124 



Chaptee 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, haK 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 aUow 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 Crustacea generally differ widely from such 

 animals as the leech, in which these systems are arranged 

 s^gmentally. 



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 are all 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. 



