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. 
I he 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 , 
lyncceus, 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 
ol 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 under- 
