CHAPTER VIII. 
EIGHTH BRANCH OF ANIMALS. 
SEA-SQUIRTS (Tunzcata). 
General Charac- 
teristics. — Though 
the adult ascidians 
possess little out- 
ward resemblance 
to the backboned 
animals, the free- 
swimming young 
(Fig. 185, @), that 
look like tadpoles, 
have a gristly cord, 
r, with a nerve-cord, 
nv, 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; 
ar a e 
Fic. 185.—Diagram of the growth of a sea- 
squirt, or ascidian. A, «, young free-swim- 
ming stage (C/avelina). a, intermediate 
stage, when first settling down. B, 4, full- 
grown sea-squirt. 7, mouth; ¢, hollow 
brain with eye; g, gill-slits; 2, heart; 7, 
rod of gristle in free-swimming form; 7v, 
nerve-cord in same; /, tail in process of 
absorption in intermediate form. (After 
Haddon.) 
