246 THE ORIGIN OF VERTEBRATES 
how natural its presence—it represents the old pharyngeal chamber 
of the palceostracan 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 IlIrd ventricle. A comparison of the two diagrams indicates 
that such a glandular organ (Pit.), Fig. 106, C, was there because the 
coxal excretory glands (¢ox. gl.), Fig. 106, B, were in a similar 
position in the paleostracan 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- 
ecetes 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 Rathke 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- 
ceetes 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 paleo- 
stracan ancestor, while the eye-muscle nerves supplied the body- 
muscles of the prosoma. 
