MOLLUSC A. 523 



the head, the fore legs, and frequently, also, to tiie breast 

 and abdomen. 



The characters furnished by the thorax are extensively 

 employed in the arrangement of the genera and species. 

 These are chiefly taken from its appearances on the back of 

 the animal. 



The Breast or third ring of the body in insects, is fre- 

 quently so much incorporated with the thorax, as to appear 

 as one organ. In other cases it seems to form a part of 

 the abdomen. In the back, it frequently exhibits a horny 

 process, termed scutelluvi, analogous in consistence to the 

 thorax, to which it is united behind. At this place of the 

 breast, the wings have their origin. On its ventral side it 

 supports the middle and hind legs. Between these, at the 

 base, is the pectoral sternum, which, in some species, ex- 

 pands into a cover for the first joint of the hind legs, and 

 in others is produced to cover a portion of the abdomen. 

 The breast contains the muscles for moving; the middle and 

 hind legs, the wings and abdomen ; and it likewise contains 

 some very strong muscles, passing from its ventral to its 

 dorsal surface, and calculated to bring these, when requir- 

 ed, nearer together. This motion is probably facilitated by 

 the number of sutures with which its skin is traversed. 

 The characters for classification are chiefly taken from the 

 scutellum and sternum. 



The Abdomen is the last portion of the body. In 

 some species, it is sessile, and intimately united with the 

 breast ; in others it is divided by a stalk. It consists of 

 rings, varying in number in different genera. These rings 

 are joined together in many species by a simple adhesion of 

 the margin, while, in others, the posterior margin of the 

 one includes the anterior margin of the otlier. The motion 

 of the whole abdomen takes place by means of muscles, 

 which arise in the breast, and, in ^hosc where it is sessile, 



