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 cut-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 
