524 PHILOSOPHY OF ZOOLOGY. 



the motion is very limited. The rings themselves are like- 

 wise susceptible of a little motion, especially when they are 

 included. The muscular fibres which change their posi- 

 tion are longitudinal, and pass from the posterior margin of 

 one joint to the anterior of the other, and are able to draw 

 the ring to one side, or pull it within the one which imme- 

 diately precedes it. 



The classical characters derived from the abdomen are 

 chiefly taken from the number of the rings, their connec- 

 tion, and the condition of their surface. The anus and ex- 

 ternal orifice of generation are situate at the termination 

 of the abdomen. These have frequently appendices, which 

 we will afterwards notice. 



These three portions of the body, in the different tribes, 

 exhibit very remarkable combinations. In some of the 

 Crustacea, the head, trunk, and abdomen, are incorporated 

 on the dorsal surface of the body. In some of the Arach- 

 nidae, the head and trunk are united, while, in others, the 

 head appears to be distinct, while the trunk and abdomen 

 are incorporated. These modifications are extensively em- 

 ployed in the methodical distribution of the groups. 



The MEMBERS of the articulated Annulosa are of two 

 kinds, wings and feet. '4fe" 



The wings, which exclusively belong to insects, vary in 

 their number, structure, and appendices. In one tribe, 

 even among these, they are wanting, and hence termed 

 Apterous; in another they are two in number, but the 

 greater number have four. These wings are either mem- 

 branaceous, and supported by corneous ribs, which form a 

 net-work in their substance ; or, where the wings are four 

 in number, the upper pair are sometimes crustaceous, ob- 

 tain the name of elytra* and serve as a covering to the in- 

 ferior ones. The ribs of the wings, improperly denominat- 

 ed nerves, in the manner of their distribution, the hairs by 



