13(> CLASS CRUSTACEA. 



when there exists only a single nucleus at the thorax (the 

 crabs proper, or hrachytiri.) Of all the raacrourous decapods 

 observed by MM. Victor Audouin and Milne Edwards, the 

 palinurus would be that whose nervous system is the naost cen- 

 tralized, and in fact, in our method, this crustaceum is but 

 little removed from the brachyuri ; but it would not be the 

 same with Palemon and the lobster, for according to them 

 the first would approach more in this respect to Palinurus 

 than the lobster does, while in our distribution this last crus- 

 taceum precedes palemon, an arrangement which appears to 

 us to be founded on many very natural characters. 



The Crustacea are apterous, or without wings, provided with 

 two eyes, with facets, but rarely with simple eyes, and com- 

 monly with four antennae ; they have, for the most part (the 

 poecilopoda excepted) three pair of jaws, (the upper two 

 which are designated under the name of mandibles compre- 

 hended) ; as many of jaw -feet, but the last four of which be- 

 come, in a great number, real feet ; and ten feet, properly so 

 called, all terminated by a single claw ; when the last two 

 pair of jaw-feet fulfil the same functions, the number of feet 

 is then fourteen. The mouth also presents, as in the insects, 

 a labrum, a ligula, but no lower lip, properly so called, or 

 comparable to that of these last. The third pair of jaw-feet, 

 or the first, closes the mouth externally, and replaces this 

 part. 



The sexual organs, or those at least of the males, are al- 

 ways double, and situated under the breast, or at the lower 

 origin of that posterior and abdominal part of the body which 

 is commonly called tail ; but they are never placed behind. 

 Their teguments are usually solid, and more or less calcareous ; 

 they change skin many times, and generally preserve their 

 primitive form and their natural activity ; for the most part, 

 they are carnivorous, aquatic, and live many years. They do 

 not become adult, or fit for generation, until after a certain 



