CRUSTACEi. 151 



CLASS I. 



CRUSTACEA. 



The Crustacea are articulated animals, with arllculated feet, re- 

 S2)iring by means of branchiae, protected in some by the borders of 

 a shell, and external in others, but which are not inclosed in special 

 cavities of the body, and which receive air from openings in the sur- 

 face of the skin. Their circulation is double, and analogous to that 

 of the MoUusca. The blood is transmitted from the heart, Avhich is 

 placed on the back, to the different parts of the body, whence it is 

 sent to the branchiae, and thence back again to the heart *. These 

 branchiae, sometimes situated at the base of the feet, or even on 

 them, at others on the inferior appendages of the abdomen, either 

 form pyramids composed of laminae in piles, or bristled with setse or 

 tufted filaments of simple ones, and even appear in some cases to 

 consist wholly of hairs. 



Some of the Zootomists, Baron Cuvier in particular, had already 

 made known to us the nervous system of various Crustacea of differ- 

 ent orders. The same subject has lately Ijeen thoroughly examined 

 by Messrs. Victor A\idouin and Milne Edwards in their third Memoir 

 on the Anatomy and Physiology of these animals — Ann. des Sc. Nat. 

 XIV, 77, — and all that is now wanting to complete their researches, 

 is the publication of those made by M. Straus on the Branchiopoda 

 and the Limuli in particular, which they have not noticed. 



" The nervous system of the Crustacea submitted to our observa- 

 tion, say they, presents itself in two very different aspects, which 

 constitute the two extremes of the modifications visible in that class. 

 Sometimes, as in the Talitrus, this apparatus is constituted by nu- 

 merous similar nervous inflations, arranged in pairs, and united by 

 cords of communication in such a way as to form two ganglionic 

 chains, separated from each other, and extending throughout the 

 length of the animal. At others, on the contrary, it is solely com- 

 posed of two ganglions or knotty enlargements, dissimilar in form, 

 volmne, and arrangement, but always simple and azygous, and 

 situated, one in the head and the other in the thorax. Such is the 

 case in the Maia. 



" These two modes of organization, at the first glance, certainly 

 seem essentially different, and if the study of the nervous system of 



* See the order Decapoda. 



