THE SKELETON OF AETICULATA. 



105 



structure. The rings differ in the several classes of this divi- 

 sion, merely as to volume, form, solidity, number of pieces, 

 and the degree of motion which one has upon another. In 

 some groups they are consolidated, so as to form a shield or 

 carapace, such as is seen in the crabs. In others, they are 

 membranous, and the body is capable of assuming various 

 forms, as in the leeches and worms generally. Fig. 75 is a 

 beautiful fossil Astacus, from the lower greensand, which exhi- 

 bits the character of the skeleton of the Crustacea. 



\Leti. Jtc 



Fig. 75. Astacus f'ectensis, from the lower greensand, Isle of Wight. 



223. A variety of appendages are attached to these rings, 

 such as jointed legs (fig. 34), or, in place of them, stiff bristles, 

 oars fringed with silken threads, wings either firm or mem- 

 branous (fig. 369), antennae, moveable pieces which perform 

 the office of jaws (fig. 195), &c. But, however diversified this 

 solid apparatus may be, it is universally the case that the 

 rings, to which every segment of the body may be referred, 

 as to a type, combine to form but a single internal cavity, in 

 which all the organs are enclosed, the nervous system, as well 

 as the organs of vegetative life ( 76). 



224. The muscles which move all these parts have this 

 peculiarity, that they are enclosed within the more solid frame- 

 work, and are not external to it, as in the vertebrata ; and 

 also that the muscular bundles, which are very considerable 

 in number, have the form of ribbons, or fleshy strips, with pa- 

 rallel fibres of remarkable whiteness. 



225. The vertebrated, like the articulated animals, have 



