PAST HISTORY OF THE INVERTEBRATES 305 



for the body. Parallel with the notochord and above it is the 

 spinal cord, lying near the dorsal wall. The characteristics 

 of structure which permit zoologists to consider Amphioxus 

 a chordate are the presence of gill slits, a notochord above 

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



Fig. 157. Photograph of living sea squirts. (Natural size) 



A, Styela; B, Cynthia 



The Sea Squirt. As far as outward appearance indicates 

 the sea squirts, Sty'ela and Cyn'thia (Fig. 157, A, B), seem 

 to have nothing in common with vertebrates. Sea squirts 

 live attached to rocks and wharves, and once attached 

 never leave the place. The body is covered with a tough 

 coat, or tunic, which gives the class its name, Tunica'ta. 

 The food and oxygen are drawn through the opening in 

 the upper tube, and the excess water and wastes are dis- 

 charged by the lower tube. There is a pharynx with gill 



