CHAPTER VIII. 



EIGHTH BRANCH OF ANIMALS. 

 SEA-SQUIRTS (Tuntcata). 



General Charac- 

 teristics. Though 

 the adult ascidians 

 possess little out- 

 ward resemblance 

 to the backboned 

 animals, the free- 

 swimming young 

 {Fig. 185, a\ that 

 look like tadpoles, 

 have a gristly cord, 

 r, with a nerve-cord, 

 n v, above it like 

 the notochord that 

 we shall see in 

 the lancelet ; con- 

 sequently, they are 

 believed to repre- 

 sent the simplest 

 phase of backboned 

 life. They are bag- 

 or barrel - shaped 

 animals, sometimes 

 growing upon stalks; 



FIG. 185. Diagram of the growth of a sea- 

 squirt, or ascidian. A, a, young free-swim- 

 ming stage (Claveh'na). aft, intermediate 

 stage, when first settling down. B, , full- 

 grown sea-squirt, m, mouth ; e, hollow 

 brain with eye ; g, gill-slits ; h, heart ; r, 

 rod of gristle in free-swimming form ; nv. 

 nerve-cord in same ; f, tail in process of 

 absorption in intermediate form. (After 

 Haddon.) 



