_ 244 THE ORIGIN OF VERTEBRATES 
pineal eye (C.Z.), is the most conspicuous opening of the olfactory 
tube (WVa.), which olfactory tube passes from the dorsal region to the 
ventral side to terminate blindly at the very spot where the infun- 
dibulum comes to the surface of the brain. Here, also, is situated 
that extraordinary glandular organ known as the pituitary body 
(Pit.). A sagittal section, then, in diagram form, of the position 
of parts in the full-grown Ammoccctes, would be represented as in 
Fig. 106, D. 
But, as argued out in the last chapter, the diagram of the adult 
Ammoccetes must be compared with that of a cephalaspidian fish; 
the diagram of the paleostracan must be compared with the larval 
condition of Ammoccetes. In other words, Fig. 106, B, must be 
compared with Fig. 106, C, which represents a section through the 
larval Ammoccetes as it would appear if it reached the adult con- 
dition without any forward growth of the upper lip or any breaking 
through of the septum between the oral and respiratory chambers. 
The striking similarity between this diagram and that of Euryp- 
terus becomes immediately manifest even to the smallest details. 
The only difference between the two, except, of course, the notochord, 
consists in the closure of the mouth opening (0), in Fig. 106, B, by 
which the olfactory passage (olf. p.) of the scorpion becomes con- 
verted into the hypophysial tube (Hy.), Fig. 106, C, and later into 
the nasal tube (Wa.), Fig. 106, D, of the full-grown Ammocetes. 
That single closure of the old mouth is absolutely all that is 
required to convert the Eurypterus diagram into the Ammoccetes 
diagram. 
Such a comparison immediately explains in the simplest manner 
a number of anatomical peculiarities which have hitherto been among 
the great mysteries of the vertebrate organization. For not only 
do the median eyes (C.#.) correspond in position in the two diagrams, 
and the infundibular tube (Jnf.) and the ventricles of the brain 
(C.C.) correspond to the cesophagus (@s.) and the cephalic stomach 
(Al.), as already fully discussed ; but even in the very place where the 
narrow cesophagus opened into the wider chamber of the pharynx 
(Ph.), there, in all the lower vertebrates, the narrow infundibular tube 
opens into the wider chamber of the membranous saccus vasculosus (sae, 
vasc.). This is the last portion of the membranous part of the tube of 
the central nervous system which has not received explanation in the 
previous chapters, and now it is seen how simple its explanation is, 
