232 



BODY OF CRUSTACEA. DIVISION INTO SEGMENTS. 



Fio. 456. SANDHOPPBR. 



they free themselves from their old shell is exceedingly singular. 

 In general they manage to get out of it without occasioning the 

 least change in its form ; and when they have quitted it, the 

 whole surface of their body is already clothed in a new casing, 

 which, however, is still soft, and does not acquire its requisite 

 solidity for some days. 



769. The body of Crustacea is composed of a series of rings, 



more or less distinct. Sometimes 

 most of these segments are simply 

 articulated one with the other, so 

 as to admit of a considerable de- 

 gree of motion, as in the Sand- 

 hopper ; sometimes they are 

 nearly all soldered together, and 

 are only distinguished by furrows 

 situated at their line of juncture ; 

 lastly, in other cases, their union 

 is still more close, and it is only by analogy that we are led to con- 

 sider the trunk, resulting from their juncture, as made up of several 

 rings, rather than of one only. Hence result, as we can easily under- 

 stand, very great differences in the form of these animals; and if we 

 compare with each other a Woodlouse (Fig. 457), a Sandhopper 

 (Fig. 456), and a Crab (Fig. 458), we might be at first led to be- 

 lieve them formed according to types en- 

 tirely different ; but a deeper study of 

 their structure shows, that the compo- 

 sition of their tegumentary skeleton is 

 essentially the same, and that the dif- 

 ferences lie almost entirely in this, 

 that most of the rings being quite dis- 

 tinct and moveable in the "Woodlouse, 

 are grown together in the Crab, and 

 that certain analogous parts do not 

 present the same proportions in both 

 animals. Thus in the Woodlouse and 



Sandhopper we find a distinct head (c) followed by a thorax 

 composed of seven rings similar to each other (t l t 8 ), and each 



- P 



Fio. 457- ONISCUS, OR 



WOODLOUSK. 



