132 



Chaptrr VIII. 



THE CEAYFISH. Astacus fluviatilis. 



The freshwater crayfish is fairly common in many of the 

 streams of England and Ireland, hiding in holes under the 

 banks. It is of a greenish grey colour ; and in form as well as 

 in internal structure it closely resembles the lobster, to which 

 it is nearly related. It moves about slowly, half walking 

 on its long thoracic legs, half swimming, but when alarmed 

 darts suddenly backwards by violent flexion of its tail or 

 abdomen. 



The hard external cuticle of the crayfish is transversely 

 jointed, to allow movements of the body and limbs ; and this 

 segmentation affects also those internal organs which are 

 directly concerned with locomotion, i.e. the muscles, and the 

 nerves and blood-vessels supplying them. No segmentation, 

 however, is to be seen in the alimentary canal or its glands, 

 and none in the excretory and reproductive systems : in this 

 respect the crayfish differs widely from such animals as the 

 leech, in which these systems are arranged segmentally. 



Each segment of the crayfish, except the last, bears a pair 

 of jointed appendages. These differ greatly in the various 

 regions of the body, but all are modifications of one common 

 type, and serially homologous with one another. The anterior 

 ones form tactile organs ; those in the region of the mouth act 

 as jaws, and the remainder as organs of locomotion. The 

 foremost pair of these latter are of great size, and end in 

 powerful pincers, used for prehension of food and for offensive 

 and defensive purposes. 



The crayfish is carnivorous, seizing its food, and tearing it 

 to pieces by means of the large pincers, aided by the smaller 



