CHAPTER VIII 



CRUSTACEANS AND RELATED FOEMS 



76. Crustaceans. — The Crustaceans (krus-ta/ shuns : 

 Latin, crusta, crust) are so-called because of their hard 

 outer covering. They belong in the same group of ani- 

 mals as the insects and are more highly developed than 

 the worms. The body consists of a limited number of 



segments, each of which usually bears 

 a pair of jointed appendages. The 

 appendages are variously modified ; 

 some aid in swimming, others in 

 securing food, and others are used 

 in walking. The jointed appendage 

 is the characteristic expressed in 

 the technical name Arthropoda (iir- 

 throp'o-da : Greek, arthros, joint ; 

 pod, root of pous, foot) given to the 

 group to which all these animals 

 belong. 



77. Crayfish. — As a typical crus- 

 tacean we have the common crayfish, 

 or " crab " as it is known away from 

 the seashore. The crayfish has nineteen pairs of append- 

 ages adapted to different kinds of work. It lives in fresh- 

 water ponds and streams where there is sufficient lime for 

 its use in building up its outside covering (exoskeleton). 



The animal is divided into two regions, the head-thorax 

 region and the abdomen. The segments of the abdomen 



86 



Figure 83. — Crayfish 

 bearing Eggs. 



