364 



EVOLUTION AND ANIMAL LIFE 



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

 begins life 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 go- 

 ing on and developing into 

 a fishlike creature, it loses 

 its notochord, its special 

 sense organs, and other 

 organs; it loses its com- 

 plexity and high organiza- 

 tion, and becomes a "mere 

 rooted bag with a double 

 neck," a thoroughly de- 

 generate 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 prawn or 

 crab, with single eye. In its next larval stage it has six 

 pairs of swimming feet, two compound eyes, and two large 

 antennae or feelers, and still lives an independent, free-swim- 

 ming life. When it makes its final change to the adult con- 

 dition, it attaches itself to some stone or shell, or pile or 

 ship's bottom, loses its compound eyes and feelers, develops 

 a protecting shell, and gives up all power of locomotion. Its 

 swimming feet become changed into grasping organs, and it 



FIG. 224. The sea squirt or tunicate. 



