Chapter Iv., the Respiratory Muscles Arose From the Dorso-Ventral Muscles

of the mesosomatic appendages. This is, of course, exactly what is found for the muscles which move the lateral eyes of the vertebrate; these muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the main evidences of segmentation in this region, are always grouped in line with the somatic muscles of spinal segments, and yet cannot be classed as longitudinal muscles. They are dorso-ventral in direction, and yet belong to the somatic system; they are exactly what one ought to find if they represent Group 3--the dorso-ventral body-muscles of the prosomatic segments of the invertebrate ancestor.

The interpretation of these muscles will be given immediately; at present I want to pass in review all the different kinds of evidence {249}of segmentation in this region afforded by the examination of the invertebrate, whether living or fossil, so as to see what clues are left if the evidence of appendages fails us. I will take in the first instance the evidence of segmentation afforded by the presence of the musculature of Group 4, even when, as in the case of many fossils, no appendages have yet been found. In such animals as Mygale and Phrynus the prosomatic carapace is seen to be marked out into a series of elevations and depressions, and upon removing the carapace we see that these elevations correspond with and are due to the large tergo-coxal muscles of the appendages; so that if such carapace alone were found fossilized we could say with certainty: this animal possessed prosomatic appendages the number of which can be guessed with more or less certainty by these indications of segments on the carapace.

In those forms, then, which are only known to us in the fossil condition, in which no prosomatic appendages have been found, but which possess, more or less clearly, radial markings on the prosomatic carapace resembling those of Phrynus or Mygale, such radial markings may be interpreted as due to the presence of prosomatic appendages, which are either entirely concealed by the prosomatic carapace or dorsal head-plate, or were of such a nature as not to have been capable of fossilization.

The group of animals in question forms the great group of animals, chiefly extinct, classified by H. Woodward under the order of Merostomata. They are divided by him into the sub-order of Eurypteridæ, which includes--(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9) Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which includes--(1) Belinurus, (2) Prestwichia, (3) Limulus.

{250}[Illustration: FIG. 107.--Phrynus Margine-Maculata.

Ce., median eyes; le., lateral eyes; glab., median plate over brain; Fo., fovea.]

cam., camerostome; pl., plastron.]

{251}The evidence of the Xiphosura and of the Hemiaspidæ conclusively shows, in Woodward's opinion, that the Merostomata are closely related to the Trilobita, and the Hemiaspidæ especially are supposed to be intermediate between the trilobites and the king-crabs. They are characterized, as also Belinurus and Prestwichia, by the absence of any prosomatic appendages, so that in these cases, as is seen in Fig. 12 (p. 30), representing Bunodes lunula, found in the Eurypterus layer at Rootziküll, we have an animal somewhat resembling Limulus in which the prosomatic appendages have either dwindled away and are completely hidden by the prosomatic carapace, or became so soft as not to be preserved in the fossilized condition. The appearance of the prosomatic carapace is, to my mind, suggestive of the presence of such appendages, for it is marked out radially, as is seen in the figure, in a manner resembling somewhat the markings on the prosomatic carapace of Mygale or Phrynus; the latter markings, as already mentioned, are due to the aponeuroses between the tergo-coxal muscles of the prosomatic appendages which lie underneath and are attached to the carapace.

A very similar radial marking is shown by Woodward in his picture of Hemiaspis limuloides, reproduced in Fig. 109, found in the Lower Ludlow beds at Leintwardine. This species has yielded the most perfect specimens of the genus Hemiaspis, which is recognized as differing from Bunodes by the possession of a telson.

It is striking to find that similar indications of segments have been found on the dorsal surface of the head-region in many of the most ancient extinct fishes, as will be fully discussed later on.

gl., glabellum.]

THE EVIDENCE OF COELOMIC CAVITIES.

In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of coelomic cavities in this region, the walls of which give origin to the striated muscles of the head, so that the question of the number of segments depends very largely upon the origin of the muscles from the walls of these head-cavities. It is therefore interesting to examine whether a similar criterion of segmentation holds good in such a segmented {252}animal as Limulus, or in the members of the scorpion group, in which the number of segments are known definitely by the presence of the appendages. In Limulus we know, from the observations of Kishinouye, that a series of coelomic cavities are formed embryologically in the various segments of the mesosoma and prosoma, in a manner exceedingly similar to their mode of formation in the head-region of the vertebrate, and he has shown that in the mesosoma a separate coelomic cavity exists for each segment, so that just as the dorso-ventral somatic muscles are regularly segmentally arranged in this region, so are the coelomic cavities, and we should be right in our estimation of the number of segments in this region by the consideration of the numerical correspondence of these cavities with the mesomatic appendages. Similarly, in the vertebrate, we find every reason to believe that a single, separate head-cavity corresponds to each of the branchial segments in the opisthotic region, and therefore we should estimate rightly the number of segments by the division of the mesoderm in this region.

In the prosomatic region of Limulus, the dorso-ventral muscles are not arranged with such absolute segmental regularity as in the mesosomatic region, and Kishinouye's observations show that the coelomic cavities in this region do not correspond absolutely with the number of prosomatic appendages. His words are:--

A pair of coelomic cavities appears in every segment except the segments of the 2nd, 3rd, and 4th appendages, in which coelomic cavities do not appear at all. At least eleven pairs of these cavities are produced. The eleventh pair belongs to the seventh abdominal segment.

The first pair of coelomic cavities is common to the cephalic lobe and the segment of the first appendage (i.e. the cheliceræ).

The second coelomic cavity belongs to the segment of the fifth appendage. It is well developed.

The ventral portion of the second coelomic cavity remains as the coxal gland.

* * * * * *

Consequently, if we were to estimate the number of segments in this region by the number of coelomic cavities we should not judge rightly, for we should find only four cavities and seven appendages, as is seen in the following table:--

Key: {253} A Prosomatic. B Mesosomatic. C.c. Coelomic cavities.

---------------------------------------------------------+--------------- LIMULUS. | VERTEBRATE. ---------+----------------+-----------------------+------+--------------- Segments.| Appendages. | Eurypterid appendages.| C.c. | Coelomic | | | | cavities. ---+-----+----------------+-----------------------+------+--------------- | 1 | Cheliceræ or | Cheliceræ | 1 | Anterior | | 1st locomotor.| | | | 2 | 2nd locomotor |} | | A | 3 | 3rd " |} Endognaths | 2 | Premandibular | 4 | 4th " |} | | | 5 | 5th " |} | | | 6 | 6th " | Ectognath | 3 |} Mandibular | 7 | Chilaria | Metastoma | 4 |} ---+-----+----------------+-----------------------+------+--------------- | 8 | Operculum |} Operculum (Genital) | 5 |} Hyoid | 9 | 1st branchial |} " (1st branchial) | 6 |} | 10 | 2nd " | 2nd branchial | 7 | 1st branchial B | 11 | 3rd " | 3rd " | 8 | 2nd " | 12 | 4th " | 4th " | 9 | 3rd " | 13 | 5th " | 5th " | 10 | 4th " | 14 | 6th " | | 11 | ---+-----+----------------+-----------------------+------+---------------

The second cavity would in reality represent four segments belonging to the 2nd, 3rd, 4th, 5th locomotor appendages, i.e. the very four segments which in the Eurypteridæ are concentrated together to form the endognaths, and we should be justified in putting this interpretation on it, because, according to Kishinouye, its ventral portion forms the coxal gland, and, according to Lankester, the coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the vertebrate, we find three head-cavities in the region which corresponds, on my theory, to the prosomatic region of Limulus, (1) the anterior cavity discovered by Miss Platt, (2) the premandibular cavity, and (3) the mandibular cavity, which, if they corresponded with the prosomatic coelomic cavities of Limulus, would represent not three segments but seven segments, as follows:--the anterior cavity would correspond to the first coelomic cavity, i.e. the cavity of the cheliceral segments in both Limulus and the Eurypteridæ; the premandibular, to the second coelomic cavity, representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in Limulus and the endognathal segments in the Eurypteridæ; and the mandibular to the 3rd and 4th coelomic cavities, representing the last locomotor and chilarial segments in Limulus, i.e. the ectognathal and metastomal segments in the Eurypteridæ.

{254}It is worthy of note that, in respect to their coelomic cavities, as in the position and origin of their nerves in the central nervous system, the first pair of appendages, the cheliceræ, retain a unique position, differing from the rest of the prosomatic appendages.

In the table I have shown how the vertebrate coelomic cavities may be compared with those of Limulus. The next question to consider is the evidence obtained by morphologists and anatomists as to the number of segments supplied by the trigeminal nerve-group; this question will be considered in the next chapter.

SUMMARY.

In Chapters IV. and V. I have dealt with the opisthotic segments of the vertebrate, including therein the segments supplied by the facial nerve, and shown that they correspond to the mesosomatic segments of the palæostracan; consequently the facial (VII.), glossopharyngeal (IX.), and vagus (X.) nerves originally supplied the branchial and opercular appendages.

In this chapter the consideration of the pro-otic segments is commenced, that is, the segments supplied by the trigeminal (V.) and the eye-muscle nerves (III., IV., VI.). I have considered the VIth nerve with the rest of the eye-muscle nerves for convenience' sake, though in reality it belongs to the same segment as the facial. Of these, that part of the trigeminal which innervates the muscles of mastication corresponds to the splanchnic segments, while the eye-muscle nerves belong to the corresponding somatic segments; but the pro-otic segments of the vertebrate ought to correspond to the prosomatic segments of the invertebrate, just as the opisthotic correspond to the mesosomatic. Therefore the motor part of the trigeminal ought to supply muscles which originally moved the prosomatic appendages, while the eye-muscles ought to have belonged to the somatic part of the same segments.

The first question considered is the number of segments which ought to be found in this region. In Limulus, the Eurypteridæ, and the scorpions there are seven prosomatic segments which carry (1) the cheliceræ, (2, 3, 4, 5) the four first locomotor appendages--the endognaths, (6) the large special appendage--the ectognath--and (7) the appendages, which in Limulus are known as the chilaria, and are small and insignificant, but in Eurypterus and other forms grow forwards, fuse together, and form a single median lip to an accessory oral chamber, which lip is known as the metastoma. Of these appendages the cheliceræ and endognaths tend to dwindle away and become mere tentacles, while the large swimming ectognath and metastoma remain strong and vigorous.

In this, the prosomatic region, the somatic segmentation is not characterized by the presence of the longitudinal muscle segments, for they do not extend into this head-region, but only by the presence of the segmental somatic {255}ventro-dorsal muscles. Among the muscles of the appendages the system of large tergo-coxal muscles is especially apparent.

From these considerations it follows that the number of segments in this region in the vertebrate ought to be seven; that the musculature supplied by the trigeminal nerve ought to represent seven ventral or splanchnic segments, of which only the last two are likely to be conspicuous; and that the musculature supplied by the eye-muscle nerves ought to be dorso-ventral in direction, which it is, and represent seven dorsal or somatic segments.

A further peculiarity of this region, both in Limulus and the scorpions, is found in the excretory organs which are known by the name of coxal glands, because they extend into the basal joint, or coxa, of certain of the prosomatic limbs. The appendages so characterized are always the four endognaths, and it follows that if these four endognaths lose their locomotor power, become reduced in size, and concentrated together to form mere tentacles, then of necessity the coxal glands will be concentrated together, and tend to form a glandular mass in the region of the mouth; in fact, take up a position corresponding to that of the pituitary body in vertebrates.

Taking all these facts into consideration, it is possible to construct a drawing of a sagittal section through the head-region of Eurypterus, which will represent, with considerable probability, the arrangement of parts in that animal. This can be compared with the corresponding section through the head of Ammocoetes.

Now, as pointed out in the last chapter, the early stage of Ammocoetes is remarkably different from the more advanced stage; at that time the septum between the oral and respiratory chambers has not yet broken through, and the olfactory or nasal tube, known at this stage as the tube of the hypophysis, is directed ventrally, not dorsally.

The comparison of the diagram of Eurypterus with that of the early stage of Ammocoetes is remarkably close, and immediately suggests not only that the single nose of the former is derived from the corresponding organ in the palæostracan, but that the pituitary body is derived from the concentrated coxal glands, and the lower lip from the metastoma. The further working out of these homologies will be discussed in the next chapter.

In addition to the evidence of segmentation afforded by the appendages, there are in this region, in Limulus and the scorpion group, three other criteria of segmentation available to us, if from any cause the evidence of appendages fails us. These are--

1. The number of neuromeres are marked out in this region of the brain more or less plainly, especially in the young animal, just as they are also in the embryo of the vertebrate.

2. The segmentation is represented here, just as in the mesosomatic region, by two sets of muscle-segments; the one somatic, consisting of the segmentally arranged dorso-ventral muscles, the continuation of the group already discussed in connection with the mesosomatic segmentation, and the other appendicular characterized by the tergo-coxal muscles. These latter segmental muscles are especially valuable, for in such forms as Mygale, Phrynus, etc., their presence is indicated externally by markings on the prosomatic carapace, and thus corresponding markings found on fossil carapaces or on dorsal head-shields can be {256}interpreted. These two sets of muscle-segments correspond in the vertebrate to the somatic and splanchnic segmentations.

3. In the vertebrate the segmentation in this region is indicated by the coelomic or head-cavities, which are cavities formed in the mesoderm of the embryo, the walls of which give origin to the striated muscles of the head. In Limulus corresponding coelomic cavities are found, which are directly comparable with those found in the vertebrate.

{257}CHAPTER VIII

THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP

The prosomatic segments of the vertebrate.--Number of segments belonging to the trigeminal nerve-group.--History of cranial segments.--Eye-muscles and their nerves.--Comparison with the dorso-ventral somatic muscles of the scorpion.--Explanation of the oculomotor nerve and its group of muscles.--Explanation of the trochlearis nerve and its dorsal crossing.--Explanation of the abducens nerve.--Number of segments supplied by the trigeminal nerves.--Evidence of their motor nuclei.--Evidence of their sensory ganglia.--Summary.

From the evidence given in the last chapter, combined with that given in