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 Chapter IV., the probability of the theory that the trigeminal group of nerves of the vertebrate have been derived from the prosomatic group of nerves of the invertebrate can be put to the test by the answers to the following morphological and anatomical questions:-- 1. Do we find in the vertebrate two segmentations in this region corresponding to the two segmentations in the branchial region, _i.e._ a somatic or dorsal series of segments, and a splanchnic or ventral series of segments? The latter would not be branchial, but rather of the nature of free tactile appendages; so that it is useless to look for or talk about gill-slits, although such appendages, being serially homologous with the branchial mesosomatic appendages, would readily give rise to the conception of branchial segments. 2. Is there morphological evidence that the trigeminal nerve is not the nerve belonging to a single segment, or even to two segments, but is really a concentration of at least six, probably seven, segmental nerves?