230 SUPPLEMENT 



nected together by more or less complicated branches, thus 

 forming a kind of reticulated web, whose tissue will perhaps 

 never be thoroughly unravelled by the art of man. 



There exist, in fact, intermediate groups between the class 

 of Crustacea and the other classes, especially those of insects 

 and arachnida ; and it is more especially the genera of the 

 families of oniscides, of asellota, of myriapoda, and pycnogo- 

 nides, which form these links. These genera have been alter- 

 nately placed by authors in one or other of these classes of 

 invertebrated animals. They form their true points of contact. 

 Nevertheless, these classes are in general very distinct, as may 

 easily be seen by a reference to our text for their comparative 

 characters. 



We shall enlarge a little here on the brief review in the 

 text, of the general form and structure of the Crustacea, endea- 

 vouring, as far as the duty of perspicuity will permit us, to 

 avoid repetition. 



The body of all insects, as we have seen (myriapoda ex- 

 cepted) is constantly divided into three very apparent parts. 

 Such, however, is not the case with the Crustacea. The head 

 is most frequently indistinct, and its position not to be recog- 

 nized but by the existence of the antennae, and the aperture 

 of the mouth. It is intimately confounded with the most con- 

 siderable part of the body, that which encloses the principal 

 viscera, and affords points of attachment for the feet. The 

 posterior portion of the body, divided into isolated segments, 

 merely contains the posterior extremity of the intestinal canal, 

 and is not provided with genuine feet. Such is the organiza- 

 tion of cancer and astacus, or, to speak more generally, of the 

 brachyurous and macrourous decapods. 



In other Crustacea, the head is decidedly detached, but there 

 is no thorax, and the whole of the body is divided into inter- 

 similar segments. This is the case with squillcc, asellce, &c. 



