ON CRUSTACEA. 237 



under which the abdomen is fixed. It is fastened by two 

 points of its middle to some appendages of the lower or sternal 

 pieces, which support it like pillars. All its inferior and an- 

 terior part is articulated with the pieces of the mouth and the' 

 first segments of the inferior face of the body. But there is a 

 breach of continuity on the sides, so as to allow the water to 

 penetrate through two clefts into the cavities of the gills. Its 

 general forms and dimensions, the smoothness or inequalities 

 of its surface, &c. vary considerably, according to the genera. 

 We may remark, however, that whatever be the irregularities 

 of the surface of that of the crabs, the arrangement of it is 

 always constant, and subjected to invariable laws. The 

 masses which they form, or the projections which they con- 

 stitute, are marked by deepened lines, more or less perceptible. 

 M. Desmarest has given them the general name of regions ; 

 and, to distinguish between them, he has added to each a 

 peculiar designation which is indicative of the organ which it 

 covers. Thus we have the gastric, the genital, the cardiac, 

 the branchial, and the hepatic regions. These vary in extent 

 and distinctness. In the macrourous Crustacea, with a very 

 slender and flexible testa, they are nearly obliterated. 



The carapace is wanting in all the isopod and amphipod 

 Crustacea, but we find it again in the sub-class of the ento- 

 mostraca. In some genera of the latter, such as daphnis, 

 lyncoeus, cypris, &c. this buckler or mantle is large, and as- 

 sumes a greater degree of solidity. It has a keel on the 

 middle, as in apus, but here this keel becomes a sort' of hinge, 

 the sides of the carapace change into valves analogous in their 

 use to those of the shells of the acephalous mollusca ; and, by 

 means of certain muscles which appertain to the dorsal region 

 of the animal, these valves can be opened or closed at plea- 

 sure. Here we have a decided link between the Crustacea 

 and the testaceous mollusca. 



The body of the Crustacea which are provided with cara- 

 pace, and particularly that of the decapods, is formed luider- 



