364 



EVOLUTION AND ANIMAL LIFE 



the simplest fishes or fishUke forms. That is, the sea squirt 

 begins hfe as a primitively simple vertebrate. It possesses 

 in its larval stage a notochord, the delicate structure which 

 precedes the formation of a backbone, extending along the 

 upper part of the body, below the spinal cord. It is found 

 in all young vertebrates, and is characteristic of the branch. 



The other organs of the 

 young tunicate are all of 

 vertebral type. But the 

 young sea squirt passes 

 a period of active and 

 free life as a little fish, 

 after which it settles down 

 and attaches itself to a 

 stone or shell or wooden 

 pier by means of suckers, 

 and remains for the rest 

 of its life fixed. Instead 

 of going on and develop- 

 ing into a fishlike creature, 

 it loses its notochord, its 

 special sense organs, and 

 other organs; it loses its 

 complexity and high or- 

 ganization and becomes, 

 a "mere rooted bag 

 with a double neck," a 

 thoroughly degenerate 

 animal. 

 A barnacle is another example of degeneration through 

 quiescence. The barnacles are crustaceans related most 

 nearly to the crabs and shrimps. The young barnacle just 

 from the egg (Fig. 225, /) is a six-legged, free-swimming 

 nauplius, much like a young shrimp, with single eye. In 

 its next larval stage it has six pairs of swimming feet, and 

 two large antennae or feelers, and still lives an independent, 

 free-swimming life. When it makes its final change to the 

 adult condition, it attaches itself to some stone or shell, or 

 pile or ship's bottom, loses its feelers, develops a protecting 

 shell, and gives up all power of locomotion. Its swim- 

 ming feet become changed into grasping organs, and it 



■fe.^U 



Fig. 224. — The sea squirt or tunicate. 



