COMMENSALISM AND PARASITISM 



345 



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 ver- 

 tebrates, and is charac- 

 teristic of the class. The 

 other organs of the young 

 tunicate are all of verte- 

 bral type. But the young 

 sea-squirt passes a period 

 of active and free life as 

 a little fish, after which 

 it settles down and at- 

 taches itself to a stone or 

 shell or wooden pier by 

 means of suckers, and re- 

 mains for the rest of its 

 life fixed. Instead of go- 

 ing on and developing 

 into a fish-like creature, it 

 loses its notochord, its 

 special sense organs, and 



other organs ; it loses its complexity and high organiza- 

 tion, 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. 211, /) is a six-legged, free-swimming 

 nauplius, very 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-swimming life. When it 



makes its final change to the adult condition, it attaches 

 23 



FIG. 210. A sea-squirt, or tunicate. 



