319 



CHAPTER XVII. 



CRUSTACEA. 



INSECTS and Arachnidans are air-breathing animals ; and, even 

 in such species of these two extensive classes as inhabit fresh 

 water, respiration is strictly aerial. No insects or spiders are 

 marine ; and consequently the waters of the ocean would be utterly 

 un tenanted by corresponding forms of Articulata, was there not 

 a class of beings belonging to this great division of the animal 

 world so organized as to be capable of respiring a watery medium, 

 and thus adapted to a residence in the recesses of the deep. 

 Examined on a large scale, the Crustaceans, upon the considera- 

 tion of which we are now entering, are marine creatures : many 

 species, it is true, are found abundantly in the lakes and ponds 

 around us, but these form rather exceptions to the general rule ; 

 and we may fairly regard this extensive group of beings as the 

 aquatic representatives of the insects and spiders, with which they 

 form a collateral series. 



(364.) The tegumentary system of the CRUSTACEA corresponds 

 in its essential structure with that of insects, and consists of a 

 vascular dermis, a coloured pigment, and a cuticular secreted 

 layer which forms the external shell or skeleton : the latter, 

 or epidermic covering, however, differs materially in texture from 

 that of other Articulata, inasmuch as it contains calcareous matter 

 in considerable abundance, and thus acquires in the larger species 

 great density and hardness. Figt 149 



As regards the mechanical arrangement 

 of the skeleton, we shall find the same 

 general laws in operation as we have ob- 

 served throughout all the annulose orders, 

 a continual centralization and progressive 

 coalescence of the different rings or ele- 

 ments composing the external integument, 

 and a strict correspondence between the 

 degree to which this consolidation is car- 

 ried and the state of the nervous system 

 within. 



In the lowest forms of the Crustacea 



