THE VERTEBRATES 



193 



The remaining three classes include a number of strange 

 marine forms which until recent years were considered as 

 worms, but which are now known to be the nearest living 

 allies of the earliest or primitive vertebrates. The rela- 

 tionship of these forms to vertebrates is manifest, not in 

 the appearance or structure of the adult stage, but only 

 during embryonic or 

 larval stages. 



The ascidians.— The 

 sea-squirts, or Ascidi- 

 ans, common on the 

 seashore, compose one 

 class of these primi- 

 tive chordate animals. 

 They possess a sim- 

 ple, sac-like body (fig. 

 95), fastened to the 

 rocks by one end, the 

 other being provided 

 with two openings, 

 one for the ingress 

 and the other for the 

 exit of water, a strong 

 current of which flows 

 constantly through the 

 body. By means of 

 this current the ascid- 

 ian obtains food. Usually sea-squirts live together in 

 large colonies, and in some cases a number of individuals 

 enclose themselves in a common gelatinous mass, forming 

 what is called a compound ascidian. 



The ascidian when born is a tiny, free-swimming, tadpole- 

 like creature with a slender finned tail. It swims about 

 freely for only a few hours, however, soon attaching itself to 

 a rock, and jn it§ further development becoming degenerate, 



Fig. 9S. An ascidian, or sea-squirt, from 

 the coast of California. (Natural size; 

 after Jordan and Kellogg.) 



