246 THE ORIGIN OF VERTEBRATES 



how natural its presence — it represents the old pharyngeal chamber 

 of the pahcostracan ancestor. 



Next among the mysteries requiring explanation is the pituitary 

 body, that strange glandular organ always found so closely attached 

 to the brain in the infundibular region that when it is detached in 

 taking out the brain it leaves the infundibular canal patent right into 

 the Illrd ventricle. A comparison of the two diagrams indicates 

 that such a glandular organ (Pit.), Fig. 106, C, was there because the 

 coxal excretory glands (cox. gl.), Fig. 106, B, were in a similar 

 position in the palseostracan ancestor — that, indeed, the pituitary 

 body is the descendant of the coxal glands. 



Finally, the diagrams not only indicate how the mesosomatic 

 appendage-nerves supplying in the one case the operculum and the 

 respiratory appendages correspond to the respiratory group of nerves, 

 VII., IX., X., supplying in the other case the thyroid, hyoid, and 

 branchial segments, but also that a similar correspondence exists 

 between the prosomatic appendage-nerves in the one case and the 

 trigeminal nerve in the other ; a correspondence which supplies the 

 reason why in the vertebrate a septum originally existed between an 

 oral and respiratory chamber. 



Such a comparison, then, leads directly to the suggestion that the 

 trigeminal nerve originally supplied the prosomatic appendages, such 

 appendages being: 1. The metastoma, which has become in Ammo- 

 ccetes the lower lip supplied by the velar or mandibular branch of the 

 trigeminal nerve (7) ; 2. The ectognath, which has become the large 

 median ventral tentacle, called by Eathke the tongue, supplied by the 

 tongue nerve (6) ; 3. The endognaths, which have been reduced to 

 tentacles and are supplied by the tentacular branch of the trigeminal 

 nerve (2, 3, 4, 5). 



I have purposely put these two diagrams of the larval Ammo- 

 ccetes and of Eurypterus before the minds of my readers at this early 

 stage of my argument, so as to make what follows more understand- 

 able. I propose now to consider fully each one of these suggestive 

 comparisons, and to see whether or no they are in accordance with 

 the results of modern research. 



In the first instance, the diagrams suggest that the trigeminal 

 nerve originally supplied the prosomatic appendages of the palseo- 

 stracan ancestor, while the eye-muscle nerves supplied the body- 

 muscles of the prosoma. 



