410 ESSENTIALS OF ZOOLOGY 



Economic Relations 



Crayfish and the entire class Crustacea, are of considerable im- 

 portance to man. The crayfish, lobster, crab, shrimp, and others 

 are used directly as food to the extent that it is an industry valued 

 at several million dollars annually in the United States. The 

 nviinerous smaller genera, like Daphnia, Cyclops, Cypris, Gammarus, 

 Aselhis, and Euhranchipus, comprise a large part of the food of 

 many of our food fish either directly or indirectly. The more minute 

 ones also feed many clams and oysters and finally end in human 

 consumption. The shrimp and crab fisheries are the most important 

 ones concerning Crustacea on the Texas coast of the Gulf of Mexico. In 

 the Mississippi Valley and on the Pacific Coast the crayfish is used ex- 

 tensively as a food. It becomes a serious pest in the cotton and 

 corn fields of Louisiana, East Texas, Mississippi, and Alabama. They 

 fill the swampy land with their burrows where they come up to the 

 surface and eat the young plants. Frequently their burrows do 

 serious damage to irrigation ditches and earthen dams. Crayfish 

 also capture numerous small fish which are either immature food 

 fish or potential food of such fish. 



Characterization of Other Crustacea 



Besides crayfish the order Decapoda includes lobster, shrimp, and 

 crab. They all have ten walking legs, for which they are named. 

 The crayfish and lobster are very similar except in size. The 

 shrimps and prawns are marine and resemble the crayfish except 

 that they do not have the great pinchers (chela) and the abdomen 

 is bent sharpl}^ downward. The crabs are quite dilferent in shape 

 in that the cephalothorax is broader than it is long, the abdomen 

 is poorly developed, and folded sharpl}^ beneath the thorax. Crabs 

 of different kinds vary in diameter from a few millimeters to several 

 inches. There are four species of swimming crabs in the Gulf of 

 IMexico, of which the hhte or edible crab (Callinectes sapidus) is the 

 most important and best known. The lady crab and calico crab 

 are also interesting species. When the blue crab is captured at 

 molting time it is called the soft-shelled crab. At other times it is 

 the hard-shelled crab. They may be caught in baited nets or on 

 pieces of meat on a line with which they are brought to the surface 

 and lifted out in a dip-net. The hermit crab (genus Pagurus) is 

 smaller and lives in empty gastropod shells by backing into the shell 



