CLASS OF CRUSTACEA. 465 



have the form of a conical papilla armed with several bristles, 

 and its feet, eight in number, differ very much from each 

 other, the four posterior feet being terminated only by 

 bristles, whilst the four anterior feet are furnished at their 

 extremity with small suckers, by means of which they can 

 adhere to the most polished bodies. 



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. 504), 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 imperfect, 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 

 alwajrs 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. 513) resembling the dermis of superior 

 animals ; and at certain points 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 present 

 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 



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