8 CRUSTACEA. 



rection ; 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 Edwards, the Palinurus was found to 

 have the venous system most centralized ; 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 Asta- 

 cini, for according to them the former approximates more 

 closely in this respect to Palinurus than the latter, while in 

 our arrangement 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 Psecilo- 

 poda excepted three pairs of jaws, the two superior ones, 

 designated by the name of mandibles, included ; as many foot- 

 jaws^), the last four of which, however, in a great many in- 

 stances, become true feet ; and ten feet properly so called, all 

 terminated by a single small nail. When the last two pairs of 

 foot-jaws exercise the same functions, the number of feet is in- 

 creased to fourteen. The mouth, as in insects, presents a 

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

 comparable to that of the latter ; the third pair of foot-jaws, 

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

 part. 



The sexual organs, at least those of the males, are always 

 double, and situated on the breast or at the inferior origin of 

 that posterior and abdominal portion of the body commonly 



(1) Auxiliary jaws, as they are termed by M. Savigny, at least when 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 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; and as supernumerary jaws, 

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

 which have been considered as mandibles, are his mandibules succtdanes. He ad- 

 mits of two auxiliary lips in the Scolopendne. 



