278 FOOT-NOTES TO EVOLUTION. 



tors, are reduced to motionless sacs, buried in the sand 

 or anchored to rocks or wharves. The evidence of their 

 origin is found in the fact that the young Tunicate is 



tadpole-shaped, with a rudimentary back- 

 Tunicates. . , . , 



bone, and has the motions and in large 



degree the structure of the fish. With the loss of power 

 of locomotion the structures on which locomotion de- 

 pends also disappear. 



Still more marked is the degeneration of parasites. 

 It is a universal rule that all creatures dependent on 

 others for support lose their power of 

 self-help. Parasitic insects lose their 

 wings and are confined to the bodies of 

 those unwillingly made their hosts. Parasitic worms 

 are the simplest of their kind. Insects feeding on the 

 juices of plants which they suck without moving be- 

 come reduced to mere living scales. 



Perhaps the most remarkable example of the degen- 

 eration of parasitism is that seen in the crustacean 



called Sacculina. This creature appears 

 Saccuhna. . ... .'.* 



as a simple sac attached to the body of 



the crab, into which its root processes or blood vessels 

 extend. When it is hatched from the egg it is similar 

 in form to a young crab, independent and free-swim- 

 ming. It soon attaches itself to some adult crab, into 

 the body of which it extends its processes. It loses its 

 power of locomotion, and the limbs all disappear. Liv- 

 ing at the expense of others, self-activity is not de- 

 manded, and its position protects it from competition to 

 which free-swimming crabs are subject. It becomes 

 degraded into a parasitic sac, with no organs except 

 a nervous ganglion, its ovaries, and root processes. 

 This is the female Sacculina, and parasitic upon this is 

 the smaller and still more degraded male of the same 

 species. 



