13(5 
CLASS CRUSTACEA. 
when there exists only a single nucleus at the thorax (the 
crabs proper, or brachyuri.) Of all the macrourous decapods 
observed by MM. Victor Audouin and Milne Edwards, the 
palinurus would be that whose nervous system is the most 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 
