244 THE ORIGIN OF VERTEBRATES 



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 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 Ammocu'tes, would bo 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 palaiostracan 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 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 (o), in Fig. 106, B, by 

 which the olfactory passage (off. ^?.) of the scorpion becomes con- 

 verted into the hypophysial tube (Sy.), 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 Euryptems 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 

 (CO.) correspond to the oesophagus (ces.) and the cephalic stomach 

 (Al), as already fully discussed ; but even in the very place wdiere 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. 

 vase). 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, 



