THE NOTOCHORD AND ALIMENTARY CANAL 439 



with a tissue resembling that of the notochord, enthusiasts have 

 immediately jumped to the conclusion that a relationship must exist 

 between it and the chordate animals ; and, accordingly, they have 

 classified such animals as follows : Amphioxus belongs to the 

 group Cephalochorda because the notochord projects beyond the 

 central nervous system ; the Tunicata are called Urochorda because 

 it is confined to the tail ; the Enteropneusta, Hemichorda, because 

 this tissue is confined to a small diverticulum of the gut, and, 

 finally, Diplochorda has been suggested for Actinotrocha and Pho- 

 ronis because two separate portions of the gut are transformed 

 in this way. 



This exaggerated importance given to any tissue resembling in 

 structure that of the notochord is believed in by many of those who 

 profess to be our teachers on this subject, the very men who can 

 deliberately shut their eyes to the plain reading of the story of the 

 pineal eyes, and say, " In our opinion this pineal organ was not an 

 eye at all." 



The only legitimate inference to be drawn from the similarity of 

 structure between the notochord and these degenerated gut-diverti- 

 cula, is that the structure of the notochord may have arisen in the 

 same way, and that therefore the notochord may once have func- 

 tioned as a gut. With cessation of its function its cells became 

 vacuolated, as in these other cases, and its lumen became filled with 

 notochordal tissue. This evidence strongly confirms the suggestion 

 that the notochord was once a digestive tube, but by no means 

 signifies that such tissue, wherever found, indicates the presence 

 of a notochord. 



In order to resemble a notochord, this tissue must possess not 

 only a definite structure but a definite position, and this position is a 

 remarkably striking and suggestive one. The notochordal tube is 

 unsegmented, although the vertebrate is markedly segmented. But 

 in all segmented animals the only unsegmented tube which extends 

 the whole length of the body, from mouth to anus, is invariably 

 the gut. In the vertebrate there are three such tubes : (1) the gut 

 itself, (2) the central canal of the nervous system, and (3) the 

 notochordal tube. 



The first is the present gut, the second the gut of the invertebrate 

 ancestor, and the third the tube in question. 



These three unsegmented tubes, extending along the whole length 



