CLASS I. 



CRUSTACEA. 



Tlie Crustacea are articulated animals, with articulated feet, 

 respiring by means of branchiie, protected in some by the bor- 

 ders of a shell, and external in others, but which are not in- 

 closed in special cavities of the body, and which receive air 

 from openings in the surface of the skin. Their circulation 

 is double, and analogous to that of the Mollusca. The blood is 

 transmitted "from the hear-t, which is placed on the back, to 

 the different parts of the body, whence it is sent to the bran- 

 chiJE, and thence back again to the heart(l). These branchiaB, 

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

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

 form pyramids composed of laminae in piles, or bristled with 

 setae 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 Crus- 

 tacea of diiferent orders. The same subject has lately been 

 thoroughly examined by Messrs Victor Audouin and Milne 

 Edwards in their third Memoir on the Anatomy and Physio- 

 logy of these animals — Ann. des Sc. Nat. XIV, 77, — and all 

 that is now w^anting to complete their researches, is the pub- 

 lication of those made by M. Straus on the Branchiopo'da and 

 the Limuli in particular, which they have not noticed. 



(1) See the order Decapoda. 



