306 



MOVEMENT IN LOWEE ANIMALS 



SO far back on the body as to make walking awkward, 

 though admirably placed for swimming. 

 All organs made for movement in the water are well 



T>opsal 



Fia. 138 — The flns of a flsh. 



adapted to pushing the body through the water. The 

 broad tail of the lobster and the flexible abdomen enable 

 it to shoot the body backward by bend- 

 ing this portion sharply forward, and 

 then slowly straightening it. The fins 

 of the fish and the flukes of the whale 

 show the same paddlelike character, while 

 the webbed feet of the frogs and aquatic 

 birds are adaptations for pushing against 

 water. In all these paddlelike organs, 

 too, the broad surface is so constructed 

 as to be opposed to the water in push- 

 ing, while by bending, it is enabled to 

 return to position without offering much 

 resistance to the water. 



An interesting modification of the water 

 motion is seen in the squid where the 

 broad tail serves simply to steer with, while the motion 

 is produced by ejecting spurts of water by contraction 



Fig . 139 — Movement 

 of the tail ttn of a 

 fish in swimming. 



