302 



GENERAL ZOOLOGY 



outside by the atrial pore (Fig. 148, 4) ; the food goes down 

 the intestine (Fig. 148, 5). 



Immediately above the intestine is the structure which 

 corresponds in position to the backbone of higher animals ; 

 it is called the notochord (Fig. 148, 7). The notochord is soft 

 throughout life, but it is sufficiently strong to act as a support- 



B ' 



FIG. 149. Photograph of Living Tunicates. Natural size 

 A, Styela stimpsoni ; Z>, Cynthia haustor 



ing skeleton for the body. Parallel with the notochord and 

 above it, is the spinal cord (Fig. 148, 8) lying near the dorsal 

 wall. The characteristics of structure which make Amphioxus 

 a vertebrate are the presence of gill-slits, a notochord above 

 the intestine, and a spinal cord dorsal to the notochord. 



The Sea-Squirt. As far as outward appearance indicates the 

 sea-squirts, Sty'ela stimpson'i and Cyn'thia, haus'tor (Fig. 149, 



