22 PHYLUM ARTHROPOD A 



abdomen, where each somite except the last, or telson, bears 

 a pair of appendages. 



The animal is capable of two sorts of locomotion. By powerful 

 strokes of the broad, finlike end of the abdomen it swims rapidly 

 backward, and it can also walk on its thoracic legs. It is well pro- 

 vided with special sense organs. Most important to it are the 

 two pairs of feelers, or antennae, which are characteristic of all 

 crustaceans, and the compound eyes on movable stalks. It also 

 possesses, in a pair of small cavities on the upper surface of the 

 basal joints of the first or shorter pair of antennae, peculiar sense 

 organs, which were formerly supposed to be ears, but are now 

 known to be balancing organs. With the aid of them the animal 

 maintains its equilibrium. 



The body of the crayfish or the lobster, as of all the higher 

 crustaceans, is made up of twenty somites, or body segments, of 

 which the thirteen anterior somites form the cephalo thorax, and 

 the seven posterior ones the abdomen. 



The Cephalothorax. The anterior five somites forming this 

 body division are cephalic, the remaining eight are thoracic, and 

 all are covered dorsally and laterally by the carapace. The pro- 

 jection running forward from the anterior end of the carapace is 

 called the rostrum. A transverse groove is seen near its middle ; 

 this is the cervical suture and marks the boundary between the 

 head and the thorax. In the crayfish two semicircular, longi- 

 tudinal grooves extend backward from the outer ends of the cer- 

 vical suture, which separate the sides of the carapace from the 

 median, dorsal portion. The sides of the carapace are called the 

 branchiostegites ; they cover lateral folds of the dorsal integu- 

 ment of the animal, which extend over the sides of the body and 

 inclose between themselves and it the spaces within which lie the 

 gills. These spaces, the gill chambers, thus communicate freely 

 with the surrounding water. Pass the handle of a scalpel or 

 other flat object beneath the lower edge of the branchiostegite 

 and it will go into the gill chamber. During life a current of 

 water passes constantly into the gill chamber along this lower 

 edge, where it bathes the gills and then passes out at the for- 

 ward end. 



