Habits and Adaftations 257 



door. Further, he has done away with his ventral fins, which 

 might otherwise make him unpopular by tickling the insides 

 of his host. Fins, it may be noted, are a favorite subject for 

 adaptation. If not done away with entirely, as in this case, 

 they are put to all kinds of uses for which they were not 

 originally intended. 



Figure 31. TRIGGER FISH 



The dotted lines diagrammatically show the locking mechanism of the 



fin-spines. 



In the common sea-robin, for instance, the pectorals be- 

 come frail legs on which it creeps over the bottom. In the 

 little mud-skipper, which spends much time out of water, 

 they have developed into such sturdy limbs that it can hop 

 about quite agilely on land in pursuit of its prey. In the 

 flying fish the pectorals have been modified into planes 

 which operate like the wings of a glider. They do not move 

 the animal forward. The forward motion is obtained in the 

 water, whereupon it shoots into the air and glides, falling 

 back into the water when the initial impetus is lost. In the 

 trigger fish the spines of the dorsal fin have been trans- 

 formed into a fairly complicated machine for defense, one 



