\'1'1 ZOOLOGY. 



CLASS OF CRUSTACEA. 



561. The Crustacea are articulated animals, properly 

 so called, having the respiration branchial or only cutaneous, 

 and a circulatory apparatus semi-vascular and semi-lacunar. 

 The crabs, craw-fish, and lobsters (Fig. 418), form the type 

 of this group, but naturalists arrange in it also a great number 

 of animals whose structure is much less complex, and whose 

 external form is different ; for in proportion as we descend in 

 the natural series of these animals, we find the general plan 

 of organization to be successively modified and simplified 

 more and more. The lowest of the Crustacea are even so im- 

 perfect, that they can only live when fixed as parasites on 

 other animals, and most naturalists have arranged them with 

 the intestinal worms. 



562. The tegumentary skeleton of the Crustacea presents 

 in general a very considerable consistence. It has almost 

 always a stony hardness, and encloses, in fact, a considerable 

 portion of the carbonate of lime. One may view this solid 

 envelope as being a kind of epidermis, for beneath it we find 

 a membrane (t, Fig. 427) resembling the dermis of superior 

 animals ; and at certain periods the hard calcareous envelope 

 is detached and thrown off, as we have already seen the 

 epidermis of reptiles separate itself from the body, and 

 the tegumentary membrane of the larvae of insects moulting 

 or renewing itself several times. It is easy to understand 

 the necessity of these moultings in animals whose whole 

 body is enclosed in a solid case, which, as it cannot grow 

 proportionately with the interior structures, would pre- 

 sent insurmountable obstacles to their development if it did 

 not fall or was thrown off at the moment when it became too 

 small to lodge the body conveniently; thus the Crustacea 

 change their external covering during the entire period of 

 their growth, and it would appear that most of these animals 

 increase in size during the whole of their lives. The manner 

 in which they throw off their old covering is very singular ; 

 in general they contrive to leave it without causing the 

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

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

 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 



