322 THE ORIGIN OF VERTEBRATES 
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 transi- 
tion 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 Ammoccetes 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 olfac- 
tory 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 paleostracan. 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 verte- 
brate and invertebrate this is a fixed position, for in the former, how- 
ever 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 branchie varies, the minimum number 
of such segments (as seen in the Pedipalpi) is never less than two. 
SUMMARY. 
The general consideration of the evidence of the number of segments, and 
their nature in the pro-otic region of the vertebrate, as given in the last 
chapter, is not incompatible with the view that the trigeminal nerve originally 
