CRUSTACEANS. 



LESSON VI. 



CLASS OF CRUSTA'CEA. Organization Moulting Circula- 

 tion Respiration Division. 



CLASS OF CRUSTA'CEA. 



1. The class of CRUSTA'CEA (from the Latin, crusta, a hard 

 covering) comprises all articulated animals, that have articulated 

 legs, and are provided with a heart, and branchiae for breathing 

 water. Crabs and cray-fish are types of this group ; but we 

 place also in it a great number of animals whose structure is 

 much more complicated, and whose external form is very dif- 

 ferent; for, in proportion as we descend in the natural series 

 formed by these creatures, we observe the same general plan 

 becomes modified, and more and more simplified. The body in 

 most of them is covered by a sort of crust of almost stony hard- 

 ness. 



2. Crusta'ceans differ greatly from anne'lidans, but resemble 

 insects and arach'nidans by having white blood, and articulated 

 legs ; and are distinguished from the two last classes, by their 

 branchial respiration, by the number of their legs, and by several 

 other characters. 



3. The body of crusta'ceans is composed of a succession of 

 rings more or less distinct. Sometimes these segments move 

 freely on each other, and at others they are so solidly joined that 

 the rings are merely indicated by ridges. Frequently the head 

 and thorax form but one piece, which is separated from the abdo- 

 men. In the lobster, for instance, the head and thorax are con- 

 founded in one mass, and the abdomen is composed of seven 

 distinct and movable rings (Jig. 61, b). It is the same in crabs, 

 except that the abdomen is smaller, and folded underneath ; but 

 in the wood-louse, the head is distinct from the thorax, which is 

 itself divided into seven movable rings. The legs, which are 

 composed of several articulations, are inserted into the thorax : 

 their number is ordinarily five or seven pairs ; lobsters and crabs 



1. What description of animals constitute the class of Crusta'cea ? 



2. How are Crusta'ceans distinguished from Anne'lidans, insects and 

 Arach'nidans ? 



3. How are Crusta'ceans characterized? 



