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.