62 Descriptive Zoology. 



ward movement, frequently bumping its body upon the 

 floor ; it evidently needs the buoyancy of the water to sup- 

 port its weight. 



How the Crayfish Swims. Swimming is the most rapid 

 action of the crayfish, and it probably seldom resorts to this 

 means of locomotion except to escape enemies. The side 

 parts of the " tail fin " are spread out as wide as possible, 

 and the whole abdomen is suddenly and forcibly bent down, 

 under, and forward. This makes a powerful stroke, and 

 drives (or rather pulls) the crayfish swiftly backward. The 

 whole of the under surface of the abdomen is concave, thus 

 getting a good hold on the water. As the animal darts 

 backward the resistance is greatly reduced by the convexity 

 and smoothness of the dorsal surface of the abdomen. 

 Again, since the big pinchers rather necessarily extend 

 forward, it would be difficult for the crayfish to move for- 

 ward with any considerable speed ; but when it darts back- 

 ward the big claws drag along in the wake without any 

 special resistance. It should be further observed that when 

 the crayfish is frightened, the chances are that it is on or 

 near the bottom, which in most cases is more or less muddy ; 

 when the powerful tail ^stroke is made, it would naturally 

 sweep close to, if not actually touch, the muddy bottom. 

 This stirs up the mud, and thus makes the water turbid 

 between the pursuer and the pursued, and greatly favors 

 the chances of escape. 



The Muscles of Locomotion. The muscles which move 

 the appendages are inclosed in the framework along the 

 ventral surface of the cephalothorax. The muscles which 

 flex and extend the abdomen in the act of swimming fill 

 most of the space in the abdomen. As would be expected, 

 the extensor muscles are much smaller than the flexors. 

 The extensors arise from the side walls of the thorax, and 



