VARIETIES OP FORM. 193 



crawling or swimming legs, the whole animal bearing a 

 great resemblance to one of the Annelides, but showing 

 a slight advance in organization. As we rise to higher 

 and more developed forms, we find a gradual concentra- 

 tion of the parts of the body, effected by the more or 

 less perfect coalition of its ring-like parts into solid 

 pieces. At the same time, one definite idea or plan 

 seems to pervade the whole class. According to this 

 idea the body of a Crustacean consists of twenty-one 

 ring-like pieces, seven of which belong to the region of 

 the head, seven to the region of the thorax, or central 

 part of the body, and seven to the abdominal region, 

 commonly called the tail. In almost every case the 

 pieces belonging to the region of the head are consider- 

 ably condensed, their pairs of legs being converted into 

 the organs of the mouth, which, in this class, as in in- 

 sects, is highly compound in structure. In many of the 

 lower Crustacea, as in the Isopoda the group to which 

 the Wood-louse belongs, and which includes a large 

 number of marine animals, which resemble Wood-lice in 

 form, the joints of the thorax are distinct from each 

 other, resembling rings, and not materially differing from 

 those of the tail. But in the higher Crustacea, as in the 

 Lobster, and still more in the Crab, the thoracic portion 

 is covered externally by a single solid shelly piece. It 

 appears like a single joint of the body, and its compound 

 nature is only indicated by the number of pairs of legs 

 which rise from its lower surface. In some species there 

 is an indication of rings on the surface of the shell, more 

 or less evident ; but in others all such tokens of compo- 

 sition are obliterated. The joints of the abdomen or 



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