THE PROSOMATIC SEGMENTS OF AMMOCCETES 319 



of the hypophysis, and the pre-oral elongation of the alimentary 

 canal. 



It is perfectly simple and easy for the olfactory tube to open into 

 any one of the other three. By opening into the infundibulum it 

 reproduces the condition of affairs seen in the scorpion ; by opening 

 into the gut it produces the actual condition of things seen in 

 Myxine and other vertebrates ; by opening into the notochordal tube 

 it would produce a transitional condition between the other two. 



The view held by Kupffer is that this nasal tube (tube of the 

 hypophysis) opened into the anterior diverticulum of the vertebrate 

 gut, and was for this reason the original mouth-tube ; then a new 

 mouth was formed, and this connection was closed, being subse- 

 quently reopened as in Myxine. My view is that this tube 

 originally opened into the infundibulum, in other words, into the 

 original gut of the pakeostracan ancestor, and was for this reason 

 the original mouth-tube, in the same sense as the olfactory passage 

 of the scorpion may be, and often is, called the mouth-tube. When, 

 with the breaking through of the septum between the oral and 

 respiratory chambers, the external opening of the oral chamber 

 became a new mouth, the old mouth was closed but the olfactory 

 tube still remained, owing to the importance of the sense of smell. 

 Subsequently, as in Myxine and the higher vertebrates, it opened 

 into the pharynx, and so formed the nose of the higher vertebrates. 



It is not, to my mind, at all improbable that during the transition 

 stage, between its connection with the old alimentary canal, as in 

 Eurypterus or the scorpions, and its blind ending, as in Ammocoetes, 

 the nasal tube opened into the tube of the notochord. This question 

 will be discussed later on when the probable significance of the 

 notochord is considered. 



The Pituitary Gland. 



Turning back to the comparison of Fig. 106, B, and Fig. 106, C, 

 which represent respectively an imaginary sagittal section through 

 an Eurypterus-like animal and through Ammocoetes at a larval 

 stage, all the points for comparison mentioned on p. 244 have now 

 been discussed with the exception of the suggested homology 

 between the coxal glands of the one animal and the pituitary 

 body of the other. 



