CRAYFISH, LOBSTERS, SPIDERS, AND INSECTS 127 



merits of the legs antl maxillipeds. The upper set is 

 attached to the mt-mbranes that connect the thoracic 

 appendages to the tliorax. Each gill is plumelike in form 

 with a stem and feathery side branches, and stands 

 vertically with the free end pointing upward. In the 

 stem are two blood vessels, one for the entrance of the 

 blood and one for its exit (Fig. 69). 



The locomotion of the crayfish. — The crayfish has two 

 methods of locomotion, walking and swimming. It walks 

 with the four posterior pairs of legs, holding the first pair, 

 which bears the claws, out in front. The crayfish, at best, 

 walks rather clumsily and usually slowly. The water aids 

 in buoying up the body, for on land this crustacean walks 

 more awkwardly still and tlie body bumps over the ground 

 with evident annoyance. 



In swimming, the l)road tail fin is spread out as wide as 

 possible and the powerful muscles of the abdomen c(jn- 

 tract very quickly and pull the tail with a sudden downward 

 and forward stroke beneath the abdomen. This action, of 

 course, always forces the crayfish in a backward direction. 

 It swims rapidly, and as the animal usually remains close 

 to the bottom, the body and dragging legs invariably stir 

 up a cloud of sediment that hides the animal from its pur- 

 suing enemies. In this backward swimming of the cray- 

 fish the unwieldy claws are prevented from retarding the 

 rapidity and celerity of its progress by being dragged vm- 

 resistingly after the body. 



The alimentary canal. — The mouth, which is situated 

 in the middle of the ventral surface of the head, opens into 

 a short gullet that leads directly upward to a large, capa- 

 cious stomach (Fig. 70). The stomach, situated in the 

 head, is divided by a constriction in the middle into two 



