in the spino-occipital region, in fact, at the junction of the cranial and spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in different {154}groups of animals, but that even in the same animal great variations are found, especially in the manner of formation of the limb-plexuses. Such marked meristic variation in the spinal nerves, in contrast to the fixed character of the cranial nerves, certainly points to a more recent formation of the former nerves.

Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog, there is evidence to show that the embryo is derived from two definite centres of growth: the first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the anterior end of the embryo; the second, one which gives rise to the growth in length of the embryo. This secondary area of proliferation coincides with the area of the primitive streak, and he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the anterior end of the primitive streak, and subsequently is found at the level of the most anterior pair of somites.

He then goes on to say--

"From these specimens it seems clear that all those parts in front of the first pair of mesoblastic somites--that is to say, the heart, the brain and medulla oblongata, the olfactory, optic, auditory organs and foregut--are developed from that portion of the unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest of the embryo is formed by the activity of the primitive streak area."

In other words, the secondary area of growth, i.e. the primitive streak area, includes the whole of the spinal cord region, while the older primary centre of growth is coincident with the cranial region.

In searching, then, for the origin of the segmental nerves, we must consider the type on which the cranial nerves are arranged rather than that of the spinal nerves.

The first striking fact occurs at the spino-occipital region, where the spinal cord merges into the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to three distinct roots, not two--a dorsal root, a ventral root, and a lateral root. This third root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system {155}continues as the roots of the glossopharyngeal nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles Bell pointed out in 1830, a system of nerves concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication, deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary body-muscles concerned with locomotion and equilibration. Also the centres from which these motor nerves arise are well defined, and form cell-masses in the central nervous system, quite separate from those which give origin to somatic muscles.

This original idea of Charles Bell, after having been ignored for so long a time, is now seen to be a very right one, and it is an extraordinary thing that his enunciation of the dual nature of the spinal roots, which was, to his mind, of subordinate importance, should so entirely have overshadowed his suggestion, that in addition to the dorsal and ventral roots, a lateral system of nerves existed, which were not exclusively sensory or exclusively motor, but formed a separate system of respiratory nerves.

Further, anatomists divide the striated muscles of the body into two great natural groups, characterized by a difference of origin and largely by a difference of appearance. The one set is concerned with the movements of internal organs, and is called visceral, the other is derived from the longitudinal sheet of musculature which forms the myotomes of the fish, and has been called parietal or somatic. The motor nerves of these two sets of muscles correspond with the lateral or respiratory and ventral roots respectively.

Finally, it has been shown that the segments of which a vertebrate is composed are recognizable in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a dorsal and ventral series of commencing muscular segments. Of these the anterior segments of the dorsal series give origin to the striated muscles of the eye which are innervated by the IIIrd (oculomotor), IVth (trochlearis), and VIth (abducens) nerves, while the posterior segments give origin to the {156}muscles from the cranium to the shoulder-girdle, innervated by the XIIth (hypoglossal) nerve. The ventral series of segments give origin to the musculature supplied by the trigeminal, facial, glossopharyngeal, and vagus nerves.

Also, the afferent or sensory nerves of the skin over the whole of this head-region are supplied by the trigeminal nerve, while the afferent nerves to the visceral surfaces are supplied by the vagus, glossopharyngeal and facial nerves.

In van Wijhe's original paper he arranged the segments belonging to the cranial nerves in the following table:--

---------+--------------------------------- Segments.| Ventral nerve-roots and muscles | derived from myotomes. ---------+---------+----------------------- 1 | III. |M. rectus superior, | | m. rectus | | internus, m. | | rectus inferior, | | m. obliquus inferior 2 | IV. |M. obliquus | | superior 3 | VI. |M. rectus externus 4 | -- | -- 5 | -- | -- 6 | -- | -- 7 | XII. |} Muscles from 8 | XII. |} cranium to 9 | XII. |}shoulder-girdle ---------+---------+-----------------------

---------+------------------+-------------------------------- Segments.| Visceral clefts.| Dorsal nerve-roots and muscles. | | ---------+------------------+--------------+----------------- 1 | | V. N. op- | | | thalmicus | | | profundus | 2 | | V. | Masticating | 1st Mandibular | | muscles. 3 | | VII.1 |{Facial muscles | | |{(VIII. is dorsal | 2nd {Hyoid1 | VII.2 |{branch of VII.) 4 | {Hyoid2 | | 5 |3rd 1st Branchial | IX. | 6 |4th 2nd " | X.1 | 7 |{5th 3rd " | X.2 |Branchial and 8 |{6th 4th " | X.3} |visceral muscles. 9 |{7th 5th " | X.4 | ---------+------------------+--------------+-----------------

As is seen in the table, van Wijhe attempts to arrange the cranial segmental nerves into dorsal and ventral roots, in accordance with the arrangement in the spinal region. In order to do this he calls the Vth, VIIth, IXth, and Xth nerves dorsal roots, although they are not purely sensory nerves, but contain motor fibres as well.

It is not accidental that he should have picked out for his dorsal roots the very nerves which form Charles Bell's lateral series of roots, inasmuch as this system of lateral roots, apart from dorsal and ventral roots, really is, as Charles Bell thought, an important separate system, dependent upon a separate segmentation in the embryo of the musculature supplied by these roots. This segmentation may receive the name of visceral or splanchnic in contradistinction to somatic, since all the muscles without exception belong to the visceral group of striated muscles.

{157}These observations of van Wijhe lead directly to the following conclusion. In the cranial region there is evidence of a double set of segments, which may be called somatic and splanchnic. The somatic segments, consisting of the outer skin and the body musculature, are doubly innervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve. But the splanchnic segments are innervated by single roots, the vagus or Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth nerve, and trigeminal or Vth nerve, which are mixed, containing both sensory and motor fibres, thus differing markedly from the arrangement of the spinal nerves.

From this sketch it follows that the arrangement seen in the spinal cord, would result from the cranial arrangement if this third system of lateral roots were left out. Further, since the cranial system is the oldest, we must search in the invertebrate ancestor for a tripartite rather than a dual system of nerve-roots for each segment; a system composed of a dorsal root supplying only the sensory nerves of the skin-surfaces, a lateral mixed root supplying the system connected with respiration with both sensory and motor fibres, and a ventral root supplying the motor nerves to the body-musculature.

COMPARISON OF THE APPENDAGE NERVES OF LIMULUS AND BRANCHIPUS TO THE LATERAL ROOT SYSTEM OF THE VERTEBRATE.

If the argument used so far is correct, and this tripartite system of nerve-roots, as seen in the cranial nerves of the vertebrate, really represents the original scheme of innervation in the palæostracan ancestor, then it follows that each segment of Limulus ought to be supplied by three nerves--(1), a sensory nerve supplying its own portion of the skin-surface of the prosomatic and mesosomatic carapaces; (2), a lateral mixed nerve supplying exclusively the appendage of the segment, for the appendages carry the respiratory organs; and (3), a motor nerve supplying the body-muscles of the segment.

It is a striking fact that Milne-Edwards describes the nerve-roots in exactly this manner. The great characteristic of the nerve-roots {158}in Limulus as in other arthropods is the large appendage-nerve, which is always a mixed nerve; in addition, there is a system of sensory nerves to the prosomatic and mesosomatic carapaces, called by him the epimeral nerves, which are purely sensory, and a third set of roots which are motor to the body-muscles, and possibly also sensory to the ventral surface between the appendages.

Moreover, just as in the vertebrate central nervous system the centres of origin of the motor nerves of the branchial segmentation are distinct from those of the somatic segmentation, so we find, from the researches of Hardy, that a similar well-marked separation exists between the centres of origin of the motor nerves of the appendages and those of the somatic muscles in the central nervous system of Branchipus and Astacus.

In the first place, he points out that the nervous system of Branchipus is of a very primitive arthropod type; that it is, in fact, as good an example of an ancient type as we are likely to find in the present day; a matter of some importance in connection with my argument, since the arthropod ancestor of the vertebrate, such as I am deducing from the study of Ammocoetes, must undoubtedly have been of an ancient type, more nearly connected with the strange forms of the trilobite era than with the crabs and spiders of the present day.

His conclusions with respect to Branchipus may be tabulated as follows:--

1. Each ganglion of the ventral chain is formed mainly for the innervation of the appendages.

2. Each ganglion is divided into an anterior and posterior division, which are connected respectively with the motor and sensory nerves of the appendages.

3. The motor nerves of the appendages arise as well-defined axis-cylinder processes of nerve-cells, which are arranged in well-defined groups in the anterior division of the ganglion.

4. A separate innervation exists for the muscles and sensory surfaces of the trunk. The trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes to them from certain cells on the dorsal surface of each lateral half of the ganglion. These cell-groups are therefore very distinct from those which give origin to the motor {159}appendage-nerves, and, moreover, are not confined to the ganglion, but extend for some distance into the interganglionic region of the nerve-cords which connect together the ganglia of the ventral chain.

Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion that in such a primitive arthropod type as my theory postulates, each segment was supplied with separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted entirely to the innervation of the pair of appendages; that also, in the central nervous system, the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two separate groups of nerve-cells. These nerve-cells in the one case were aggregated into well-defined groups, which formed the centres for the motor nerves of the markedly segmented muscles of the appendages, and in the other case formed a system of more diffused cells, less markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented somatic muscles.

Such an arrangement suggests that in the ancient arthropod type a double segmentation existed, viz. a segmentation of the body, and a segmentation due to the appendages. Undoubtedly, the segments originally corresponded absolutely as in Branchipus, and every appendage was attached to a well-defined separate body-segment. In, however, such an ancient type as Limulus, though the segmentation may be spoken of as twofold, yet the number of segments in the prosomatic and mesosomatic regions are much more clearly marked out by the appendages than by the divisions of the soma; for, in the prosomatic region such a fusion of somatic segments to form the tergal prosomatic carapace has taken place that the segments of which it is composed are visible only in the young condition, while in the mesosomatic region the separate somatic segments, though fused to form the mesosomatic carapace, are still indicated by the entapophysial indentations.

Clearly, then, if the mesosomatic branchial appendages of forms related to Limulus were reduced to the branchial portion of the appendage, and that branchial portion became internal, just as is known to be the case in the scorpion group, we should obtain an animal in which the mesosomatic region would be characterized by a segmentation predominantly branchial, which might be termed, as in vertebrates, the branchiomeric segmentation, but yet would show {160}indications of a corresponding somatic or mesomeric segmentation. The nerve supply to these segments would consist of--

1. The epimeral purely sensory nerves to the somatic surface, equivalent in the vertebrate to the ascending root of the trigeminal.

2. The mixed nerves to the internal branchial segments, equivalent in the vertebrate to the vagus, glossopharyngeal, and facial.

3. The motor nerves to the somatic muscles, equivalent in the vertebrate to the original nerve-supply to the somatic muscles belonging to these segments, i.e. to the muscles derived from van Wijhe's 4th, 5th, and 6th somites.

Further, the centres of origin of these appendage-nerves would form centres in the central nervous system separate from the centres of the motor nerves to the somatic muscles, just as the centres of origin of the motor parts of the facial, vagus, and glossopharyngeal nerves form groups of cells quite distinct from the centres for the hypoglossal, abducens, trochlear, and oculomotor nerves.

In fact, if the vertebrate branchial nerves are looked upon as the descendants of nerves which originally supplied branchial appendages, then every question connected with the branchial segmentation, with the origin and distribution of these nerves, receives a simple and adequate solution--a solution in exact agreement with the conclusion that the vertebrate arose from a palæostracan ancestor.

It would, therefore, be natural to expect that the earliest fishes breathed by means of branchial appendages situated internally, and that the evidence for such appendages would be much stronger in them than in more recent fishes.

Although we know nothing of the nature of the respiratory apparatus in the extinct fishes of Silurian times, we have still living, in the shape of Ammocoetes, a possible representative of such types. If, then, we find, as is the case, that the respiratory apparatus of Ammocoetes differs markedly from that of the rest of the fishes, and, indeed, from that of the adult form or Petromyzon, and that that very difference consists in a greater resemblance to internal branchial appendages in the case of Ammocoetes, then we may feel that the proof of the origin of the branchial apparatus of the vertebrate from the internal branchial appendages of the invertebrate has gained enormously.

{161}THE RESPIRATORY CHAMBER OF AMMOCOETES.

In order to make clear the nature of the branchial segments in Ammocoetes, I have divided the head-part of the animal by means of a longitudinal horizontal section into halves--ventral and dorsal--as shown in Figs. 63 and 64. These figures are each a combination of a section and a solid drawing. The animal was slit open by a longitudinal section in the neighbourhood of the gill-slits, and each half was slightly flattened out, so as to expose the ventral and dorsal internal surfaces respectively. The structures in the cut surface were drawn from one of a series of horizontal longitudinal sections taken through the head of the animal. These figures show that the head-region of Ammocoetes consists of two chambers, the contents of which are different. In front, an oral or stomodæal chamber, which contains the velum and tentacles, is enclosed by the upper and lower lips, and was originally separated by a septum from the larger respiratory chamber, which contains the separate pairs of branchiæ. A glance at the two drawings shows clearly that Rathke's original description of this chamber is the natural one, for he at that time, looking upon Ammocoetes branchialis as a separate species, described the branchial chamber as containing a series of paired gills, with the gill-openings between consecutive gills. His branchial unit or gill, therefore, was represented by each of the so-called diaphragms, which, as seen in Figs. 63, 64, are all exactly alike, except the first and the last. Any one of these is represented in section in Fig. 65, and represents a branchial unit in Rathke's view and in mine. Clearly, it may be described as a branchial appendage which projects into an open pharyngeal chamber, so that the series of such appendages divides the chamber into a series of compartments, each of which communicates with the exterior by means of a gill-slit, and with each other by means of the open space between opposing appendages.

Each of these appendages possesses its own cartilaginous bar (Br. cart.), as explained in Chapter III.; each possesses its own branchial or visceral muscles (coloured blue in Figs. 63 and 64), separated absolutely from the longitudinal somatic muscles (coloured dark red in Figs. 63 and 64) by a space (Sp.) containing blood and peculiar fat-cells, etc. Each possesses its own afferent branchial blood-vessel from the ventral aorta, and its own efferent vessel to the dorsal aorta (Fig. 65, a. br. and v. br.). Each possesses its own segmental nerve, which supplies its own branchial muscles and no others with motor fibres, and sends sensory fibres to the general surface of each appendage, as also to the special sense-organs in the shape of the epithelial pits (S., Fig. 65) arranged along the free edges of the diaphragms; each of these nerves possesses its own ganglion--the epibranchial ganglion.

{162}[Illustration: FIG. 63.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.

Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching. Tent., tentacles; Tent. m.c., muco-cartilage of tentacles; Vel. m.c., muco-cartilage of the velum; Hy. m.c., muco-cartilage of the hyoid segment; Ps. br., pseudo-branchial groove; Br. cart., branchial cartilages; Sp., space between somatic and splanchnic muscles; Th. op., orifice of thyroid; H., heart.]

{163}[Illustration: FIG. 64.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.

Tr., trabeculæ; Pit., pituitary space; Inf., infundibulum; Ser., median serrated flange of velar folds.]

{164}[Illustration: FIG. 65.--SECTION THROUGH BRANCHIAL APPENDAGE OF AMMOCOETES.

br. cart., branchial cartilage; v. br., branchial vein; a. br., branchial artery; b.s., blood-spaces; p., pigment; S., sense-organ; c., ciliated band; E., I., external and internal borders; m. add., adductor muscle; m.c.s., striated constrictor muscle; m.c.t., tubular constrictor muscle; m. and m.v., muscles of valve.]

br. cart., branchial cartilage; v.br., branchial vein; b.s., blood-spaces formed by branchial artery; P., pigment; m1, posterior entapophysio-branchial muscle; m2, anterior entapophysio-branchial muscle; m3_, external branchial muscle.]

The work of Miss Alcock has shown that the segmental branchial nerve supplies solely and absolutely such an appendage or branchial {165}segment, and does not supply any portion of the neighbouring branchial segments. The nerve-supply in Ammocoetes gives no countenance to the view that the original unit was a branchial pouch, the two sides of which each nerve supplied, but is strong evidence that the original unit was a branchial appendage, which was supplied by a single nerve with both motor and sensory fibres.

Any observer having before him only this picture of the respiratory chamber of Ammocoetes, upon which to base his view of a vertebrate respiratory chamber, would naturally look upon the branchial unit of a vertebrate as a gilled appendage projecting into the open cavity of the anterior part of the alimentary canal or pharynx. This is not, however, the usual conception. The branchial unit is ordinarily described as a gill-pouch, which possesses two openings or slits, an internal one into the lumen of the alimentary canal, and an external one into the surrounding medium. This view is based upon embryological evidence of the following character:--

The alimentary canal of all vertebrates forms a tube stretching the whole length of the animal; the anterior part of this tube becomes pouched on each side at regular intervals, and the walls of each pouch becoming folded form the respiratory surfaces or gills. The openings of these separate pouches into the central lumen of the gut form the internal gill-pouch openings; the other extremity of the pouch approaches the external surface of the animal, and finally breaks through to form a series of external gill-pouch openings.

From the mesoblastic tissue, between each gill-pouch, there is formed a supporting cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting framework for the internal gill-slit, so that the gills are situated externally to them; the more primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in position, so that the gills are situated internally to them.

From this description of the mode of formation of the respiratory apparatus in water-breathing vertebrates the conception has arisen of the gill-pouch as the branchial unit, a conception which is absolutely removed from all idea of a branchial unit such as is found in an arthropod, viz. an appendage.

This conception of spaces as units pervades the whole of embryology, and is the outcome of the gastrula theory--a theory which {166}teaches that all animals above the Protozoa are derived from a form which by invagination of its external surface formed an internal cavity or primitive gut. From pouches of this gut other cavities were said to be formed, called coelomic cavities, and thus arose the group of coelomatous animals. To speak of the developmental history of animals in terms of spaces; to speak of the atrophy of a cavity as though such a thing were possible, is, to my mind, the wrong way of looking at the facts of anatomy. It resembles the description of a net as a number of holes tied together with string, which is not usually considered the best method of description.

There are two ways in which a series of pouches can be formed from a simple tube without folding, either by a thinning at regular intervals of the original tissue surrounding the tube, or by the ingrowth into the tube of the surrounding tissue at regular intervals, thus--

A, by the thinning of the mesoblast at intervals. B, by the ingrowth of mesoblast at intervals. Ep., epiblast; Mes., mesoblast; Hy., hypoblast.]

In the first case (A) the formation of a pouch is the significant act, and therefore the branchial segments might be expressed in terms of pouches. In the second case (B) the formation of a pouch is {167}brought about in consequence of the ingrowth of the mesoblastic tissues at intervals; here, although the end-result is the same as in the first case, the pouch-formation is only secondary, the true branchial unit is the mesoblastic ingrowth.

The evidence all points directly to the second method of formation. Thus Shipley, in his description of the development of the lamprey, says--

"The gill-slits appear to me to be the result of the ventral downgrowth of mesoblast taking place only at certain places, these forming the gill-bars. Between each downgrowth the hypoblastic lining of the alimentary canal remains in contact with the epiblast; here the gill-opening subsequently appears about the twenty-second day."

Dohrn describes and gives excellent pictures of the growth of the diaphragms, as the Ammocoetes grows in size, pictures which are distinctly reminiscent of the corresponding illustrations given by Brauer of the growth of the internal gills in the scorpion embryo.

Another piece of evidence confirmatory of the view that the branchial segments are really of the nature of internal appendages, as the result of which gill-pouches are formed, is given by the presence in each of these branchial bars or diaphragms of a separate coelomic cavity. From the walls of this cavity the branchial muscles and cartilaginous bar are formed.

Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by the formation of somites, which consist of a series of divisions of the coelom, of which the walls form a series of muscular and skeletal segments. In the head-region, as already mentioned, such coelomic divisions form two rows--a dorsal and a ventral set. From the walls of the dorsal set the somatic musculature is formed. From those of the ventral set the branchial musculature. From the latter also the branchial cartilaginous bars are formed. Thus Shipley, in his description of the development of the lamprey, says: "The mesoblast between the gills arranges itself into head-cavities, and the walls of these cavities ultimately form the skeleton of the gill-arches."

Similarly, in the arthropod, the segments in the embryo are marked out by a series of coelomic cavities and Kishinouye has described in Limulus a separate coelomic cavity for every one of the mesosomatic or branchial segments, and he states that in Arachnida {168}the segmental coelomic cavities extend into the limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult condition is reached.

The whole evidence thus points strongly to the conclusion that the true branchial segmental units are the branchial bars or diaphragms, not the pouches between them.

It is possible to understand why such prominence has been given to the conception of the branchial unit as a gill-pouch rather than as a gill-appendage, when the extraordinary change of appearance in the respiratory chamber of the lamprey which occurs at transformation, is taken into consideration. This change is of a very far-reaching character, and consists essentially of the formation of a new alimentary canal in this region, whereby the pharyngeal chamber of Ammocoetes is cut off posteriorly from the alimentary canal, and is confined entirely to respiratory purposes, its original lumen now forming a tube called the bronchus, which opens into the mouth and into a series of branchial pouches.

In Fig. 68 I give diagrammatic illustrations taken from Nestler's paper to show the striking change which takes place at transformation, (A) representing three branchial segments of Ammocoetes, and (B) the corresponding three segments of Petromyzon. The corresponding parts in the two diagrams are shown by the cartilages (br. cart.), the sense-organs (S), and the branchial veins (V. br.); the corresponding diaphragms are marked by the figures 1, 2, 3 respectively. As is clearly seen, it is perfectly possible in the latter case to describe the respiratory chamber, as Nestler has done, as divided into a series of separate smaller chambers--the gill-pouches--by means of a series of diaphragms or branchial bars. The surface of these gill-pouches is in part thrown into folds for respiratory purposes, and each gill-pouch opens, on the one hand, into the bronchus (Bro.), and, on the other, to the exterior by means of the gill-slit. The branchial unit in Petromyzon is, therefore, according to Nestler and other morphologists, the folded opposed surfaces of two contiguous diaphragms, and each one of the diaphragms is intersegmental between two gill-pouches.

Nestler then goes on to describe the arrangement in Ammocoetes in the same terms, although there is no bronchus or gill-pouch, but only an open chamber into which these gill-bearing diaphragms project, which open chamber serves both for the passage of food and {169}of the water for respiration. This is manifestly the wrong way to look at the matter: the adult form is derived from the larval, not vice versâ, and the transformation process shows exactly how the gills, in Rathke's sense, come together to form the bronchus and so make the gill-pouches of Petromyzon.

When we bear in mind that almost all observers consider that the internal branchiæ of the scorpion group are directly derived from branchial appendages of a kind similar to those of Limulus, it is evident that a branchial appendage such as that of Ammocoetes might also have arisen from such an appendage, because in various respects it is easier to compare the branchial appendage of Ammocoetes, than that of the scorpion group, with that of Limulus.

In both figures the branchial cartilages (br. cart.), the branchial view (V. br.), and the sense-organs (S), are marked out in order to show corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial segment are not distinguished, being represented a uniform black colour. Bro., the bronchus into which each gill-pouch opens.]

In the case of the scorpions, various suggestions have been made as to the manner in which such a conversion may have taken place. The most probable explanation is that given by Macleod, in which {170}each of the branchiæ of the scorpion group is directly compared with the branchial part of the Limulus appendage which has sunk into and amalgamated with the ventral surface.

According to this view, the modification which has taken place in transforming the branchial Limulus-appendage into the branchial scorpion-appendage is a further stage of the process by which the Limulus branchial appendage itself has been formed, viz. the getting rid of the free locomotor segments of the original appendage, thus confining the appendage more and more to the basal branchial portion. So far has this process been carried in the scorpion that all the free part of the appendage has disappeared; apparently, also, the intrinsic muscles of the appendage have vanished, with the possible exception of the post-stigmatic muscle, so that any direct comparison between the branchial appendages of Limulus and the scorpions is limited to the comparison of their branchiæ, their nerves, and their afferent and efferent blood-vessels.

In the case of Ammocoetes the comparison must be made not with air-breathing but with water-breathing scorpions, such as existed in past ages in the forms of Eurypterus, Pterygotus, Slimonia, and with the crowd of trilobite and Limulus-like forms which were in past ages so predominant in the sea; forms in some of which the branchial appendages had already become internal, but which, from the very fact of these forms being water-breathers, probably resembled, in respect of their respiratory apparatus, Limulus rather than the present-day scorpion.

On the assumption that the branchial appendages of Ammocoetes, like the branchial appendages of the scorpion group, are to a certain extent comparable with those of Limulus, it becomes a matter of great interest to inquire whether the mode in which respiration is effected in Ammocoetes resembles most that of Limulus or of the scorpion.

THE ORIGIN OF THE BRANCHIAL MUSCULATURE.

The difference between the movements of respiration in Limulus and those of the scorpions consists in the fact that, although in both cases respiration is effected mainly by dorso-ventral muscles, these muscles are not homologous in the two cases: in the former, the dorso-ventral appendage-muscles are mainly concerned, in the latter, the dorso-ventral somatic muscles.

{171}The paper by Benham gives a full description of the musculature of Limulus, and according to his arrangement the muscles are divided into two sets, longitudinal and dorso-ventral. Of the latter, each mesosomatic segment possesses a pair of dorso-ventral muscles, attached to the mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. 58, Dv.). These muscles are called by Benham the vertical mesosomatic muscles. I shall call them the somatic dorso-ventral muscles, in contradistinction to the dorso-ventral muscles of the branchial appendages. Of the latter, the two chief are the external branchial (Fig. 66, m3) and the posterior entapophysio-branchial (Fig. 66, m1); a third muscle is the anterior entapophysio-branchial (Fig. 66, m2). Of these muscles, the posterior entapophysio-branchial (m1) is closely attached along the branchial cartilaginous bar up to its round-headed termination on the anterior surface of the appendage. The anterior entapophysio-branchial muscle (m2_) is attached to the branchial cartilage near the entapophysis.

In the case of the scorpion, as described by Miss Beck, the branchial appendage has become reduced to the branchiæ, and the intrinsic appendage-muscles have entirely disappeared, with the possible exception of the small post-stigmatic muscle; on the other hand, the dorso-ventral somatic muscles, which are clearly homologous with the corresponding muscles of Limulus, have remained, and become the essential respiratory muscles.

Of these two possible types of respiratory movement it is quite conceivable that in the water-breathing scorpions of olden times and in their allies, the dorso-ventral muscles of their branchial appendages may have continued their rôle of respiratory muscles, and so have given origin to the respiratory muscles of the ancestors of Ammocoetes.

The respiratory muscles of Ammocoetes are three in number, and have been described by Nestler and Miss Alcock as the adductor muscle, the striated constrictor muscle, and the tubular constrictor muscle (Fig. 65, m. add., m.c.s., and m.c.t.). Of these, the constrictor muscle (Fig. 71, m. con. str.) is in close contact with its cartilaginous bar, while the adductor (Fig. 71, m. add.) is attached to the cartilage only at its origin and insertion, and the tubular muscles (Fig. 71, m. con. tub.) have nothing whatever to do with the cartilage at all, being attached ventrally to the connective tissue in the neighbourhood {172}of the ventral aorta (V.A.), and dorsally to the mid-line between the dorsal aorta (D.A.) and the notochord.

The close relationship of the constrictor muscle to the cartilaginous branchial bar does not favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the scorpion. It is, however, directly in accordance with the view that this muscle is homologous with one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle (m1, Fig. 66) of the Limulus appendage, especially when the homology of the Ammocoetes branchial cartilage with the Limulus branchial cartilage is borne in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the Ammocoetes branchial segment as more likely to have been derived from dorso-ventral muscles which belonged originally to a branchial appendage, such as we see in Limulus, than from dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in Limulus and scorpion. In other words, I am inclined to hold the view that the somatic dorso-ventral muscles have disappeared in this region in Ammocoetes, while dorso-ventral appendage-muscles have been retained, i.e._ the exact reverse to what has taken place in the air-breathing scorpion.

I am especially inclined to this view because of the manner in which it fits in with and explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates into parietal and visceral, such a division has received general acceptance and, as far as the head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van Wijhe, i.e. to the somatic and splanchnic striated muscles according to my nomenclature. Of these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammocoetes and of the same character throughout; the somatic musculature, on the other hand, is clearly divisible, in the head region, into two sets--a spinal and a cranial set. The somatic muscles innervated by the spinal set of nerves, including in this term the spino-occipital or so-called hypoglossal nerves, are in Ammocoetes most sharply defined from all the other muscles of the body. They form the great dorsal and ventral longitudinal {173}body-muscles, which extend dorsally as far forward as the nose and are developed embryologically quite distinctly from the others, being formed as muscle-plates (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated musculature of the Ammocoetes must be divided into (1) the visceral muscles; (2) the longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. Of these the 1st, on the view just stated, represent the original appendage-muscles; the 2nd belong to the spinal region, and will be considered with that region; the 3rd represent the original segmental dorso-ventral somatic muscles, which are so conspicuous in the musculature of the Limulus and the scorpion group.

The discussion of this last statement will be given when I come to deal with the prosomatic segments of Ammocoetes. I wish, here, simply to point out that van Wijhe has shown that the eye-muscles develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and therefore represent the somatic muscles belonging to those segments, while no development of any corresponding muscles takes place in the 4th, 5th, and 6th segments; so that if the eye-muscles represent a group of dorso-ventral somatic muscles, such muscles have been lost in the 4th, 5th, and 6th segments. The latter segments are, however, the glossopharyngeal and vagus segments, the branchial musculature of which is derived from the ventral segments of the mesoderm. In other words, van Wijhe's observations mean that the dorso-ventral somatic musculature has been lost in the branchial or mesosomatic region, while the dorso-ventral appendage musculature has been retained, and that, therefore, the mode of respiration in Ammocoetes more closely resembles that of Limulus than of Scorpio.

In addition to these branchial muscles, another and very striking set of muscles is found in the respiratory region of Ammocoetes--the so-called tubular muscles. These muscles are of great interest, but as they are especially connected with the VIIth nerve, their consideration is best postponed to the chapter dealing with that nerve.

Also, in connection with the vagus group of nerves, special sense-organs are found in the skin covering this mesosomatic region, the so-called epithelial pit-organs (Ep. pit., Fig. 71). They, too, are of {174}great interest, but their consideration may also better be deferred to the chapter dealing with those special sense-systems known as the lateral line and auditory systems.

COMPARISON OF THE BRANCHIAL CIRCULATION IN AMMOCOETES AND LIMULUS.

Closely bound up with the respiratory system is the nature of the circulation of blood through the gills. Before, therefore, proceeding to the consideration of the segments in front of those which carry branchiæ, it is worth while to compare the circulation of the blood in the gills of Limulus and of Ammocoetes respectively.

In all the higher vertebrates the blood circulates in a closed system of capillaries, which unite the arterial with the venous systems. In all the higher invertebrates this capillary system can hardly be said to exist; the blood is pumped from the arterial system into blood spaces or lacunæ, and thus comes into immediate contact with the tissues. From these it is collected into veins, and so returned to the heart. There is, in fact, no separate lymph-system in the higher invertebrates; the blood-system and lymph-system are not yet differentiated from each other. This also is the case in Ammocoetes; here, too, in many places the blood is poured into a lacunar space, and collected thence by the venous system; a capillary system is only in its commencement and a lymph-system does not yet exist. In this part of its vascular system Ammocoetes again resembles the higher invertebrates more than the higher vertebrates.

This resemblance is still more striking when the circulation in the respiratory organs of the two animals is compared. A branchial appendage is essentially an appendage whose vascular system is arranged for the special purpose of aerating blood. In the higher vertebrates such a purpose is attained by the pulmonary capillaries, in Limulus by the division of the posterior surface of the basal part of the appendage into thin lamellar plates, the interior of each of which is filled with blood. The two surfaces of each lamella are kept parallel to each other by means of fibrous or cellular strands forming little pillars at intervals, called by Macleod "colonettes." A precisely similar arrangement is found in the scorpion gill-lamella, as seen in Fig. 69, A, taken from Macleod. In Ammocoetes there are no well-defined branchial capillaries, but the blood circulates, as in {175}the invertebrate gill, in a lamellar space; here, also, as Nestler has shown, the opposing walls of the gill-lamella are held in position by little pillar-like cells, as seen in Fig. 69, B, taken from his paper.

In this representative of the earliest vertebrates the method of manufacturing an efficient gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time when vertebrates first appeared. This similarity indicates a close resemblance between the circulatory systems of the two groups of animals, and therefore, to the superficial inquirer, would indicate an homology between the heart of the vertebrate and the heart of the higher invertebrate; but the former is situated ventrally to the gut and the nervous system, while the latter is composed of a long vessel which lies in the mid-dorsal line immediately under the external dorsal covering. Indeed, this ventral position of the heart in the one group of animals and its dorsal position in the other, combined with the corresponding positions of the central nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most important fact which ought to make us hesitate before accepting the homology of the dorsal heart of the arthropod with the ventral heart of the vertebrate--The heart in all invertebrates is a systemic heart, i.e. drives the arterial blood to the different organs of the body, and then the veins carry it back to the respiratory organ, from whence it passes to the heart.

A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of Ammocoetes (after Nestler).]

The only exception to this scheme is found in the vertebrate where the heart is essentially a branchial heart, the blood being {176}driven from the heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body. The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to the ventral venous system. This constitutes a very strong reason for believing that the systemic heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the vertebrate heart arise?

Let us first see how the blood is supplied to the gills in Limulus.

L.V.S., longitudinal venous sinus, or collecting sinus; a. br., branchial arteries; V.p., veno-pericardial muscles; P., pericardium.]

In Limulus the blood flows into the lamellæ from sinuses or blood-spaces (b.s., Fig. 66) at the base of each of the lamellæ, which sinuses are filled by a vessel which may be called the branchial artery, since it is the afferent branchial vessel. On each side of the middle line of the ventral surface of the body a large longitudinal venous sinus exists, called by Milne-Edwards the venous collecting sinus, L.V.S., (Fig. 70 and Fig. 58), which gives off to each of the branchial appendages on that side a well-defined afferent branchial vessel--the branchial artery (a. br.). The blood of the branchial artery flows into the blood-spaces between the anterior and posterior laminæ of the appendage and thence into the gill-lamellæ, from which it is collected into an efferent vessel or branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which carries it back to the dorsal heart. The position of the branchial artery and vein is shown in Fig. 66, which represents a section through the branchial appendage of Limulus at right angles to the cartilaginous branchial bar (br. cart.), just as Fig. 65 represents a section through the {177}branchial appendage of Ammocoetes at right angles to the cartilaginous branchial bar.

Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and Benham concur in showing that in both Limulus and the scorpion group a striking and most useful connection exists between the heart and these two collecting venous sinuses, in the shape of a segmentally arranged series of muscular bands (V.p., Fig. 70 and Fig. 58), attached, on the one hand, to the pericardium, and on the other to the venous collecting sinus on each side. These muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,' are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in the scorpion as 'ligaments contractiles,' and considers that they play an important part in assisting the pulmonary circulation; for, he says, "en mettant a nu une portion du coeur, on remarque que ces battements se font sentir sur les ligaments contractiles, et determinent sur les poches pulmonaires une pression qui fait aussitot refluer et remonter le sang dans les vaisseaux pneumocardiaques." Lankester, in discussing the veno-pericardial muscles of Limulus and of the scorpions, says that these muscles probably contract simultaneously with the heart and are of great importance in assisting the flow through the pulmonary system. More recently Carlson has investigated the action of these muscles in the living Limulus and found that they act simultaneously with the muscles of respiration.

Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group, and we may fairly assume that a similar arrangement existed in the extinct forms from which I imagine the vertebrate to have arisen. The further consideration of this group of muscles will be given in Chapter IX.

Passing now to the condition of the branchial blood-vessels of Ammocoetes, we see that the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery which arises segmentally from the ventral aorta (V.A.), as seen in Fig. 71 (taken from Miss Alcock's paper). From the gill-lamellæ the blood is collected into an efferent or branchial vein (v. br.), which {178}runs, as seen in Fig. 65, along the free edge of the diaphragm, and terminates in the dorsal aorta.

The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries arise segmentally.

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.]

From this description it is clear that the vascular supply of the branchial segment of Ammocoetes would resemble most closely the vascular supply of the Limulus branchial appendage, if the ventral aorta of the former was derived from two longitudinal veins, homologous with the paired longitudinal venous sinuses of the latter.

{179}A priori, such a derivation seems highly improbable; and yet it is precisely the manner in which embryology teaches us that the heart and ventral aorta of the vertebrate have arisen.

THE ORIGIN OF THE INVERTEBRATE HEART AND THE ORIGIN OF THE VERTEBRATE HEART.

Not only does the vertebrate heart differ from that of the invertebrate, in that it is branchial while the latter is systemic, but also it is unique in its mode of formation in the embryo. In the Appendiculata the heart is formed as a single organ in the mid-dorsal line by the growth of the two lateral plates of mesoblast dorsalwards, the heart being formed where they meet. In Mammalia and Aves, the heart and ventral aorta commence as a pair of longitudinal veins, one on each side of the commencing notochord.

If the embryo be removed from the yolk, the surface of the embryo covering these two venous trunks can be spoken of as the ventral surface of the embryo at that stage, and indeed we find that in the present day there is an increasing tendency to speak of this surface as the ventral surface of the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays great stress on the importance of surface views and when the embryo has been removed from the yolk, figures and speaks of its ventral surface. So, also, Locy and Neal find that the best method of seeing the early segments of the embryo is to remove the embryo from the yolk, and examine what they speak of as a ventral view. At the period, then, before the formation of the throat, we may say that on the ventral surface of the embryo a pair of longitudinal venous sinuses are found, one on each side of the mid-ventral line, which are in the same position with respect to the mid-axis of the embryo as are the longitudinal venous sinuses in Limulus.

The next step is the formation of the throat by the extension of the layers of the embryo laterally to meet in the mid-line and so form the pharynx, with the consequence that a new ventral surface is formed; these two veins, as is well known, travel round also, and, meeting together in the new mid-ventral line, form the subintestinal vein, the heart, and the ventral aorta.

What is true of Mammalia and Aves, has been shown by P. Mayer to be true universally among vertebrates, so that in all cases the heart and ventral aorta have arisen by the coalescence in the new mid-ventral {180}line of two longitudinal venous channels, which were originally situated one on each side of the notochord, in what was then the ventral surface of this part of the embryo. This history is especially instructive in showing how the pharyngeal region is formed by the growing round of the lateral mesoblast, i.e. the muscular and other mesoblastic tissues of the branchial segments, and how the two longitudinal veins take part in this process. The phylogenetic interpretation of this embryological fact seems to be, that the new ventral surface of the vertebrate in this region is formed, not only by the branchial appendages, but also by the growth ventrally of that part of the original ventral surface which covered each longitudinal venous sinus.

The following out of the consecutive clues, which one after the other arise in harmonious succession as the necessary sequence of the original working hypothesis, brings even now into view the manner in which the respiratory portion of the alimentary canal arose, and gives strong hints as to the position of that part of the arthropod which gave origin to the notochord. Here I will say no more at present, for the origin of the new alimentary canal of the vertebrate and of the notochord will be more fittingly discussed as a whole, after all the other organs of the vertebrate have been compared with the corresponding organs of the arthropod.

C.N.S., central nervous system; nc., notochord; m., myotome.

The lower half of figure shows comparative position of the longitudinal venous sinus (L.V.S.) in Limulus. C.N.S., central nervous system; Al., alimentary canal; H., heart; m., body-muscles.]

The strong evidence that the vertebrate heart was formed from a pair of longitudinal venous sinuses on the ventral side of the central canal, carries with it the conclusion that the original single median dorsal heart of the arthropod is not represented in the vertebrate, {181}for the dorsal aorta cannot by any possibility represent that heart.

Although it is not now functional the original existence of so important an organ as a dorsal heart may have left traces of its former presence; if so, such traces would be most likely to be visible in the lowest vertebrates, just as the median eyes are much more evident in them than in the higher forms. In Fig. 58 the position of the dorsal heart is shown in Limulus, and in Fig. 70 the shape and extent of this dorsal heart is shown. It extends slightly into the prosomatic region, and thins down to a point there, runs along the length of the animal and finally thins down to a point at the caudal end.

The heart is surrounded by a pericardium, from which at regular intervals a number of dorso-ventral muscles pass, to be inserted into the longitudinal venous sinus on each side. These veno-pericardial muscles are absolutely segmental with the mesosomatic segments, and are confined to that region, with the exception of two pairs in the prosomatic region. Their homologies will be discussed later.

Any trace of a heart such as we have just described must be sought for in Ammocoetes between the central nervous system and the mid-line dorsally. Now, in this very position a large striking mass of tissue is found, represented in section in Fig. 73, f. It forms a column of similar tissue along the whole mid-dorsal region, except at the two extremities; it tapers away in the caudal region, and headwards grows thinner and thinner, so that no trace of it is seen anterior to the commencement of the branchial region. It resembles in its dorsal position, in its shape, and in its size a dorsal heart-tube such as is seen in Limulus and elsewhere, but it differs from such a tube in its extension headwards. The heart-tube of Limulus ceases at the anterior end of the mesosomatic region, this fat-column of Ammocoetes at the posterior end. In its structure there is not the slightest sign of anything of the nature of a heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat, staining intensely black with osmic acid. Nowhere else in the whole body of Ammocoetes is such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply fat-cells, such as might easily have taken the place of some other previously existing organ.

{182}I do not know how to decide the question which thus arises. Supposing, for the sake of argument, that this column of fat-cells has really taken the place of the original dorsal heart, what criterion would there be as to this? The heart ex hypothesi having ceased to function, the muscular tissue would not remain, and the space would be filled up, presumably with some form of connective tissue. As likely as not, the connective tissue might take the form of fatty tissue, the storage of fat being a physiological necessity to an animal, while at the same time no special organ has been developed for such a purpose, but fat is being laid down in all manner of places in the body.

This dorsal fat-column, as it is seen in Ammocoetes, is not found in the higher vertebrates, so that it possesses, at all events, the significance of being a peculiarity of ancient times before the vertebrate skeletal column was formed.

I mention it here in connection with my view as to the origin of vertebrates, because there it is, in the very place where the dorsal heart ought to have been. For my own part, I should not have expected that a muscular organ such as the heart would leave any trace of itself if it disappeared, so that its absence in the dorsal region of the vertebrate does not seem to me in the slightest degree to invalidate my theory.

sp. c., spinal cord; gl., glandular tissue filling the spinal canal; sk., Gegenbaur's skeletogenous cells; p., pigment.]

{183}SUMMARY.

From the close similarity of structure and position between the branchial skeleton of Limulus and of Ammocoetes, as given in the preceding chapter, it logically follows that the branchiæ of Ammocoetes must be homologous with the branchiæ of Limulus. But the respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that the branchiæ of Ammocoetes, and consequently of the vertebrates, must have been derived from branchial appendages, and as they are internal, not external, such branchial appendages must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no external respiratory appendages have been discovered in any of the sea-scorpions.

In the vertebrates--and it is especially well shown in Ammocoetes--a double segmentation exists in the head-region, a body or somatic segmentation, and a branchial or splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric segmentations. The nerves which supply the latter segments form a very well-marked group (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves from the very beginning.

The system of cranial segmental nerves is older than the spinal system, and cannot, therefore, be derived from it, but can be arranged as a system supplying two segments, somatic and splanchnic, which differ in the following way: Each somatic segment is supplied by two roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic segment possesses only one root, which is mixed in function.

The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been unexplained, immediately receive a straightforward and satisfactory explanation if the splanchnic or branchiomeric segments owe their origin to a system of appendages after the style of those of Limulus.

In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial region of the vertebrate, the nerves which supply the somatic segments arise from separate sensory and motor roots, while the single nerve which supplies each appendage contains all the fibres for the appendage, both motor and sensory.

It follows from this that the branchial segments supplied by the vagus and glossopharyngeal nerves ought to have arisen from appendages bearing branchiæ.

Although the evidence of such appendages has entirely disappeared in the higher vertebrates, together with the disappearance of branchiæ, and is not strikingly apparent in the higher gill-bearing fishes, yet in Ammocoetes, so great is the difference here from all other fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of these appendages possesses its own mixed nerve, glossopharyngeal or vagus, {184}its own cartilage, its own set of visceral muscles, its own sense-organs, just as do the respiratory appendages of Limulus.

The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or appendage between the pouches. Embryology shows how each such appendage grows inwards, how a coelomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in scorpions.

We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the present day somewhat in form, but they are in many respects closely allied to Limulus. The present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral somatic muscles, while those of Limulus are the appendage-muscles.

The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion, being water-breathers, even although their respiratory appendages were no longer free but sunk in below the surface of the body. The probability that such was the case is increased after consideration of the method of breathing in Ammocoetes, for the respiratory muscles of the latter animal are directly comparable with the muscles of the respiratory appendages of Limulus, and are not somatic. Even the gills themselves of Ammocoetes are built up in the same fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate heart, which is so strikingly different from that of all invertebrate hearts, in that it originates as a branchial and not as a systemic heart, and is formed by the coalescence of two longitudinal veins.

The origin of these two longitudinal veins is immediately apparent if the vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal venous sinuses, situated on each side of the middle line of the animal in an exactly corresponding position to that of the two longitudinal veins, which come together to form the heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of its blood-supply in the vertebrate still further points to the origin of vertebrates from the Palæostraca.

{185}CHAPTER V

THE EVIDENCE OF THE THYROID GLAND

The value of the appendage-unit in non-branchial segments.--The double nature of the hyoid segment.--Its branchial part.--Its thyroid part.--The double nature of the opercular appendage.--Its branchial part.--Its genital part.--Unique character of the thyroid gland of Ammocoetes--Its structure.--Its openings.--The nature of the thyroid segment.--The uterus of the scorpion.--Its glands.--Comparison with the thyroid gland of Ammocoetes.--Cephalic genital glands of Limulus.--Interpretation of glandular tissue filling up the brain-case of Ammocoetes.--Function of thyroid gland.--Relation of thyroid gland to sexual functions.--Summary.

I have now given my reasons why I consider that the glossopharyngeal and vagus nerves were originally the nerves belonging to a series of mesosomatic branchial appendages, each of which is still traceable in the respiratory chamber of Ammocoetes, and gives the type-form from which to search for other serially homologous, although it may be specially modified, segments.

As long as the branchial unit consisted of the gill-pouch the segments of the head-region were always referred to such units, hence we find Dohrn and Marshall picturing to themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the anterior end of the body. Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the segmentation were sound, his interpretation only was in fault, because he was obliged to express his segments in terms of the gill-pouch unit. Once abandon that point of view and take as the unit a branchial appendage, then immediately we see that in the region in front of the branchiæ we may still have segments {186}homologous to the branchial segments, originally characterized by the presence of appendages, but that such appendages need never have carried branchiæ. The new mouth may have been formed by such appendages, which would express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant in calling it a branchial sense-organ.

THE FACIAL NERVE AND THE FOREMOST RESPIRATORY SEGMENT.

This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid segments, for embryologically there is evidence of two segments. As already mentioned, the facial nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the pro-otic group of nerves forms one of the main reasons why this group has been supposed to have originally supplied gill-pouch segments, for the hyoid segment is clearly associated with branchiæ.

When, however, we examine Ammocoetes (cf. Figs. 63 and 64) it is clear that the foremost of the segments forming the respiratory chamber, which must be classed with the rest of the mesosomatic or opisthotic segments, is that supplied by the facial nerves.

An examination of this respiratory chamber shows clearly that there are six pairs of branchial appendages or diaphragms, which are all exactly similar to each other. These are those already considered, the foremost of which are supplied by the IXth or glossopharyngeal nerves. Immediately anterior to this glossopharyngeal segment is seen in the figures the segment supplied by the VIIth or facial nerves. It is so much like the segments belonging to the glossopharyngeal and vagus nerves as to make it certain that we are dealing here with a branchial segment, composed of a pair of branchial appendages similar to those in the other cases, except that the cartilaginous bar is here replaced by a bar of muco-cartilage and the branchiæ are confined to the posterior part of each appendage. The anterior portion is, as is seen in Fig. 74, largely occupied by blood-spaces, but in addition carries the ciliated groove (ps. br.) called by Dohrn 'pseudo-branchiale Rinne.' This groove leads directly into the thyroid gland, which is a large bilateral organ situated in the middle line, as seen in Fig. 80 and Fig. 85. As shown by Miss Alcock, the facial nerve supplies this thyroid gland, as well as the posterior hyoid branchial segment, and, as pointed out by Dohrn, there is every reason to consider this thyroid gland as indicative of a separate segment, especially when van Wijhe's statement that the hyoid segment is in reality double is taken into account.

{187}[Illustration: FIG. 74.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.

Somatic muscles coloured red. Branchial and visceral muscles coloured blue. Tubular constrictor muscles distinguished from striated constrictor muscles by simple hatching. Tent., tentacles; Tent. m.c., muco-cartilage of tentacles; Vel. m.c., muco-cartilage of the velum; Hy. m.c., muco-cartilage of the hyoid segment; Ps. br., pseudo-branchial groove; Br. cart., branchial cartilages; Sp., space between somatic and splanchnic muscles; Th. op., orifice of thyroid; H., heart.]

{188}The evidence, then, of Ammocoetes points directly to this conclusion: The facial nerves represent the foremost of the mesosomatic group of nerves, and supply two segments, which have amalgamated with each other. The most posterior of these, the hyoid segment, is a branchial segment of the same character as those supplied by the vagus and glossopharyngeal nerves; represents, therefore, the foremost pair of branchial appendages. The anterior or thyroid segment, on the other hand, differs from the rest in that, instead of branchiæ, it carries the thyroid gland with its two ciliated grooves. If this segment, which is the foremost of the mesosomatic segments, also indicates a pair of appendages which carry the thyroid gland instead of branchiæ, then it follows that this pair of appendages has joined together in the mid-line ventrally and thus formed a single median organ--the thyroid gland. If, then, we find that the foremost of the mesosomatic appendages in the Palæostraca was really composed of two pairs of appendages, of which the most posterior carried branchiæ, while the anterior pair had amalgamated in the mid-line ventrally, and carried some special organ instead of branchiæ, then the accumulation of coincidences is becoming so strong as to amount to proof of the correctness of our line of investigation.

THE FIRST MESOSOMATIC SEGMENT IN LIMULUS AND ITS ALLIES.

What, then, is the nature of the foremost pair of mesosomatic appendages in Limulus. They differ from the rest of the mesosomatic appendages in that they do not carry branchiæ, and instead of being {189}separate are joined together in the mid-line ventrally to form a single terminal plate-like appendage known as the operculum. On its posterior surface the operculum carries the genital duct on each side.

So also in the scorpion group, the operculum is always found and always carries the genital ducts.

A survey of the nature of the opercular appendage demonstrates the existence of three different types--

1. That of Limulus, in which the operculum is free, and carries only the terminations of the genital ducts. In this type the duct on each side opens to the exterior separately (Fig. 75).

Ut., terminal chamber, or uterus.]

2. The type of Scorpio, Androctonus, Buthus, etc., in which the operculum is not free, but forms part of the ventral surface of the body-wall, but, like Limulus, carries only the terminations of the genital ducts. In this type the duct on each side terminates in a common chamber (vagina or uterus), which communicates with the exterior by a single external median opening. This common chamber, or uterus (Ut.), extends the whole breadth of the operculum (as seen in Fig. 76), and is limited to that segment.

3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the Pedipalpi, in which the operculum forms a part of the ventral surface of the body wall, but no longer covers only the termination of the genital apparatus. It really consists of two parts, a median anterior, which covers the terminal genital apparatus, {190}and a lateral posterior, which covers the first pair of gills, or lung-books, as they are called. In this type (Fig. 77) the genital ducts terminate in a common chamber or uterus, the nature of which will be further considered.

As has been pointed out by Blanchard, the terminal genital organs of the scorpions and the Pedipalpi vary considerably in the different genera, especially the male genital organs. The general type of structure is the same, and consists in both male and female of vasa deferentia, which come together to form a common chamber before the actual opening to the exterior. This common chamber has been called in the female scorpion the vagina, or in Thelyphonus the uterus. I shall use the latter term, in accordance with Tarnani's work, and the corresponding chamber in the male will be the uterus masculinus.

A considerable discussion has taken place about the method of action of the external genital organs in the members of the scorpion tribe, into which it is hardly necessary to enter here. The evidence points to the conclusion that in all these forms the operculum covers a median single chamber or uterus, into which the genital ducts open on each side, the main channels of emission being provided with a massive chitinous internal framework. We may feel certain that in the old extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, and that therefore in them also the median portion of the operculum covered a median chamber or uterus composed of the amalgamation of the terminations of the two genital ducts, which were originally separate, as in Limulus.

Opercular segment is marked out by thick black line. Ut. Masc., uterus masculinus; Int. Op., internal opening of uterus into genital chamber; Ext. Op., common external opening to genital chamber (Gen. Ch.) and pulmonary chamber.]

The observations of Schmidt, Zittel, and others show that the {191}operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, etc., belonged to the type of Thelyphonus, rather than to that of Limulus or Scorpio. In Fig. 78 I give a picture from Schmidt of the ventral aspect of Eurypterus, and by the side of it a picture of the isolated operculum. Schmidt considers that there were five branchiæ-bearing segments constituting the mesosoma, the foremost of which formed the operculum. Such operculum is often found isolated, and is clearly composed of two lateral appendages fused together in the middle line, of such a nature as to form a median elongated tongue, which lies between and separates the first three pairs of branchial segments. This median tongue, together with the anterior and median portion of the operculum, concealed, in all probability, according to Schmidt, the terminal parts of the genital organs, just as the median part of the operculum in Phrynus and Thelyphonus conceals the complicated terminal portions of the genital organs. The posterior part of the operculum, like that of Phrynus and Thelyphonus, carried the first pair of branchiæ, so Schmidt thinks from the evidence of markings on some specimens.

The segments and appendages on the right are numbered in correspondence with the cranial system of lateral nerve-roots as found in vertebrates. M., metastoma. The surface ornamentation is represented on the first segment posterior to the branchial segments. The opercular appendage is marked out by dots.]

Apparently an opercular appendage of this kind is in reality the result of a fusion of the genital operculum with the first branchial appendage in forms such as the scorpion; for, in order that the tergal plates may correspond in number with the sternal in Eurypterus, etc., it is necessary to consider that the operculum is composed of two sternites joined together. Similarly in Thelyphonus, Phrynus, etc., this numerical correspondence is only observed if the operculum is looked upon as double.

A restoration of the mesosomatic region of Eurypterus, viewed {192}from the internal surface, might be represented by Fig. 79, in which the thick line represents the outline of the opercular segment, and the fainter lines the succeeding branchial segments. The middle and anterior part of the opercular segment carried the terminations of the genital organs; these I have represented, in accordance with our knowledge of the nature of these organs in the present-day scorpions, as a median elongated uterus, bilaterally formed, from which the genital ducts passed, probably as in Limulus, towards a mass of generative gland in the cephalic region, and not as in Scorpio or Thelyphonus, tailwards to the abdominal region.

The opercular segment is marked out by the thick black line. The segments II.-VI. bear branchiæ, and segment I. is supposed in the male to carry the uterus masculinus (Ut. Masc.) and the genital ducts.]

It is possible that in Holm's representation of Eurypterus, Fig. 104, the genital duct on each side is indicated.

THE THYROID GLAND OF AMMOCOETES.

If we compare this mesosomatic region of Eurypterus with that of Ammocoetes, the resemblance is most striking, and gives a meaning to the facial nerve which is in absolute accordance with the interpretation already given of the glossopharyngeal and vagus nerves. In both cases the foremost respiratory or mesosomatic segment is double, the posterior lateral part alone bearing the branchiæ, while the median and anterior part bore in the one animal the uterus and genital ducts, in the other the thyroid gland and ciliated grooves. We are driven, therefore, to the conclusion that this extraordinary and unique organ, the so-called thyroid gland of Ammocoetes, which exists only in the larval condition and is got rid of as soon as the adult sexual organs are formed, shows the very form and position of the uterus of this invertebrate ancestor of Ammocoetes. What, then, is the nature of the thyroid gland in Ammocoetes?

{193}Throughout the vertebrate kingdom it is possible to compare the thyroid gland of one group of animals with that of another without coming across any very marked difference of structure right down to and including Petromyzon. When, however, we examine Ammocoetes, we find that the thyroid has suddenly become an organ of much more complicated structure, covering a much larger space, and bearing no resemblance to the thyroid glands of the higher forms. At transformation the thyroid of Ammocoetes is largely destroyed, and what remains of the gland in Petromyzon becomes limited to a few follicles resembling those of other fishes. The structure and position of this gland in Ammocoetes is so well known that it is unnecessary to describe it in detail. For the purpose, however, of making my points clear, I give in Fig. 80 the position and appearance of the thyroid gland (Th.) when the skin and underlying laminated layer has been removed by the action of hypochlorite of soda. On the one side the ventral somatic muscles have been removed to show the branchial cartilaginous basket-work.

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

The series of transverse sections in Fig. 81 represents the nature of the organ at different levels in front of and behind the opening into the respiratory chamber; and in Fig. 82 I have sketched the appearance of the whole gland, viewed so as to show its opening into the respiratory chamber and its posterior curled-up termination.

{194}[Illustration: FIG. 81.--SAMPLES FROM A COMPLETE SERIES OF TRANSVERSE SECTIONS THROUGH THE THYROID GLAND OF AMMOCOETES.

Sections 1 and 2 are anterior to the thyroid opening, Th. o.; sections 3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening before the commencement of the curled portion.]

{195}The series of transverse sections (1-6, Fig. 81) show that we are dealing here with a central glandular chamber, C (Fig. 81 (6) and Fig. 82), which opens by the thyroid duct (Th. o.) into the pharyngeal chamber, and is curled upon itself in its more posterior part. This central chamber divides, anteriorly to the thyroid orifice, into two portions, A, A[prime] (Fig. 82), giving origin to two tubes, B, B[prime], which lie close alongside of, and extend further back than, the posterior limit of the curled portion of the central chamber, C. The structure of the central chamber, C, and, therefore, of the separate coils, is given in both Schneider's and Dohrn's pictures, and is represented in Fig. 81 (6), which shows the peculiar arrangement and character of the glandular cells typical of this organ, and also the nature of the central cavity, with the arrangement of the ciliated epithelium. The structure of each of the lateral tubes, B, is different from that of the central chamber, in that only half the central chamber is present in them, as is seen by the comparison of the tube B with the tube C in Fig. 81 (5 and 6), so that we may look upon the central chamber, C, as formed of two tubes, similar in structure to the tubes B, which have come together to form a single chamber by the partial absorption of their walls, the remains of the wall being still visible as the septum, which partially divides the chamber, C, into halves.

In the walls of each of these tubes is situated a continuous glandular line, the structure of the glandular elements being specially characterized by the length of the cells, by the large spherical nucleus situated at the very base of each cell, and by the way in which the cells form a wedge-shaped group, the thin points of all the wedge-shaped cells coming together so as to form a continuous line along the chamber wall. This free termination of the cells of the gland in the lumen of the chamber constitutes the whole method for the secretion of the gland; there is no duct, no alveolus, nothing but this free termination of the cells.

Moreover, sections through the portion A, A[prime] (Fig. 82) show that here, as in the central chamber, C, four of these glandular lines open into a common chamber, but they are not the same four as in the case of the central chamber, for if we name these glandular lines on the left side a b, a[prime] b[prime] (Fig. 81), and on the right side c d, c[prime] d[prime], then the central chamber has opening into it the glands a b, c d, while the chambers of A and A[prime] have opening into them respectively a b, a[prime] b[prime], and c d, c[prime] d[prime]. Further, the same series of sections shows that the glands a and b are continuous with the glands a[prime] and b[prime] respectively across the apex of A, and similarly on the other side, so that the two glandular rows a b are continuous with the two glandular rows a[prime] b[prime], and we see that the {196}cavity of the portion A or A[prime] is formed by the bending over of the tube or horn, B or B[prime], with the partial absorption of the septum so formed between the tube and its bent-over part. If, then, we uncoil the curled-up part of C, and separate the portion, B, on each side from the chamber, C, we see that the so-called thyroid of Ammocoetes may be represented as in Fig. 83, i.e. it consists of a long, common chamber, C, which, for reasons apparent afterwards, I will call the palæo-hysteron, which opens, by means of a large orifice, into the respiratory or pharyngeal chamber. The anterior end of this chamber terminates in two tubes, or horns, B, B[prime], the structure of which shows that the median chamber, C, is the result of the amalgamation of two such tubes, and consequently in this chamber, or palæo-hysteron, the glandular lines are symmetrically situated on each side.

C, central chamber; A, A[prime], anterior extremity; B, B[prime], posterior extremity; Th. o., thyroid opening into respiratory chamber; Ps. br., Ps. br[prime]., ciliated grooves, Dohrn's pseudo-branchial grooves.]

Any explanation, then, of the thyroid gland of Ammocoetes, must {197}take into account the clear evidence that it is composed of two tubes, which have in part fused together to form an elongated central chamber, in part remain as horns to that chamber, and that in its walls there exist lines of gland-cells of a striking and characteristic nature.

Further, this central chamber, with its horns, is not a closed chamber, but is in communication with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber, as is well known, opens into the respiratory chamber by a funnel-shaped opening--the so-called thyroid duct (Th. o.). In the second place, there exist two ciliated grooves (Ps. br., Ps. br[prime].), the pseudo-branchial grooves of Dohrn, which have direct communication with the thyroid chamber. The manner in which these grooves communicate with the thyroid chamber has never, to my knowledge, been described previously to my description in the Journal of Physiology and Anatomy; it is very instructive, for, as I have there shown, each groove enters into the corresponding lateral horn, so that, in reality, there are three openings into the thyroid chamber or palæo-hysteron--a median opening into the central chamber, and a separate opening into each lateral horn.

The system of ciliated grooves on the inner ventral surface of the respiratory chamber of Ammocoetes was originally described by Schneider as consisting of a single median groove, which extends from the opening of the thyroid to the posterior extremity of the branchial chamber, and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run headwards and diverge from each other, becoming more and more lateral, and more and more dorsal, till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon the whole of this system as a single system, for he speaks of "a ciliated groove, which extends from the orifice of the stomach (i.e. anterior intestine) to the orifice of the thyroid, then divides into two, and runs forward right and left of the median ridge, etc." Dohrn rightly separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. 81 (6)) from the paired pseudo-branchial grooves; the former is a shallow depression which opens into the rim of the thyroid orifice, while the latter has a much more intimate connection with the thyroid gland itself.

{198}A series of sections, such as is given in Fig. 81, shows the relation of this pair of ciliated grooves to the thyroid better than any elaborate description. In the first place, it is clear that they remain separate up to their termination--they do not join in the middle line to open into the thyroid duct; in the second place, they are separate from the thyroid orifice--they do not terminate at the rim of the orifice, as is the case with the median groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig. 81 (2)), gradually moving further and further away from the actual opening of the duct into the pharyngeal chamber. During the whole of their course on the wall of the funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of the duct. The direction of the groove (Ps. br.) shifts as it passes deeper and deeper towards the thyroid, until at last, as seen in Fig. 81 (3 and 4), it is continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or turned-down horn, as I have called it. In other words, the median chamber opens into the pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the central chamber, owing to their being semi-canals, and not complete canals. If they were originally canals, and not grooves, then the thyroid of Ammocoetes would be derived from an organ composed of a large, common glandular chamber, which opened into the respiratory chamber by means of an extensive median orifice, and possessed anteriorly two horns, from each of which a canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.

Dohrn has pointed out that a somewhat similar structure and topographical arrangement is found in Amphioxus and the Tunicata, the gland-cells being here arranged along the hypobranchial groove to form the endostyle and not shut off to form a closed organ, as in the thyroid of Ammocoetes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and in Amphioxus represents the remnants of the more elaborate organ in Ammocoetes, and that, therefore, in order to explain the meaning of these organs in the former animals, we must first find out their meaning in Ammocoetes. Dohrn, however, goes further than this; for just as he considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammocoetes-like form, so he considers Ammocoetes to have arisen {199}from a degenerated Selachian; therefore, in order to be logical, he ought to show that the thyroid of Ammocoetes is an intermediate downward step between the thyroid of Selachians and that of Amphioxus and the Tunicates. Here, it seems to me, his argument utterly breaks down; it is so clear that the thyroid of Petromyzon links on to that of the higher fishes, and that the Ammocoetes thyroid is so immeasurably more complicated and elaborate a structure than is that of Petromyzon, as to make it impossible to believe that the Ammocoetes thyroid has been derived by a process of degeneration from that of the Selachian. On the contrary, the manner in which it is eaten up at transformation and absolutely disappears in its original form is, like the other instances mentioned, strong evidence that we are dealing here with an ancestral organ, which is confined to the larval form, and disappears when the change to the higher adult condition takes place. Dohrn's evidence, then, points strongly to the conclusion that the starting-point of the thyroid gland in the vertebrate series is to be found in the thyroid of Ammocoetes, which has given rise, on the one hand, to the endostyle of Amphioxus and the Tunicata, and on the other, to the thyroid gland of Petromyzon and the rest of the Vertebrata.

The evidence which I have just given of the intimate connection of the two pseudo-branchial grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that morphologically these two grooves and the thyroid must be considered together. His explanation is that the whole system represents a modified pair of branchial segments distinct from those belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in various papers, is, I think, right in so far as the separateness of the thyroid segment is concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid gland is a modified pair of gills.

We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial segments, without any necessity of deriving the thyroid gland from a pair of gills.

{200}The evidence that such a median segment has been interpolated ventrally between the foremost pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial segments are marked out on each side by the ventral border of the cartilaginous basket-work; and it is well known, as seen in Fig. 80, that whereas this cartilaginous framework on the two sides meets together in the middle ventral line in the posterior branchial region, it diverges in the anterior region so as to form a tongue-shaped space between the branchial segments on the two sides. This space is covered over with a plate of muco-cartilage which bears on its inner surface the thyroid gland.

In addition to this evidence that we are dealing here with a ventral tongue-like segment belonging to the facial nerve which is interpolated between the foremost branchial segments, we find the most striking fact that at transformation the whole of this muco-cartilaginous plate disappears, the remarkable thyroid gland of the {201}Ammocoetes is eaten up, and nothing is left except a small, totally different glandular mass; and now the cartilaginous basket-work meets together in the middle line in this region as well as in the more posterior region. In other words, the striking characteristic of transformation here is the destruction of this interpolated segment, and the resulting necessary drawing together ventrally of the branchial segments on each side.

VII. 1, thyroid part of segment; VII. 2, hyoid or branchial part; 3-9, succeeding branchial segments belonging to IXth and Xth nerves; V, the velar folds; Ps. br., Dohrn's pseudo-branchial groove; Th. o., thyroid opening; C, curled portion of thyroid.]

Moreover, another most instructive piece of evidence pointing in the same direction is afforded by the behaviour of the ventral epithelial {202}pits, as determined by Miss Alcock. Although there is no indication on the ventral surface of the skin of any difference between the anterior and posterior portions of the respiratory region, yet when the ventral rows of the epithelial pits supplied by each branchial nerve are mapped out, we see how the most anterior ones diverge more and more from the mid-ventral line, following out exactly the limits of the underlying muco-cartilaginous thyroid plate (Fig. 84).

The whole evidence strongly leads to the conclusion that the thyroid portion of the facial segment was inserted as a median tongue between the foremost branchial segments on each side, and that, therefore, the whole facial segment, consisting as it does of a thyroid part and a hyoid or branchial part, may be represented as in Fig. 85, which is obtained by splitting an Ammocoetes longitudinally along the mid-dorsal line, so as to open out the pharyngeal chamber and expose the whole internal surface. The facial segment is marked out by shading lines, the glosso-pharyngeal and vagus segments and the last of the trigeminal segments being indicated faintly. The position of the thyroid gland is indicated by oblique lines, C being the curled portion.

THE UTERUS OF THE SCORPION GROUP.

Seeing how striking is the arrangement and the structure of the glandular tissue of this thyroid, how large the organ is and how absolutely it is confined to Ammocoetes, disappearing entirely as such at transformation, we may feel perfectly certain that a corresponding, probably very similar, organ existed in the invertebrate ancestor of the vertebrate; for the transformation process consists essentially of the discarding of invertebrate characteristics and the putting on of more vertebrate characters; also, so elaborate an organ cannot possibly have been evolved as a larval adaptation during the life of Ammocoetes. We may therefore assert with considerable confidence that the thyroid gland was the palæo-hysteron, and was derived from the uterus of the ancient palæostracan forms. If, then, it be found that a glandular organ of this very peculiar structure and arrangement is characteristic of the uterus of any living member of the scorpion group, then the confidence of this assertion is greatly increased.

In Limulus, as already stated, the genital ducts open separately {203}on each side of the operculum, and do not combine to form a uterus; I have examined them and was unable to find any glandular structure at all resembling that of the thyroid gland of Ammocoetes. I then turned my attention to the organs of the scorpion, in which the two ducts have fused to form a single uterus.

C, cavity of chamber. A portion of the epithelial lining of the channels of emission is drawn above the section of the uterus.]

{204}[Illustration: FIG. 87.--LONGITUDINAL SECTION THROUGH THREE OF THE CONES OF THE UTERINE GLANDS OF THE SCORPION.]

I there found that both in the male and in the female the genital ducts on each side terminate in a common chamber or uterus, which underlies the whole length of the operculum, and opens to the exterior in the middle line, as shown in Fig. 76. In transverse section, this uterus has the appearance shown in Fig. 86, i.e. it is a large tube, evidently expansible, lined with a chitinous layer and epithelial cells belonging to the chitinogenous layer, except in two symmetrical places, where the uniformity of the uterine wall is interrupted by two large, remarkable glandular structures. The structure of these glands is better shown by means of sagittal sections. They are composed of very long, wedge-shaped cells, each of which possesses a large, round nucleus at the basal end of the cell (Fig. 87). These cells are arranged in bundles of about eight to ten, which are separated from each other by connective tissue, the apex of each conical bundle being directed into the cavity of the uterus; where this brush-like termination of the cells reaches the surface, the chitinous layer is absent, so that this layer is, on surface view, seen (Fig. 88 (b)) to be pitted with round holes over that part of the internal surface of the uterus where these glands are situated. Each of these holes represents the termination of one of these cone-shaped wedges of cells. If the section is cut across at right angles to the axis of these cones, then its appearance is represented in Fig. 88 (a), and shows well the arrangement of the blocks of cells, separated from each other by connective tissue. When the section passes through the basal part of the cones, and only in that case, then the nuclei of the cells appear, often in considerable numbers in one section, as {205}is seen in Fig. 89. In Fig. 88 the section shows at b the holes in the chitin in which the cones terminate, and then a series of layers of sections through the cones further and further away from their apices.

These conical groups of long cells, represented in Fig. 87, form on each side of the uterus a gland, which is continuous along its whole length, and thus forms a line of secreting surface on each side, just as in the corresponding arrangement of the glandular structures in the thyroid of Ammocoetes. This uterus and glandular arrangement is found in both sexes; the gland is, however, more developed in the male than in the female scorpion.

The resemblance between the structure of the thyroid of Ammocoetes and the uterus of the scorpion is most striking, except in two respects, viz. the nature of the lining of the non-glandular part of the cavity--in the one case ciliated, in the other chitinous--and the place of exit of the cavity, the thyroid of Ammocoetes opening into the respiratory chamber, while the uterus of Scorpio opens direct to the exterior.

With respect to the first difference, the same difficulty is met {206}with in the comparison of the ciliated lining of the tube in the central nervous system of vertebrates with the chitinous lining of the intestine in the arthropod. Such a difference does not seem to me either unlikely or unreasonable, seeing that cilia are found instead of chitin in the intestine of the primitive arthropod Peripatus. Also the worm-like ancestors of the arthropods almost certainly possessed a ciliated intestine. Finally, the researches of Hardy and McDougall on the intestine of Daphnia point directly to the presence of a ciliated rather than a chitinous epithelial lining of the intestine in this animal--all evidence pointing to the probability that in the ancient arthropod forms, derived as they were from the annelids, the intestine was originally ciliated and not chitinous. It is from such forms that I suppose vertebrates to have sprung, and not from forms like the living king-crabs, scorpions, Apus, Branchipus, etc. I only use them as illustrations, because they are the only living representatives of the great archaic group, from which the Crustacea, Arachnida, and Vertebrata all took origin.

The second difference is more important, and is at first sight fatal to any comparison between the two organs. How is it possible to compare the uterus of the scorpion, which opens on the surface by an external genital opening, with the thyroid of Ammocoetes, which opens by an internal opening into the respiratory chamber? However close may be the histological resemblance of structure in the two cases, surely such a difference is too great to be accounted for.

It is, however, to be remembered that the operculum of Scorpio covers only the terminal genital apparatus, and does not, therefore, resemble the operculum of the presumed ancestor of Ammocoetes, which, as already argued, must have resembled the operculum of Thelyphonus with its conjoint branchial and genital apparatus, rather than that of Scorpio. Before, therefore, making too sure of the insuperable character of this difficulty, we must examine the uterus of the Pedipalpi, and see the nature of its opening.

The nature of the terminal genital organs in Thelyphonus has been described to some extent by Blanchard, and more recently by Tarnani. The ducts of the generative organs terminate, according to the latter observer, in the large uterus, which is found both in the male and female; he describes the walls of the uterus in the female as formed of elongated glandular epithelium, with a strongly-developed porous, chitinized intima. In the male, he says that the {207}epithelium of the uterus masculinus and its processes is extraordinarily elongated, the chitin covering being thick. In these animals, then, the common chamber or uterus into which the genital ducts empty, which, like the corresponding chamber in the scorpion, occupies the middle region of the operculum, is a large and conspicuous organ. Further, and this is a most striking fact, the uterus masculinus does not open direct to the exterior, but into the genital cavity, "which lies above the uterus, so that the latter is situated between the lower wall of the genital cavity and the outer integument." The opening, therefore, of the uterus is not external but internal, into the large internal space known as the genital cavity. The arrangement is shown in Fig. 91, taken from Tarnani's paper, which represents a diagrammatic sagittal section through the exit of the male genital duct. Yet another most striking fact is described by Tarnani. This genital cavity is continuous with the pulmonary or gill cavities on each side, so that instead of a single opening for the genital products and one on each side for each gill-pouch, as would be the case if the arrangement was of the same kind as in the scorpion, there is a single large chamber, the genital chamber, common to both respiratory and genital organs.

The thick line is the operculum, composed of two segments, I. and II. Ut. Masc., uterus masculinus; Gen. Ch., genital chamber; Int. Op., internal opening; Ext. Op., external opening common to the genital and respiratory organs.]

This genital chamber, according to Tarnani, opens to the exterior by a single median opening between the operculum and the succeeding segment; similarly, a communication from side to side exists between the second pair of gill-pouches. I have been able to examine Hypoctonus formosus and Thelyphonus caudatus, and in both cases, in both male and female, the opening to the exterior of the common chamber for respiration and for the genital products was {208}not a single opening, as described by Tarnani in Thelyphonus asperatus, but on each side of the middle line, a round orifice closed by a lid, like the nest of the trapdoor spider, led into the common genital chamber (Gen. Ch.) into which both uterus and gills opened. In Fig. 77 I have endeavoured to represent the arrangement of the genital and respiratory organs in the male Thelyphonus according to Tarnani's and my own observations.

If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the operculum was fused with the first branchial appendage, among which must be included the old sea-scorpions, then it is most significant that their uterus should open internally into a cavity which was continuous with the respiratory cavity. Thus not only the structure of the gland, but also the arrangement of the internal opening into the respiratory, or, as it became later, the pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammocoetes represents the uterus of the extinct Eurypterus-like ancestor.

Into this uterus the products of the generative organs were poured by means of the vasa deferentia, so that there was not a single median opening or duct in connection with it, but also two side openings, the terminations of the vasa deferentia. These are described by Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral character, although the body of the organ is median and single; these ducts then pass within the body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs which are situated in these animals, as in other scorpions, in the abdomen, so that the direction of the ducts from the generative glands to the uterus is headwards. If, however, we examine the condition of affairs in Limulus, we find that the main mass of the generative material is cephalic, forming with the liver that dense glandular mass which is packed round the supra-oesophageal and prosomatic ganglia, and round the stomach and muscles of the head-region. From this cephalic region the duct passes out on each side at the junction of the prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum, near the middle line, as is indicated in Fig. 75.

We have, therefore, two distinct possible positions for the genital ducts among the group of extinct scorpion-like animals, the one from the cephalic region to the operculum, and the other from the abdominal region to the operculum.

{209}THE GENERATIVE GLANDS OF LIMULUS AND ITS ALLIES.

The whole argument, so far, has in every case ended with the conclusion that the original scorpion-like form with which I have been comparing Ammocoetes resembled in many respects Limulus rather than the present-day scorpions, and therefore in the case also of the generative organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that they were cephalic in position rather than abdominal. If this were so, then the duct on each side, starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge mass of generative gland lying within the prosomatic carapace, just as I have represented in the figure of Eurypterus (Fig. 79), a course which would take much the same direction as the ciliated groove in Ammocoetes.

We ought, therefore, on this supposition, to expect to find the remains of the invertebrate generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in the abdomen.

Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is composed of two separate glands, the generative glands and the hepatico-pancreatic glands--the so-called liver--and surrounds closely the central nervous system and the alimentary canal. From the generative glands proceed the genital ducts to terminate on the posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic stomach, and carry the fluid by means of which the food is digested, for, in all these animals, the active digesting juices are formed in the so-called liver, and not in the cells of the stomach or intestine.

It is a very striking fact that the brain of Ammocoetes is much too small for the brain-case, and that the space between brain and brain-case is filled up with a very peculiar glandular-looking tissue, which is found in Ammocoetes and not elsewhere. Further, it is also striking that in the brain of Ammocoetes there should still exist the remains of a tube extending from the IVth ventricle to the surface at the conus post-commissuralis, which can actually be traced right into this tissue on the outside of the brain (see Fig. 13, a-e, Pl. XXVI., in my paper in the Quarterly Journal of Microscopical Science). {210}This, in my opinion, is the last remnant of one of the old liver-ducts which extended from the original stomach and intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the pineal eye and its nerve, in Fig. 31, also in Fig. 22, and separately in Fig. 92. It is composed of large cells, with a badly staining nucleus, closely packed together with lines of pigment here and there between the cells; this pigment is especially congregated at the spot where the so-called liver-duct loses itself in this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is not true; peculiar it certainly is, but fatty it is not.

This tissue has been largely described as a peculiar kind of connective tissue, which is there as packing material, for the purpose of steadying a brain too small for its case. On the face of it such an explanation is unscientific; certainly for all those who really believe in evolution, it is out of the question to suppose that a brain-case has been laid down in the first instance too large for the brain, in order to provide room for a subsequent increase of brain; just as it is out of the question to suppose that the nervous system was laid down originally as an epithelial tube in order to provide for the further development of the nervous system by the conversion of more and more of that tube into nervous matter. Yet this latter proposition has been seriously put forward by professed believers in evolution and in natural selection.

This tissue bears no resemblance whatever to any form of connective tissue, either fatty or otherwise. By every test this tissue tells as plainly as possible that it is a vestige of some former organ, presumably glandular, which existed in that position; that it is not there as packing material because the brain happened to be too small for its case, but that, on the contrary, the brain is too small for its case, because the case, when it was formed, included this organ as well as the brain; in other words, this tissue {211}is there because it is the remnant of the great glandular mass which so closely surrounds the brain and alimentary canal in animals such as Limulus. In my paper in the Quarterly Journal of Microscopical Science, in which I was comparing the tube of the vertebrate nervous system with the alimentary canal of the invertebrate, I spoke of this tissue as being the remnant of the invertebrate liver. At the same time the whole point of my argument was that the glandular material surrounding the brain of Limulus was made up of two glands--liver and generative gland--so that this tissue might be the remnant of either one or the other, or both. All I desired, at that time, was to point out the glandular appearance of this so-called packing tissue, which surrounded the brain-region of Ammocoetes, in connection with the fact that the brain and alimentary canal of Limulus were closely surrounded with a glandular mass composed partly of liver, partly of the generative gland. At present, I think these large cells found round the brain in Ammocoetes are much more likely to be the remnant of the generative gland than of the liver; the size of the cells and their arrangement recalls Owen's picture of the generative gland in Limulus, and seeing how important all generative glands are in their capacity of internal secreting glands, apart entirely from the extrusion of the ripe generative products, and how unimportant is an hepato-pancreas when the alimentary canal is closed, it is much more likely that of the two glands the former would persist longer than the latter. It may be that all that is left of the old hepato-pancreas consists of the pigment so markedly found in between these cells, especially at the place where the old liver-duct reaches the surface of the brain; just as the only remnant of the two pineal eyes in the higher vertebrates is the remains of the pigment, known as brain-sand, which still exists in the pineal gland of even the highest vertebrate. This, however, is a mere speculation of no importance. What is important is the recognition of this tissue round the brain as the remnant of the glandular mass round the brain of animals such as Limulus. Still further confirmation of the truth of this comparison will be given when the origin of the auditory organ comes up for discussion.

I conclude, therefore, from the evidence of Ammocoetes, that the generative glands in the ancestral form were situated largely in the cephalic region, and suggest that the course and direction of the ciliated pseudo-branchial grooves on each side indicate the direction of the {212}original opercular ducts by which the generative products were conveyed to the uterine chamber, i.e. to the chamber of the thyroid gland, and thence to the common genital and respiratory cavity, and so to the exterior.

It is easy to picture the sequence of events. First, the generative glands, chiefly confined to the cephalic region, communicating with the exterior by separate ducts on the inner surface of the operculum as in Limulus. Then, in connection with the viviparous habit, these two oviducts fused together to form a single chamber, covered by the operculum, which opened out to the exterior by a single opening as in Scorpio: or, in forms such as Eurypterus, in which the operculum had amalgamated with the first branchial appendage and possessed a long, tongue-like ventral projection, the amalgamated ducts formed a long uterine chamber which opened internally into the genital chamber--a chamber which, as in Thelyphonus, was common with that of the two gill-chambers, while at the same time the genital ducts from the cephalic generative material opened into two uterine horns which arose from the anterior part of the uterus, as in Thelyphonus.

Such an arrangement would lead directly to the condition found in Ammocoetes, if the generative material around the brain lost its function, owing to a new exit for generative products being formed in the posterior part of the body. The connection of the genital duct with this cephalic gland being then closed and cut off by the brain-case, the position of the oviducts would still be shown by the ciliated grooves opening into the folded-down thyroid tube, i.e. the folded-down horns of the uterus; the uterus itself would remain as the main body of the thyroid and still open by a conspicuous orifice into the common respiratory chamber. Next, in the degeneration process, we may suppose that not only the oviducts opened out to form the ciliated groove, but that the uterine chamber itself also opened out, and thus formed the endostyle of Amphioxus and of the Tunicata.

It might seem at first sight improbable that a closed tube should become an open groove, although the reverse phenomenon is common enough; the difficulty, however, is clearly not considered great, for it is precisely what Dohrn imagines to have taken place in the conversion of the thyroid of Ammocoetes into the endostyle of Amphioxus and the Tunicata; it is only carrying on the same idea a stage further to see in the open, ciliated groove of Ammocoetes the remains of the closed genital duct of Limulus and its allies.

{213}Such is the conclusion to which the study of the thyroid gland in Ammocoetes seems to me to lead, and one cannot help wondering why such an unused and rudimentary organ should have remained after its original function had gone. Is it possible to find out its function in Ammocoetes?

THE FUNCTION OF THE THYROID GLAND IN AMMOCOETES.

The thyroid gland has been supposed to secrete mucus into the respiratory chamber for the purpose of entangling the particles of food, and so aiding in digestion. I see no sign of any such function; neither by the thionin method, nor by any other test, have Miss Alcock and myself ever been able to see any trace of mucous secretion in the thyroid, and, indeed, the thyroid duct is always remarkably free from any sign of any secretion whatever. Not only is there no evidence of any mucous secretion in the thyroid of the fully developed Ammocoetes, but also no necessity for such secretion from Dohrn's point of view, for so copious a supply of mucus is poured out by the glands of the branchiæ, along the whole pharyngeal tract, especially from the cells of the foremost or hyoid gills, as to mix up with the food as thoroughly as can possibly be needed. Further, too, the ciliated pharyngeal bands described by Schneider are amply sufficient to move this mixed mass along in the way required by Dohrn. Finally, the evidence given by Miss Alcock is absolutely against the view that the thyroid takes any part in the process of digestion, while, on the other hand, her evidence directly favours the view that these glandular branchial mucus-secreting cells play a most important part in the digestive process.

In Fig. 93, A is a representation of the respiratory tissue of a normal gill; B is the corresponding portion of the first or hyoid gill, in which, as is seen, the whole of the respiratory epithelium is converted into gland-tissue of the nature of mucous cells.

To sum up, the evidence is clear and conclusive that the Ammocoetes possesses in its pharyngeal chamber mucus-secreting glands, which take an active part in the digestive process, which do not in the least resemble either in structure or arrangement the remarkable cells of the thyroid gland, and that the experimental evidence that the latter cells either secrete mucus or take any part in digestion is so far absolutely negative. It is, of course, possible, that they {214}may contain mucin in the younger developmental stages, and therefore possible that they might at that stage secrete it; they certainly, however, show no sign of doing so in their more adult condition, and cannot be compared in the very faintest degree to the glandular cells of the pharyngeal region. It is also perfectly possible for gland-cells belonging to a retrograde organ to become mucus-secreting, and so to give rise to the cells of Amphioxus and the Tunicata.

If, then, these cells were not retained for digestive purposes, what was their function? To answer this question we must first know the function of the corresponding gland-cells in the uterus of the scorpion, which undoubtedly secreted into the cavity of the uterus and took some part in connection with the generative act, and certainly not with digestion. What the function of these cells is or in what way they act I am unable at present to say. I can only suppose that the reason why the thyroid gland has persisted throughout the vertebrate kingdom, after the generative tissues had found a new outlet for their products in the body-cavity of the posterior region, is because it possessed some important function in addition to that connected with the exit of the products of the generative organs; a function which was essential to the well-being, or even to the life of the animal. We do not know its function in the scorpion, or the nature of its secretion in that animal. We know only that physiology at the present day has demonstrated clearly that the actual external secretion of a gland may be by no means its most important function; in addition, glands possess what is called an internal secretion, viz. a {215}secretion into the blood and lymph, and this latter secretion may be of the most vital importance. Now, the striking fact forces itself prominently forward, that the thyroid gland of the higher vertebrates is the most conspicuous example of the importance of such internal secretion. Here, although ductless, we have a gland which cannot be removed without fatal consequences. Here, in the importance of its internal secretion, we have a reason for the continued existence of this organ; an organ which remains much the same throughout the Vertebrata down to and including Petromyzon, but, as is seen at transformation, is all that remains of the more elaborate, more extensive organ of Ammocoetes. Surely we may argue that it is this second function which has led to the persistence of the thyroid, and that its original form, without its original function, is seen in Ammocoetes, because that is a larval form, and not a fully-developed animal. As soon as the generative organs of Petromyzon are developed at transformation, all trace of its connection with a genital duct vanishes, and presumably its internal secretory function alone remains.

Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it is in the neck, should have any sympathy with sexual functions if it was originally a gland concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary, likely enough if it originated from a glandular organ in connection with the sexual organs of the palæostracan ancestor of the vertebrate.

Freund has shown, and shown conclusively, that there is an intimate connection between the condition of the thyroid gland and the state of the sexual organs, not only in human beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty, may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may become so extreme as to threaten suffocation, or even cause death; that the period of puberty and the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic goitre especially is associated with a special disease connected with the uterus.

{216}SUMMARY.

Step by step in the preceding chapters the evidence is accumulating in favour of the origin of vertebrates from a member of the palæostracan group. In a continuously complete and harmonious manner the evidence has throughout been most convincing when the vertebrate chosen for the purpose of my arguments has been Ammocoetes.

So many fixed points have been firmly established as to enable us to proceed further with very great confidence, in the full expectation of being able ultimately to homologize the Vertebrata with the Palæostraca even to minute details.

Perhaps the most striking and unexpected result of such a comparison is the discovery that the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the evidence that it is difficult to see how the homology can be denied.

In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the operculum, which always bears the terminal generative organs and is fused in the middle line. In many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed of two segments fused together: an anterior one which carried the uterus, and a posterior one which carried the first pair of branchiæ.

In the other animal (Ammocoetes) the foremost segments of the mesosomatic or respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal, for the facial supplies two segments fused together; the anterior one, the thyroid segment, carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of branchiæ.

Just as in Eurypterus the fused segment, carrying the uterus on its internal surface, forms a long median tongue which separates the most anterior branchial segments on each side, so also the fused segment carrying the thyroid forms in Ammocoetes a long median tongue, which separates the most anterior branchial segments on each side.

Finally, and this is the most conclusive evidence of all, this thyroid gland of Ammocoetes is totally unlike that of any of the higher vertebrates, and, indeed, of the adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a comparison valid with respect to its shape, but also with respect to its structure, which is absolutely unique among vertebrates, and very different to that of any other vertebrate gland, but resembles in a striking manner a glandular structure found in the uterus, both of male and female scorpions.

The generative glands in Limulus, together with the liver-glands, form a large glandular mass, situated in the head-region closely surrounding the central nervous system, so that the genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in the abdominal region, so that their ducts pass headwards to the operculum.

Probably in the Palæostraca the generative mass was situated in the cephalic region as in Limulus, and it is probable that the remnant of it still exists in {217}Ammocoetes in the shape of the peculiar large cells packed together, with pigment masses in between them, which form such a characteristic feature of the glandular-looking material, which fills up the space between the cranial walls and the central nervous system.

Finally, the relationship which has been known from time immemorial to exist between the sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without any explanation, may possibly be the last reminiscence of a time when the thyroid glands were the uterine glands of the palæostracan ancestor.

The consideration of the facial nerve, and the segments it supplies, still further points to the origin of the Vertebrata from the Palæostraca.

{218}CHAPTER VI

THE EVIDENCE OF THE OLFACTORY APPARATUS

Fishes divided into Amphirhinæ and Monorhinæ.--Nasal tube of the lamprey.--Its termination at the infundibulum.--The olfactory organs of the scorpion group.--The camerostome.--Its formation as a tube.--Its derivation from a pair of antennæ.--Its termination at the true mouth.--Comparison with the olfactory tube of Ammocoetes.--Origin of the nasal tube of Ammocoetes from the tube of the hypophysis.--Direct comparison of the hypophysial tube with the olfactory tube of the scorpion group--Summary.

In the last chapter I finished the evidence given by the consideration of the mesosomatic or opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with that of previous chapters, and not only confirms the conclusion that vertebrates arose from some member of the Palæostraca, but helps still further to delimit the nature of that member. It is almost startling to see how the hypothesis put forward in the second chapter, suggested by the consideration of the nature of the vertebrate central nervous system and of the geological record, has received stronger and stronger confirmation from the consideration of the vertebrate optic apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland. All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony will be found upon consideration of the rest of the vertebrate organs.

Following naturally upon the segments supplied by the opisthotic (mesosomatic) cranial nerves, we ought to consider now the segments supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments belonging to the trigeminal nerve-group in the vertebrate, and in the invertebrate the segments of the prosoma with their characteristic appendages. There are, however, in all vertebrates in this foremost cranial region, in addition to the optic nerves, two other well-marked nerves of special sense, the olfactory and the auditory. Of these, the former are in the same class as the optic nerves, for they arise {219}in the vertebrate from the supra-infundibular nerve-mass, and in the invertebrate from the supra-oesophageal ganglia. The latter arise in the vertebrate from the infra-infundibular nerve-mass, and, as the name implies, are situated in the region where the pro-otic nerves are contiguous to the opisthotic, i.e. at the junction of the prosomatic and mesosomatic nerve-regions.

The chapter dealing with the evidence given by the olfactory nerves and the olfactory apparatus ought logically to have followed immediately upon the one dealing with the optic apparatus, seeing that both these special sense-nerves belong to the supra-infundibular segments in the vertebrate and to the supra-oesophageal in the invertebrate.

I did not deal with them in that logical sequence because it was necessary for their understanding to introduce first the conception of modified appendages as important factors in any consideration of vertebrate segments; a conception which followed naturally after the evidence afforded by the skeleton in Chapter III., and by the branchial segments in Chapter IV. So, too, now, although the discussion of the prosomatic segmentation ought logically to follow immediately on that of the mesosomatic segmentation, I have determined to devote this chapter to the evidence of the olfactory organs, because the arguments as to the segments belonging to the trigeminal nerve-group are so much easier to understand if the position of the olfactory apparatus is first made clear.

In all vertebrates the nose is double and opens into the pharynx, until we descend to the fishes, where the whole group Pisces has been divided into two subsidiary groups, Monorhinæ and Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening. The Monorhinæ include only the Cyclostomata--the lampreys and hag-fishes.

In the lampreys the single olfactory tube ends blindly, while in the hag-fishes it opens into the pharynx. In the lamprey, both in Petromyzon and Ammocoetes, the opening of this nasal tube is a conspicuous object on the dorsal surface of the head in front of the transparent spot which indicates the position of the right median eye. It is especially significant, as showing the primitive nature of this median olfactory passage, that a perfectly similar opening in the {220}same position is always found in the dorsal head-shields of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X.

All the evidence points to the conclusion that the olfactory apparatus of the vertebrate originated as a single median tube, containing the special olfactory sense-epithelium, which, although median and single, was innervated by the olfactory nerve of each side. The external opening of this tube in the lamprey is dorsal. How does it terminate ventrally?

The ventral termination of this tube is most instructive and suggestive. It terminates blindly at the very spot where the infundibular tube terminates blindly and the notochord ends. After transformation, when the Ammocoete becomes the Petromyzon, the tube still ends blindly, and does not open into the pharynx as in Myxine; it, however, no longer terminates at the infundibulum, but extends beyond it towards the pharynx.

This position of the nasal tube suggests that it may originally have opened into the tube of the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in value by the fact that in the larval Amphioxus the tube of the central nervous system is open to the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is situated at the base of a pit, known as the olfactory pit because it is supposed to represent the olfactory organ of other fishes.

Following the same lines of argument as in previous chapters, this suggestion indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates consisted of a single median olfactory tube or passage, which led directly into the oesophagus and was innervated, though single and median, by a pair of olfactory nerves which arose from the supra-oesophageal ganglia. Let us see what is the nature of the olfactory organs among arthropods, and whether such a suggestion possesses any probability.

THE OLFACTORY ORGANS OF THE SCORPION GROUP.

At first sight the answer appears to be distinctly adverse, for it is well known that in all the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often called the antennules, are olfactory in function, and these are free-moving, bilaterally {221}situated, independent appendages. Still, even here there is the striking fact that the nerves of these olfactory organs always arise from the supra-oesophageal ganglia, although those to the second pair of antennæ arise from the infra-oesophageal ganglia, just as the olfactory nerves of the vertebrate arise from the supra-infundibular brain-mass. Not only is there this similarity of position, but also a similarity of structure in the olfactive lobes of the brain itself of so striking a character as to cause Bellonci to sum up his investigations as follows:--

"The structure and connections of the olfactive lobes present the same fundamental plan in the higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does of separate groups, may each one be called an olfactory glomerulus."

He attributes this remarkable resemblance to a physiological necessity that similarity of function necessitates similarity of structure, for he considers it out of the question to suppose any near relationship between arthropods and vertebrates.

Truly an interesting remark, with the one fallacy that relationship is out of the question.

The evidence so far has consistently pointed to some member of the palæostracan group as the ancestor of the vertebrates--a group which had affinities both to the crustaceans and the arachnids; indeed, many of its members resembled scorpions much more than they resemble crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely than any others to give a clue to the position of the desired olfactory organs. In these animals and their allies paired olfactory antennæ are not present, either in the living land-forms or the extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in Pterygotus, etc. (Fig. 8), represent the cheliceræ, and correspond, therefore, to the second pair of antennæ in the crustaceans.

What, then, represents the olfactory antennæ in the scorpions? The answer to this question has been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have combined together to form a hollow tube at the base of which the mouth of the animal is situated, so that the food passes along this olfactory passage before it reaches the mouth. This organ is often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in position and appears, therefore, to be an unpaired organ; its paired {222}character is, of course, evident enough, for it is innervated by a pair of nerves, and these nerves, as ought to be the case, arise from the supra-oesophageal ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig. 94).

Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of appendages similar to the other appendages. This last observation of Croneberg has been confirmed by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar terms, without, however, referring to Croneberg's paper. Croneberg further shows that this foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower lip, for from the basal part of the camerostome there projects on each side of the pharynx a dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were, a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral entosclerite of Lankester and Miss Beck.

cam., camerostome; pr. ent., pre-oral entosclerite; l.l., dependent portion of camerostome; ph., pharynx; al., alimentary canal; n. op., median optic nerves; pl., plastron; v.c., ventral nerve chain; 2, 3, second and third appendages.]

At the base of the tubular passage formed by this modified first pair of antennæ the true mouth is found opening directly into the dilated pharynx, the muscles of which enable the act of suction to be carried out. The narrow oesophagus leads out from the pharynx and is completely surrounded by the supra- and infra-oesophageal nerve masses.

Huxley also describes the mouth of the scorpion in precisely the same position (cf. o, Fig. 96).

{223}In order to convey to my readers the antennæ-like character of the camerostome in Galeodes (Fig. 101), and its position, I give a figure (Fig. 94) of the organ from its dorsal aspect, after removal of the cheliceræ and their muscles. A side view of the same organ is given in Fig. 95 to show the feathered termination of the camerostome, and the position of the dependent accessory portion (l.l.) (Croneberg's 'untere Anhang') with its single long antenna-like feather. In both figures the alimentary canal (al.) is seen issuing from the conjoined supra- and infra-oesophageal mass.

As is seen in the figures, the bilateral character of the rostrum, as Croneberg calls it, is apparent not only in its feathered extremity but also in its chitinous covering, the softer median dorsal part (left white in figure) being bounded by two lateral plates of hard chitin, which meet in the middle line near the extremity of the organ. In all the members of the scorpion group, as is clearly shown in Croneberg's figures, the rostrum or camerostome is built up on the same plan as in Galeodes, though the antenna-like character may not be so evident.

gl. supr. oes., supra-oesophageal ganglion; gl. infr. oes., infra-oesophageal ganglion. The rest of the lettering same as in Fig. 94.]

When we consider that the first pair of antennæ in the crustaceans are olfactory in function, Croneberg's observations amount to this--

In the arachnids and their allies the first pair of antennæ form a pre-oral passage or tube, olfactory in function; the small mouth, which opens directly into the pharynx, being situated at the end of this olfactory passage.

{224}Croneberg's observations and conclusions are distinctly of very great importance in bringing the arachnids into line with the crustaceans, and it is therefore most surprising that they are absolutely ignored by Lankester and Miss Beck in their paper published in 1883, in which Latreille only is mentioned with respect to this organ, and his term "camerostome," or upper lip, is used throughout, in accordance with the terminology in Lankester's previous paper. That this organ is not only a movable lip or tongue, but essentially a sense-organ, almost certainly of smell and taste, as follows from Croneberg's conclusions, is shown by the series of sections which I have made through a number of young Thelyphonus (Fig. 102).

I give in Fig. 96 a sagittal median section through the head-end of the animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is seen the median eye (ce.), o is the mouth, Ph. the pharynx, oes. the narrow oesophagus, compressed between the supra-oesophageal (supr. oes.) and infra-oesophageal (infr. oes.) brain mass, which opens into the large alimentary canal (Al.); Olf. pass. is the olfactory passage to the mouth, lined with thick-set, very fine hairs, which spring from the hypostome (Hyp.) as well as from the large conspicuous camerostome (Cam.), which limits this tube anteriorly. The space between the camerostome and the median eye is filled up by the massive cheliceræ, which are not shown in this section, as they begin to appear in the {225}sections on each side of the median one. The muscles of the pharynx and the muscles of the camerostome are attached to the pre-oral entosclerite (pr. ent.). The post-oral entosclerite is shown in section as post. ent. The dorsal blood-vessel, or heart, is indicated at H.

In Fig. 97 I give a transverse section through another specimen of the same litter, to show the nature of this olfactory tube when cut across. Both sections show most clearly that we are dealing here with an elaborate sense-organ, the surface of which is partly covered with very fine long hairs, partly, as is seen in the figure, is composed of long, separate, closely-set sense-rods (bat.), well protected by the long hairs which project on every side in front of them, which recall to mind Bellonci's figure of the 'batonnets olfactives' on the antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the effect that this camerostome is innervated by nerves from the supra-oesophageal ganglia which are clearly bilateral, seeing that they arise from the ganglion on each side and then unite to pass into the camerostome; in other words, paired olfactory nerves from the supra-oesophageal ganglia.

These facts demonstrate with wonderful clearness that in one group of the Arthropoda the olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads directly into the mouth and so to the oesophagus of the animal, and, strikingly enough, this group, the Arachnida, is the very one to which the scorpions belong.

If for any cause the mouth o in Fig. 96 were to be closed, then the olfactory tube (olf. pass.) might still remain, owing to its importance as the organ of smell, and the olfactory tube would terminate blindly at the very spot where the corresponding tube does terminate in the vertebrate, according to the theory put forward in this book.

THE OLFACTORY TUBE OF AMMOCOETES.

In all cases where there is similarity of topographical position in the organs of the vertebrate and arthropod we may expect also to find similarity of structure. At first sight it would appear as though such similarity fails us here, for a cross-section of the olfactory tube in Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance to the section across the olfactory passage of a young Thelyphonus given in Fig. 97.

{226}[Illustration: FIG. 97.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE OF A YOUNG THELYPHONUS.

1 and 2, sections of first and second appendages.]

cart., nasal cartilage.]

{227}As is seen, it is difficult to see any connection between these folds of olfactory epithelium and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of the lamprey, remarkable changes take place at transformation, and examination of the same tube in Ammocoetes demonstrates that the elaborate structure of the adult olfactory organ is actually derived from a much simpler form of organ, represented in Fig. 99. Here, in Ammocoetes, the section is no longer strikingly different from that of the Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the rest of the vertebrates put together.

cart., nasal cartilage.]

Still, even now the similarity between the two organs is not complete, for the tube in the lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids, and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does the development of the individual recapitulate the development of the race, that this very discrepancy becomes converted into a triumphant vindication of the {228}correctness of the theory advocated in this book, as soon as we turn our attention to the development of this nasal tube in the lamprey.

We must always remember not only the great importance of a larval stage for the unriddling of problems of ancestry, but also the great advantage of being able to follow more favourably any clues as to past history afforded by the development of the larva itself, owing to the greater slowness in the development of the larva than of the embryo. Such a clue is especially well marked in the course of development of Ammocoetes according to Kupffer's researches, for he finds that when the young Ammocoetes is from 5 to 7 mm. in length, some time after it has left the egg, when it is living a free larval life, a remarkable series of changes takes place with considerable rapidity, so that we may regard the transformation which takes place at this stage, as in some degree comparable with the great transformation which occurs when the Ammocoetes becomes a Petromyzon.

All the evidence emphasizes the fact that the latter transformation indicates the passage from a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the former transformation is of the same character, it would indicate the passage from the Palæostracan to the Cephalaspid.

What is the nature of this transformation process as described by Kupffer?

It is characterized by two most important events. In the first place, up to this time the oral chamber has been cut off from the respiratory chamber by a septum--the velum--so that no food could pass from the mouth to the alimentary canal. At this stage the septum is broken through, the oral chamber communicates with the respiratory chamber, and the velar folds of the more adult Ammocoetes are left as the remains of the original septum. The other striking change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from a ventral to a dorsal position. Fig. 100, taken from Kupffer's paper, represents a sagittal section through an Ammocoetes 4 mm. long; l.l. is the lower lip, u.l. the upper lip, and, as is seen, the short oral chamber is closed by the septum, vel. Opening ventrally is a tube called the tube of the hypophysis, Hy., which extends close up to the termination of the infundibulum. On the anterior surface of this tube is the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with great {229}rapidity and thickens considerably, thus forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the full-grown Ammocoetes, it becomes the dorsal opening of the nasal tube, as already described. Here, then, in the hypophysial tube we have the original position of the olfactory tube of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to find that such a hypophysial tube is characteristic of the embryological development of every vertebrate, whatever may be the ultimate form of the external nasal orifices.

The single median position of the olfactory organ in the Cyclostomata, in contradistinction to its paired character in the rest of the vertebrates, has always been a stumbling-block in the way of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a single organ and not as a paired organ.

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.]

On the other hand, the two olfactory nerves of Ammocoetes compare absolutely with the olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of Ammocoetes arose from a pair of bilateral organs, which have fused in the middle line.

{230}[Illustration: FIG. 101.--Galeodes. (From the Royal Natural History.)]

{231}The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all cases double.

The evidence of the olfactory organs in the vertebrate not only confirms, in a most striking manner, the theory of the origin of the {232}vertebrate from the Palæostracan, but points indubitably to an origin from a scorpion-like rather than a crustacean-like stock. To complete the evidence, it ought to be shown that the ancient sea-scorpions did possess an olfactory passage similar to the modern land-scorpions. The evidence on this question will come best in the next chapter, where I propose to deal with the prosomatic appendages of the Palæostracan group.

SUMMARY.

The vertebrate olfactory apparatus commences as a single median tube which terminates dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and terminates blindly at the infundibulum in Ammocoetes. The dorsal position of the nasal opening is not the original one, but is brought about by the growth of the upper lip. The nasal tube originally opened ventrally, and was at that period of development known as the tube of the hypophysis.

The evidence of Ammocoetes thus goes to show that the olfactory apparatus started as an olfactory tube on the ventral side of the animal, which led directly up to, and probably into, the oesophagus of the original alimentary canal of the palæostracan ancestor.

Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a tube or olfactory passage, which leads directly into the mouth and oesophagus of the animal.

This olfactory passage is very conspicuous in all members of the scorpion group, and, like the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those supplying the first pair of antennæ in crustaceans as to their origin from the supra-oesophageal ganglia.

This nasal passage, or tube of the hypophysis, corresponds in structure and in position most closely with the olfactory tube of the scorpion group, the only difference being that in the latter case it opens directly into the oesophagus, while in the former, owing to the closure of the old mouth, it cannot open into the infundibulum.

The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most interesting, for, whereas the former points indubitably to an ancestor having scorpion-like affinities, the structure of the lateral eyes points distinctly to crustacean, as well as arachnid, affinities.

Taking the two together the evidence is extraordinarily strong that the vertebrate arose from a member of the palæostracan group with marked scorpion-like affinities.

{233}CHAPTER VII

THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES

Comparison of the trigeminal with the prosomatic region.--The prosomatic appendages of the Gigantostraca.--Their number and nature.--Endognaths and ectognath.--The metastoma.--The coxal glands.--Prosomatic region of Eurypterus compared with that of Ammocoetes.--Prosomatic segmentation shown by muscular markings on carapace.--Evidence of coelomic cavities in Limulus.--Summary.

The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the arthropod in the last chapter is confirmatory proof of the soundness of the proposition put forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a segmentation implies a definite series of body-segments, corresponding to the mesomeric segmentation of the vertebrate, and a definite series of appendages corresponding to the splanchnic segmentation of the vertebrate.

Of the foremost segments belonging to the supra-oesophageal region characterized by the presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of the vertebrate, belonging, as they all do, to the supra-infundibular region.

Of the infra-oesophageal segments belonging to the prosoma and mesosoma respectively, the correspondence between the mesosomatic segments carrying the branchial appendages and the uterus, with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been fully proved in previous chapters.

There remain, then, only the segments of the prosomatic region to be considered, a region which, both in the vertebrate and invertebrate, is never respiratory in function but always masticatory, such {234}mastication being performed in Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of the prosomatic appendages specially modified for that purpose, and in the vertebrates by the masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic appendages.

The investigations of van Wijhe and of all observers since the publication of his paper prove that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear nerves--nerves which supply muscles moving the lateral eyes.

In accordance, then, with the evidence afforded by the nerves of the branchial segments, it follows that the muscles supplied by the motor part of the trigeminal ought originally to have moved the appendages belonging to a series of prosomatic segments. On the other hand, the eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore have been grouped originally in a segmental series corresponding to the prosomatic appendages.

The evidence for and against this conclusion will be the subject of consideration in this and the succeeding chapters. At the outset it is evident that any such comparison necessitates an accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature of the corresponding appendages.

In all this group of animals, the evidence as to the number of segments in either the prosomatic or mesosomatic regions is given by--

1. The number of appendages.

2. The segmental arrangement of the muscles of the prosoma or mesosoma respectively.

3. The segmental arrangement of the coelomic or head-cavities.

4. The divisions of the central nervous system, or neuromeres, together with their outgoing segmental nerves.

It follows, therefore, that if from any cause the appendages are not apparent, as is the case in many fossil remains, or have dwindled {235}away and become insignificant, we still have the muscular, coelomic, and nervous arrangements left to us as evidence of segmentation in these animals, just as in vertebrates.

In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve; (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres. The main difference between these two regions in Limulus consists in the closer aggregation of the prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the prosomatic region of the brain.

The number of prosomatic segments in Limulus is not evident by examination of the prosomatic carapace, so that the most reliable guide to the segmentation of this region is given by the appendages, of which one pair corresponds to each prosomatic segment.

The number of such segments, according to present opinion, is seven, viz.:--

(1) The foremost segment, which bears the cheliceræ.

(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and

(7) The last segment, to which belongs a small abortive pair of appendages, known by the name of the chilaria, situated between the last pair of locomotor appendages and the operculum or first pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing (Fig. 103).

Of these seven pairs of appendages, the significance of the first and the last has been matter of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether their nerves belong to the infra-oesophageal group, or are in reality supra-oesophageal.

It is instructive to observe the nature and the anterior position of this pair of appendages in the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to be represented by the small non-chelate antennæ seen in Fig. 8, B and C (p. 27), taken from Woodward. If such is the case, then these figures show that a pair of appendages is missing in each {236}of these forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his restorations were throughout consistent. Schmidt, in Pterygotus osiliensis has shown that the true number was six, not five, as seen in his restoration given in Fig. 8, A (p. 27).

The gnathic bases of the appendages have been separated from those of the other side to show the promesosternite or endostoma (End.).]

With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm, however, in his recent beautiful preparations from Schmidt's specimens and others collected at Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as those of Limulus. I reproduce his figure (Fig. 104) showing the small chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.

{237}So, also, since Woodward's monograph, Laurie has discovered in Slimonia acuminata a small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions were in the same position and of the same character as the cheliceræ of the scorpions.

In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise from the foremost prosomatic ganglia, and the reason why they appear to belong to the supra-oesophageal brain-mass has been made clear by Brauer's investigations on the embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the dorsal side of the oesophagus during development, thus becoming pseudo-supra-oesophageal, though in reality belonging to the infra-oesophageal ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second pair of antennæ in the crustacea.

{238}I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are also exceptional in position.

Next for consideration come the chilaria (7 in Fig. 103), which Lankester did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their very appearance, with their spinous hairs corresponding to those of the other gnathites and their separate nerve-supply, all point distinctly to their being a modified pair of appendages, and, indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found embryologically that they arise in the same way as the rest of the prosomatic appendages, and belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer has found that in the scorpion there is in the embryo a segment, whose appendages degenerate, which is situated between the segment bearing the last pair of thoracic appendages and the genital operculum--a segment, therefore, comparable in position to the chilarial segment of Limulus.

Coming now to the five locomotor appendages, we find that they resemble each other to a considerable extent in most cases, with, however, certain striking differences. Thus in Limulus they are chelate, with their basal joints formed as gnathites, except in the case of the fifth appendage, in which the extremity is modified for the purpose of digging in the sand. In Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no prehensile power, and in most cases could have had no appreciable share in locomotion, called by Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being withdrawn entirely under the hood. The fifth appendage is usually different, being a large swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the ectognath.

Finally, in Drepanopterus Bembycoides, as stated by Laurie, all five locomotor appendages are built up after the same fashion, the last one not being formed as a paddle-shaped organ or elongated as {239}in Stylonurus, but all five possess no special locomotor or prehensile power. According to Laurie this is a specially primitive form of the group.

It is significant to notice from this sketch that with the absence of special prehensile terminations such as chelæ, or the absence of special locomotor functions such as walking or swimming, these appendages tend to dwindle and become insignificant, taking up the position of mere feelers round the mouth, and at the same time are concentrated and pressed closely together, so that their appendage-nerves must also be close together.

This sketch therefore shows us that--

Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the time when the vertebrates first appeared, in very many cases dwindling away; the latter especially no longer functioned as locomotor appendages, but were becoming more and more mere palps or tentacles situated round the mouth, which could by no possibility afford any help to locomotion.

On the contrary, the sixth pair of appendages--the ectognaths--remained powerful, being modified in many cases into large oar-like limbs by which the animal propelled itself through the water.

It is a striking coincidence that those ancient fishes, Pterichthys and Bothriolepis, should have possessed a pair of large oar-like appendages.

At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of appendages, remained strong and vigorous. What about the seventh pair, the chilaria of Limulus?

Of all the prosomatic appendages these are the most interesting from the point of view of my theory, for whereas in the scorpion of the present day they have dwindled away and left no trace except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and become a much more important organ than the chilaria of Limulus.

In all these animals a peculiarly striking and unique structure is found in this region known by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to be formed by the fusion of the two chilarial appendages.

All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seen {240}between the basal joints of all the pairs of locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium, and served more effectually to enclose the posterior part of the buccal orifice, being found exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski, and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths worked.

Quite recently Gerhard Holm has worked over again the very numerous specimens of Eurypterus Fischeri, which are obtainable at Rootziküll, and has thrown new light on the relation of the metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus was not the metastoma, for when the metastoma is removed another plate (End., Fig. 105, B) situated {241}internally to it is disclosed, which, in his view, corresponds to the sternite between the bases of the pro-somatic appendages in Limulus, i.e. to the sternite called by Lankester, the pro-mesosternite (End., Fig. 103). This inner plate formed with the metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the bases of the ectognaths worked. In other words, the removal of the metastoma discloses in Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus, or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name of endostoma.

To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in the direction of the throat, and forms, therefore, the lower lip (Hyp., Fig. 105, B) of the passage of the mouth (olf. p.). This membrane bears upon its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently, in his preliminary communication, he describes this lower lip as forming, in all probability, an olfactory organ; in his full communication he repudiates this suggestion, because he thinks it unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated at the base of an olfactory passage, he would have recognized that his first suggestion is in striking accordance with the nature of the entrance to the mouth in other scorpions.

That Eurypterus also possessed a camerostome (cam.) seems to follow of necessity from its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal section of one of these old palæostracan forms would be obtained by the combination of actual sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a due recognition of Holm's researches. Such a section is represented in Fig. 105, B, in which I have drawn the central nervous system and its nerves, the median eyes (C.E.), the olfactory organs (Cam.), the pharynx (Ph.), oesophagus (oes.), and alimentary canal (Al.), but have not tried to indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), and {242}the foremost mesosomatic segments by numbers (8-13). I have placed the four endognaths and the nerves going to them close together, and made them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and have indicated the position and size of the large ectognath, with its separate nerve, by (6). If among the ancient Eurypterus-like forms, which were living at the time when vertebrates first appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five pairs of tentacles in the lowest vertebrate.

This prosomatic or oral chamber, as it might be called, was limited posteriorly by the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines that the gill-chambers, instead of being separate, are united to form one large respiratory chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid part of the facial) would run, as shown in the figure, close together in the first part of their course, and would separate when the ventral surface was reached, to pass headwards and tailwards respectively.

THE COXAL GLANDS.

One more characteristic of these appendages requires mention, and that is the excretory glands situated at the base of the four endognaths known as the coxal glands. These glands are the main excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their name, coxal {243}glands; and, seeing the importance of the excretory function, it is likely enough that they would remain, even when the appendages themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal glands would also be concentrated, so that in the diagram (Fig. 105) they would rightly be grouped together in the position indicated (cox. gl.).

Such a diagram indicates the position of all the important organs of the head-region except the special organs for taste and hearing. These, for the sake of convenience, I propose to take separately, in order at this stage of my argument not to overburden the simplicity of the comparison I desire to make with too much unavoidable detail.

THE PROSOMATIC REGION OF AMMOCOETES.

Let us now compare this diagram with that of the corresponding region in Ammocoetes and see whether or no any points of similarity exist.

With respect to this region, as in so many other instances already mentioned, Ammocoetes occupies an almost unique position among vertebrates, for the region supplied by the trigeminal nerve--the prosomatic region--consists of a large oral chamber which was separated from the respiratory chamber in the very young stage by a septum which is subsequently broken through, and so the two chambers communicate.

This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the circular mouth; of these, the ventral pair are large, fused together, and attached to the lower lip.

All the muscles belonging to this oral chamber are of the visceral type, and are innervated by the trigeminal nerve. In accordance with the evidence obtained up to this point this means that such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor, similarly to the oral chamber just figured for Eurypterus.

This chamber in the full-grown Ammocoetes is not only open to the respiratory chamber, but is bounded by the large upper lip (U.L., Fig. 106, D). On the dorsal surface of this region, in front of the {244}pineal eye (C.E.), is the most conspicuous opening of the olfactory tube (Na.), which olfactory tube passes from the dorsal region to the ventral side to terminate blindly at the very spot where the infundibulum comes to the surface of the brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body (Pit.). A sagittal section, then, in diagram form, of the position of parts in the full-grown Ammocoetes, would be represented as in Fig. 106, D.

But, as argued out in the last chapter, the diagram of the adult Ammocoetes must be compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with the larval condition of Ammocoetes. In other words, Fig. 106, B, must be compared with Fig. 106, C, which represents a section through the larval Ammocoetes as it would appear if it reached the adult condition without any forward growth of the upper lip or any breaking through of the septum between the oral and respiratory chambers. The striking similarity between this diagram and that of Eurypterus becomes immediately manifest even to the smallest details. The only difference between the two, except, of course, the notochord, consists in the closure of the mouth opening (o), in Fig. 106, B, by which the olfactory passage (olf. p.) of the scorpion becomes converted into the hypophysial tube (Hy.), Fig. 106, C, and later into the nasal tube (Na.), Fig. 106, D, of the full-grown Ammocoetes. That single closure of the old mouth is absolutely all that is required to convert the Eurypterus diagram into the Ammocoetes diagram.

Such a comparison immediately explains in the simplest manner a number of anatomical peculiarities which have hitherto been among the great mysteries of the vertebrate organization. For not only do the median eyes (C.E.) correspond in position in the two diagrams, and the infundibular tube (Inf.) and the ventricles of the brain (C.C.) correspond to the oesophagus (oes.) and the cephalic stomach (Al.), as already fully discussed; but even in the very place where the narrow oesophagus opened into the wider chamber of the pharynx (Ph.), there, in all the lower vertebrates, the narrow infundibular tube opens into the wider chamber of the membranous saccus vasculosus (sac. vasc.). This is the last portion of the membranous part of the tube of the central nervous system which has not received explanation in the previous chapters, and now it is seen how simple its explanation is, how natural its presence--it represents the old pharyngeal chamber of the palæostracan ancestor.

{245}[Illustration: FIG. 106.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH B, EURYPTERUS; C, LARVAL AMMOCOETES; D, FULL-GROWN AMMOCOETES.]

{246}Next among the mysteries requiring explanation is the pituitary body, that strange glandular organ always found so closely attached to the brain in the infundibular region that when it is detached in taking out the brain it leaves the infundibular canal patent right into the IIIrd ventricle. A comparison of the two diagrams indicates that such a glandular organ (Pit.), Fig. 106, C, was there because the coxal excretory glands (cox. gl.), Fig. 106, B, were in a similar position in the palæostracan ancestor--that, indeed, the pituitary body is the descendant of the coxal glands.

Finally, the diagrams not only indicate how the mesosomatic appendage-nerves supplying in the one case the operculum and the respiratory appendages correspond to the respiratory group of nerves, VII., IX., X., supplying in the other case the thyroid, hyoid, and branchial segments, but also that a similar correspondence exists between the prosomatic appendage-nerves in the one case and the trigeminal nerve in the other; a correspondence which supplies the reason why in the vertebrate a septum originally existed between an oral and respiratory chamber.

Such a comparison, then, leads directly to the suggestion that the trigeminal nerve originally supplied the prosomatic appendages, such appendages being: 1. The metastoma, which has become in Ammocoetes the lower lip supplied by the velar or mandibular branch of the trigeminal nerve (7); 2. The ectognath, which has become the large median ventral tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3. The endognaths, which have been reduced to tentacles and are supplied by the tentacular branch of the trigeminal nerve (2, 3, 4, 5).

I have purposely put these two diagrams of the larval Ammocoetes and of Eurypterus before the minds of my readers at this early stage of my argument, so as to make what follows more understandable. I propose now to consider fully each one of these suggestive comparisons, and to see whether or no they are in accordance with the results of modern research.

In the first instance, the diagrams suggest that the trigeminal nerve originally supplied the prosomatic appendages of the palæostracan ancestor, while the eye-muscle nerves supplied the body-muscles of the prosoma.

{247}As these appendages did not carry any vital organs such as branchiæ, but were mainly locomotor and masticatory in function, it follows that their disappearance as such would be much more complete than that of the mesosomatic branchial appendages. Most probably, then, in the higher vertebrates no trace of such appendages might be left; consequently the segmentation due to their presence would be very obscure, so that in this region the very reverse of what is found in the region of the vagus nerve would be the rule. There branchiomeric segmentation is especially evident, owing to the persistence of the branchial part of the branchial appendages; here, owing to the disappearance of the appendages, the segmentation is no longer branchiomeric, but essentially mesomeric in consequence of the persistence of the somatic eye-muscles.

In addition to the evidence of the appendages themselves, the number of prosomatic segments is well marked out in all the members of the scorpion group by the divisions of the central nervous system into well-defined neuromeres in accordance with the appendages, a segmentation the reminiscence of which may still persist after the appendages themselves have dwindled or disappeared. In accordance with this possibility we see that one of the most recent discoveries in favour of a number of segments in the head-region of the vertebrate is the discovery in the early embryo of a number of partial divisions in the brain-mass, forming a system of cephalic neuromeres which may well be the rudiments of the well-defined cephalic neuromeres of animals such as the scorpion.

THE EVIDENCE OF THE PROSOMATIC MUSCULATURE.

Even if the appendages as such become obscure, yet their muscles might remain and show evidence of their presence. The most persistent of all the appendage-muscles are the basal muscles which pass from coxa to carapace and are known by the name of tergo-coxal muscles. They are large, well marked, segmentally arranged muscles, dorso-ventral in direction, and, owing to their connecting the limb with the carapace, are likely to be retained even if the appendage dwindles away.

The muscular system of Limulus and Scorpio has been investigated by Benham and Miss Beck under Lankester's direction, and the conclusions to which Lankester comes are these--

{248}The simple musculature of the primitive animal from which both Limulus and the scorpions arose consisted of--

1. A series of paired longitudinal dorsal muscles passing from tergite to tergite of each successive segment.

2. A similar series of paired longitudinal ventral muscles.

3. A pair of dorso-ventral muscles passing from tergite to sternite in each segment.

4. A set of dorso-ventral muscles moving the coxa of each limb in its socket.

5. A pair of veno-pericardial muscles in each segment.

Of these groups of muscles, any one of which would indicate the number of segments, Groups 1 and 2 do not extend into the prosomatic region, and Group 5 extends only as far as the heart extends in the case of both Limulus and the Scorpion group; so that we may safely conclude that in the Palæostraca the evidence of somatic segmentation in the prosomatic region would be given, as far as the musculature is concerned, by the dorso-ventral somatic muscles (Group 3), and of segmentation due to the appendages by the dorso-ventral appendage musculature (Group 4).

Therefore, if, as the evidence so far indicates, the vertebrate has arisen from a palæostracan stock, we should expect to find that the musculature of the somatic segments in the region of the trigeminal nerve did not resemble the segmental muscles of the spinal region, was not, therefore, the continuation of the longitudinal musculature of the body, but was dorso-ventral in position, and that the musculature of the splanchic segments resembled that of the vagus region, where, as pointed out in