ANIMALIA ARTICULATA. 113 



CRUSTACEA. 



Body divided into rings more or less distinct, movable, of 

 considerable consistency (horny or calcareous), and having a 

 double series of articulated membranes constituting one or 

 more, frequently two pairs of antennae ; several jaws or other 

 organs serving for prehension of food ; several pairs of nata- 

 tory or ambulatory feet (in general five or seven pairs); ner- 

 vous system composed either, of a great number of similar 

 nervous swellings, disposed by pairs, and united so as to 

 form two gangl ionic chains, distant from each other, and ex- 

 tending throughout the length of the body, or of two ganglions 

 dissimilar in form, volume and disposition, but always simple 

 and single, and situated one on the head, the other on the 

 thorax. Circulation . complete ; heart aortic; respiration by 

 branchiae; eyes compound, in a few cases simple, and de- 

 scending, by successive modifications of organization, to pe- 

 diculated eyes with facets^ composed each of an hexagonal 

 or square corneal, a conical crystalline, or vitreous body, and 

 a gelatinous substance, surrounded, as well as the crystalline, 

 with a colouring matter, and considered as an expansion of 

 the optic nerve. Almost always there is ah auditory appa- 

 ratus which consists in a small tubercle situated between the 

 mouth and the base of the external antennae, including a 

 vessel filled with an aqueous liquid. Three remarkable 

 orders. 



ORDER DECAPODA. 



Rings of the head and thorax soldered together and con- 

 cealed under an enormous carapax which extends to the ab- 

 p 



