(32 CRUSTACEA. 



Most of the swimming Cancroidea have this ridge prominent, and the 

 lamellar maxilliped branch is rather narrow. But in a Lupa (L. 

 cribraria), this ridge is wanting, and the lamellar branch is quite 

 broad, covering the praelabial plate to its centre. This is a striking 

 instance of a wide discrepancy among species that have hitherto been 

 referred to a single genus. The completeness with which this lamel- 

 lar branch of the first maxillipeds is adapted to cover the efferent 

 passage, varies much in different families ; it is not always wide, 

 as in the L. cribraria, when the ridge alluded to is wanting. 



The larger part of the Brachyura have the characters just described. 

 But in another part, the efferent passage, instead of passing over the 

 outer portions of the prcelabial plate, passes inward and makes its exit 

 by the middle portion, which is prolonged forward. These are the 

 Leucosoidea or Oxystomata (sharp-mouthed Crustacea) . The narrow 

 anterior limit of the mouth forms the place of exit for the- pair of effe- 

 rent passages; and by means of a ridge, and the same lamellar maxil- 

 liped branch, here much elongated and narrow, the efferent passage is 

 made a complete tube, well inclosed. This passage, it will be perceived, 

 passes inside of the ridge in the Leucosoidea, and outside of it (or out- 

 side of the position it would occupy, if there is none) in the other 

 Brachyura. The afferent passage, in such cases, may occupy the outer 

 portion of the buccal area (that is, the same part that is the efferent 

 in all other Brachyura), or it may enter from a point posterior to the 

 mouth, in which case it is like other Brachyura. 



The branchial system also varies in the number of branchiae, 

 though not in their structure. But this variation in number is only 

 a variation in the amount of surface exposed to aeration, or rather, in 

 the number of subdivisions. It is of less moment than the striking 

 difference in the mode of carrying on the branchial function, between 

 the Leucosoidea and other Brachyura. The triangular mouth of the 

 Oxystomes, is hence, a significant characteristic, not as to impde of 

 nutrition, but of aeration. De Haan, in his system, has well exhi- 

 bited this prominent peculiarity of the Leucosoidea, and has drawn 

 out with more strictness the limits of this group than is done by 

 Edwards. 



The system of reproduction, next to the branchial, affords characters 

 of the widest bearing in classification. But these characters are 

 observed in the position of the external parts, rather than in internal 

 peculiarities. The connexion of the male verges with the posterior 



