This chamber in the full-grown Ammocoetes is not only open to the respiratory chamber, but is bounded by the large upper lip (_U.L._, Fig. 106, D). On the dorsal surface of this region, in front of the {244}pineal eye (_C.E._), is the most conspicuous opening of the olfactory tube (_Na._), which olfactory tube passes from the dorsal region to the ventral side to terminate blindly at the very spot where the infundibulum 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 Ammocoetes, would be represented as in Fig. 106, D. But, as argued out in the last chapter, the diagram of the adult Ammocoetes must be compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with the larval condition of Ammocoetes. In other words, Fig. 106, B, must be compared with Fig. 106, C, which represents a section through the larval Ammocoetes as it would appear if it reached the adult condition 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 Eurypterus 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 (_o_), in Fig. 106, B, by which the olfactory passage (_olf. p._) of the scorpion becomes converted into the hypophysial tube (_Hy._), Fig. 106, C, and later into the nasal tube (_Na._), Fig. 106, D, of the full-grown Ammocoetes. That single closure of the old mouth is absolutely all that is required to convert the Eurypterus diagram into the Ammocoetes 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.E._) correspond in position in the two diagrams, and the infundibular tube (_Inf._) and the ventricles of the brain (_C.C._) correspond to the oesophagus (_oes._) and the cephalic stomach (_Al._), as already fully discussed; but even in the very place where the narrow oesophagus 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_ (_sac. 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, how natural its presence--it represents the old pharyngeal chamber of the palæostracan ancestor. {245}[Illustration: FIG. 106.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH B, EURYPTERUS; C, LARVAL AMMOCOETES; D, FULL-GROWN AMMOCOETES.]