152 CRUSTACEA. 



the Crustacea were limited to these two animals, it would be ex- 

 tremely difficult to recognize tlie analogy between the central nervous 

 mass in the thorax of the Maia, and the two ganglionic chains which 

 occupy 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 Cymothoae*, the Phyllosomae-j-, AstacusJ, 

 Palsemon, and Palinurus. They have also supported their positions 

 by the observations of Cuvier, and those of M. Treviranus. Tlie 

 consequence deduced by thein is, that notwithstanding this difference, 

 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 approximation 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 witli Pala-mon 

 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 wliich appears 

 to VIS 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 antennae. 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. 



t Stomapoda. 



X For this subgenus and the two following subgenera, see the Decapoda 

 Macroura. 



§ Avxiliary Jans, as they are termed by M. ?a\-igny, at least vrhen speaking 

 of the Crustacea Decapoda. As the two superior ones, in the Amphipoda and 

 Isopoda, form a sort of lip, he there calls them the aiixilian/ Up. He distinguishes 

 the jaws in Phalangium, a gemis of Arachnides, as yrincijHtJ javs ; those which are 

 attached to the palpi — false palpi, according to him ; and as supernumerary juus, 

 those which are attached to the first four feet. Those parts of the same animals 

 ■which have been considered as mandibles, arc his ma»dibulcs succedanes. He admits 

 of two auxiliary lips in the Scolopendrae. 



