THE PROSOMATIC SEGMENTS OF AMMOCGTES 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 paleostracan 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 Ammoceetes, 
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 PIrvirary 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 Ammoccetes 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. 
