Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata. Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor appendages. When these appendages became reduced in size and purely tactile they were compressed and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary consequence of the concentration of the coxæ of the endognaths, the coxal gland also became concentrated, {322}and took up a situation close against the pharynx, as represented in Fig. 106, B. When, then, the old mouth closed, and the pharynx became the _saccus vasculosus_, the coxal gland remained in close contact with the _saccus vasculosus_, and became the pituitary body, thus giving the reason why there is always so close a connection between the pituitary body and the infundibular region. Whatever was the condition of the digestive tracts at the transition stage between the arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was ultimately closed. The method of its closure was exceedingly simple and evident. The membranous cranium was in process of formation by the extension of the plastron laterally and dorsally; a slight growth of the same tissue in the region of the mouth would suffice to close it and thus separate the infundibulum from the olfactory tube. As evidence that such was the method of closure, it is instructive to see how in Ammocoetes the glandular tissue of the pituitary body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the nasal tube is embedded in this same thickened mass of the cranial wall--how, in fact, both coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron, by means of which the mouth was closed. I have now passed in review the nature of the evidence which justifies a comparison between the segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic segments of the palæostracan. For the convenience of my readers I have put these conclusions into tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however variable may be the number of branchial segments which the vagus supplies, the second branchial segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in the Pedipalpi) is never less than two. {323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND IN AMMOCOETES (_i.e._ IN CEPHALASPIDS).