CHAPTER X. 



CLASS OF CRUSTACEA. 



767. CRUST ACE A. are Articulated animals, respiring by Iranchice 

 or external gills, or by the general surface, and possessing 

 a circulating apparatus, and separate sexes. Crabs and Lob- 

 sters are the types of this group ; but a great number of animals 

 of a much less complicated structure, and of a different external 

 form, are also classed with these ; for as we descend the 

 natural series formed by these animals, we see the same general 

 plan of structure gradually modified and simplified. The 

 lowest Crustacea are even so imperfect, that they can only exist, 

 attached like parasites, on other animals : whence most Natura- 

 lists have placed them with the Intestinal Worms. 



768. The tegumentary skeleton of Crustacea generally pos- 

 sesses a very considerable degree of firmness. It has nearly 

 always a stony hardness ; and indeed contains a very consider- 

 able proportion of carbonate of lime. We may look upon this 

 solid envelope as a kind of epidermis ; for beneath it we find a 

 membrane like the true skin of higher animals ; and at certain 

 times it detaches itself and falls off, in the same manner as the 

 epidermis of Reptiles separates itself from their bodies ( 471), 

 and as we have also seen the enveloping membrane of the larvae 

 of Insects renew itself several times. We can easily understand 

 the necessity of this change, in animals whose whole bodies are 

 inclosed in a solid case ; which, not being able to grow like the 

 interior parts, would oppose an invincible obstacle to their 

 development, if it could not be thrown off, as soon as it has 

 become too small to lodge them commodiously. Thus the 

 Crustacea change their skin during the whole time of their 

 growth ; and it would seem that the greater part of these 

 animals grow during their whole life. The manner in which 



