232 



BODY OF CRUSTACEA. DIVISION INTO SEGMENTS. 



Fro. 564. SANimorrER. 



they free themselves from their old shell is very singular. In 

 general they 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, but this is still 

 soft, and does not acquire its requisite solidity for some days. 

 844. 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. Hence result very great differences of form ; and if we 

 compare with each other a Woodlouse (Fig. 565), a Sandhopper 

 (Fig. 564), and a Crab (Fig. 566), we might be at first led to be- 

 lieve them formed according to types en- 

 \\ / tirely different ; but a deeper study of 



~ = \jjf 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 which are quite 

 distinct 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 



Sand-hopper we find a distinct head (c) followed by a thorax 

 composed of seven rings similar to each other (t l 2 ), and each 

 ring provided with a pair of legs ( p, pp,) ; and at the posterior 

 part of the body we see an abdomen, also composed of seven seg- 



FIG. 565. ONISCTJS, OR 

 WOODLOUSE. 



