32 2 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 vasculosis, 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 iu 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 paheostracan. 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 branchiae varies, the minimum number 

 of such segments (as seen in the Pedipalpi) is never less than two. 



Summary. 



Tlie general consideration of the evidence of the number of segments, and 

 their nature in the pro-otic reg-ion of the vertebrate, as given in the last 

 chapter, is not incompatible with the view that the trigeminal nerve originally 



