XVIII. UROCHORDA AND VERTEBRATA 



Branch XII. — Chobdata (continued) 



Class — UROCHORDA (with notochord) 



Sea Squirts 



Sea squirts. — The animals of this class are not well 

 known to most of us. They are called ascidians as well 

 as tunicates. They are marine animals, some species 

 of which live a free-swinmiing life, while others are attached 

 to rocks and are unable to 

 move from place to place 

 any more than the sponges. 

 Like the sponges, many of 

 them reproduce by bud- 

 ding, thus forming large 

 colonies of individuals. 

 Other species pass a soli- 

 tar}' existence. 



In general, their bodies 

 are barrel-shaped (Fig. 143), 

 and covered with a flexible, 

 leathery mantle or tunic, 

 composed of two layers, the 

 inner one of which u.sually 

 contains many longitudinal 

 and transverse muscles. In 

 most species there are two i'i«. 143. 



203 



-Sea stjuirt or ascidian. 



