CLASS OF CRUSTACEA. 467 



slightest deformation, and when they quit it the whole sur- 

 face of their body is already covered with a new sheath ; hut 

 this is still soft, and only acquires its necessary solidity at 

 the end of some days. 



The bodies of Crustacea are composed of a series of rings 

 more or less distinct. Sometimes the most of these segments 

 are simply articulated with each other, and have a tolerably 

 wide range of mobility. Sometimes they are all united to- 

 gether, and can be distinguished only by grooves at the points 

 of junction. Finally, at other times, their union is still 

 more complete, and it is only by analogy that the zoologist 

 is induced to consider the segment resulting from their fusion 

 as composed of several rings rather than one. There results, 

 as might he expected, very wide differences in the general 

 form of these animals ; and if we 



compare together a cloportes (Fig. ^^-^ / 



505) or wood louse, a talitrus 

 (Fig. 169),* and a crab (Fig. c 

 506), for example, one might be t l 

 led at first sight to believe them 

 to be constructed on wholly diffe- 

 rent types; but a deeper study 

 of their structure shows that the 

 composition of their tegumentary *' 

 skeleton is essentially the same, 

 and that the differences depend on 

 this, that the greater number of 

 the rings, completely distinct and Fi - 505 '^^l e ^ 0niscus)> 

 moveable in the oniscus, are 



united to each other in the crabs, and that certain analogous 

 parts do not present in these two genera the same proportions. 

 Thus, in the oniscus (Fig. 505) or in the talitrus (Fig. 169) 

 we find a distinct head (c), followed by a thorax composed of 

 seven rings, resembling each other (t l , t 7 ), and carrying each 

 a pair of limbs (p, pp) ; finally, at the posterior part of the 

 body we find an abdomen (ab), formed also of seven segments, 

 whose size diminishes rapidly, but whose form is nearly the 

 same as in the thorax. In a crab, on the contrary (Fig. 506), 

 the head is not distinct from the thorax, and forms with all 

 this section of the body a single segment covered with a large 

 solid buckler, named shell, or carapace ; finally, the abdomen 



* Oniscus locustra; talitrus: Lat. Of the genus Grammarina; family 

 Gammarus ; band-hopper. K. K. 



H H 2 



