The comparison of the diagram of Eurypterus with that of the early stage of Ammocœtes 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 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 cœlomic 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 cœlomic cavities are found, which are directly comparable with those found in the vertebrate.
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 Chapter IV., the probability of the theory that the trigeminal group of nerves of the vertebrate have been derived from the prosomatic group of nerves of the invertebrate can be put to the test by the answers to the following morphological and anatomical questions:—
1. Do we find in the vertebrate two segmentations in this region corresponding to the two segmentations in the branchial region, i.e. a somatic or dorsal series of segments, and a splanchnic or ventral series of segments? The latter would not be branchial, but rather of the nature of free tactile appendages; so that it is useless to look for or talk about gill-slits, although such appendages, being serially homologous with the branchial mesosomatic appendages, would readily give rise to the conception of branchial segments.