CRUSTACEANS. 141 



subject of various species. But to admit of progi-essive increment, it is 

 frequently renovated, and especially during earlier age. Wliile it subsists 

 in perfection, the subject does not gTOW, for apparently the only period 

 of increment is restricted to preparation for its exuviation, or casting 

 of the integument. Whence the importance of this physical alternative 

 is obvious. The precise period of the commencement and cessation of 

 its occurrence are alike unknown. Undoubtedly it commences soon, 

 and terminates when the full dimensions are attained ; for, in the light 

 that we consider its purpose, there is no further use for the change. 



The shells of living Crustaceans are of various colours, green, red. 

 or yellow. "Wlien cast, they dry, and the colour fades. Tlie more vivid 

 are consequently converted to another hue, principally reddish. The 

 same tendency is often exhibited by some still investing the living sub- 

 ject ; also white patches become gi-een on successive exu^'iations. 



Those young animals which are originally pure white, are darkened 

 by supervening patches ; and possibly the whole integuments are ulti- 

 mately converted to the colour usually distinguishing their species. But 

 the only Crustacean I have seen white in the early stages, is the Cancer 

 pfujurus, or Common Crab. 



Many naturalists have viewed the animals, or most of those in- 

 cluded in this section, as nearly akin to insects. Some are terrestrial, 

 others are acquatic. Only the latter are considered here. 



The shell is composed of numerous parts, of which one, the princi- 

 pal, investing the intestinal organs, is called the carapace. Naturahsts 

 compare the parts of the body to segments, whereof they enumerate 

 twenty-one composing the Common Crab. The parts are connected by 

 a kind of cartilage, which are liberated by its continued maceration, 

 when then- former articulation may be examined in detail. Various re- 

 markable pieces Avill be then found among them. 



Though composed of so many portions, the whole integument ge- 

 nerally separates entire on exuviation. 



This process is not confined to Crustaceans alone. The integuments 

 of numerous other animals, especially those belonging to the Insect, the 

 Ophidian and Batrachian tribes, are also cast and renovated. But the 



