386 



CEUSTACEA. 



Fig. 195. 



ing 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 Crusta- 

 ceans, upon the consideration 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. 



(982.) 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 ac- 

 quires in the larger species great density and hardness. 



(983.) As regards the mechanical ar- 

 rangement of the skeleton, we shall find the 

 same general laws in operation as we have 

 observed throughout all the Annulose orders 

 a continual centralization and progres- 

 sive coalescence of the different rings or 

 elements composing the external integu- 

 ment, and a strict correspondence between 

 the degree to which this consolidation is 

 carried and the state of the nervous system 

 within. 



(984.) In the lowest forms of the Crus- 

 tacea we have, in fact, a repetition of the 

 condition of the skeleton met with in the 

 Myriapoda, or in the larva state of many 

 insects, the whole body being composed of 

 a series of similar segments, to which are 



appended external articulated members of the simplest construction 

 (fig. 195). 



(985.) The number of rings or segments composing the body varies 

 in different species ; but such variation would seem, from the interesting 

 researches of Milne -Edwards and Audouin concerning the real organi- 

 zation of articulated tegumentary skeletons, to be rather apparent than 

 real, inasmuch as the discoveries of these distinguished naturalists go far 

 to prove that, whatever the state of consolidation in which the integu- 

 ment is found, the same number of elements or rings may be proved to 

 have originally existed before, by their union, they became no longer 

 distinguishable as separate segments. 



(986.) The normal number of these elements Milne-Edwards con- 



