152 
CRUSTACEA. 
the Crustacea were limited to these two animals, it would be ex- 
tremely difficult to recognize the analogy between the central nervous 
mass in the thorax of the Maia, and the two ganglionic chains which 
occui)y the same region of the body in the Talitrus. But if we re- 
member the various facts detailed in this memoir, we necessarily 
arrive at this remarkable result.” 
They were led to it by the exact and careful study of the nervous 
system of various intermediate Crustacea, forming so many links of 
the series, such as the Cymothoce * * * § , the Phyllosomse f , Astacus :|;, 
Palaemon, and Palinurus. They have also supported their positions 
by the observations of Cuvier, and those of M. Treviranus. The 
consequence deduced by them is, that notwithstanding this ditterence, 
the nervous system of the Crustacea is formed of the same elements, 
which, insulated in some and uniformly distributed throughout the 
length of the body, present in others, various degrees of centraliza- 
tion, at first from without inwardly, and then in a longitudinal 
direction ; and that finally, this apiJroximation in all directions is 
carried to its extreme point, when it is reduced to a single nucleus in 
the thorax — as in Cancer, properly so called, or the Brachyura. Of 
all the Decapoda Macroura examined by Messrs. Audouin and Ed- 
wards, the Palinurus was found to have the venous system most cen- 
tralized ; and in fact, that animal in our system is but little removed 
from the Brachyura. But this should not be the case with Palsemon 
and the Astacini, for according to them the former approximates more 
closely in this respect to Palinurus than the latter, while in our ar- 
rangement the second precede the first, a disposition which appears 
to us to be founded on several very natural characters. 
The Crustacea are apterous or deprived of wings, furnished with 
compound eyes, though rarely with simple ones, and usually with 
four antennse. They have mostly — the Paecilopoda excepted — three 
pairs of jaws, the two superior ones, designated by the name of man- 
dibles, included ; as many foot-jaws § , the last four of which, how- 
ever, in a great many instances, became true feet ; and ten feet pro- 
perly so called, all terminated by a single small nail. When the last 
* Isopoda. 
•p Stomapoda. 
X For this subgenus and the tvo following subgenera, see the Decapoda 
^Macroura. 
§ Auxiliary jaws, as they are termed by M. Savigny, at least w'hen speaking 
of the Crustacea Decapoda. As the two superior ones, in the Arnphipoda and 
Isopoda, form a sort of lip, he there calls them the auxiliary lip. He distinguishes 
the jaws in Phalangium, a genus of Arachnides, as principal jaws ; those which are 
attached to the palpi — false palpi, according to him ; aud as supernumerary jaws, 
those which are attached to the first four feet. Those parts of the same anin)als 
which have been considered as mandibles, are his 7nan(iibules succ^danh. He admits 
of two auxiliary lips in the Seolopendrse. 
