BRACHYURA. gj 



some peculiarities in the palpus; but even for generic distinctions, they 

 may be too much relied upon. 



We have observed above, that the mode of articulation between the 

 third and fourth segments of the outer maxillipeds distinguishes the 

 Grapsoidea. Yet it should be understood, that while this is true, this 

 is not the most prominent characteristic of the Grapsoidea. 



The mouth area, besides subserving the purposes of the mouth, is 

 also, the place for the passage of the waters used by the branchiae 

 in aeration. The current flows over the praelabial plate, beneath 

 the maxillipeds. This function involves modifications in the buccal 

 organs which are of great importance. But the considerations con- 

 nected with this point properly relate to the system of aeration. 



The brancliial system is one from which we should particularly ex- 

 pect important distinctions and valuable characteristics of the highest 

 significance; and such distinctions exist. They are at the basis of 

 some of the primary subdivisions, as exhibited in the system of Milne 

 Edwards, and to a large extent, also, in the system of De Haan. A 

 large group of species, the Leucosoidea (Oxystomata of Edwards), 

 have the buccal area narrowing forward, sometimes nearly to a point ; 

 and this is not due to any peculiar modification of the nutritive 

 system, but to an adaptation of the buccal area to certain peculiar 

 modes of aeration. In these adaptations in different groups, the part of 

 the buccal organs especially devoted to the branchial system (apart 

 from the basal appendages or " fouets") is the inner branch of the first 

 pair of maxillipeds (or third maxillse). This lamellar branch covers 

 the efferent branchial current, forming a covered passage for it, and, as 

 well illustrated by De Haan, it is especially devoted to this purpose, the 

 water flowing beneath it to the anterior margin of the buccal area, 

 where it passes out. This thin plate, which is but a small appendage 

 to the first maxillipeds, has hence a high functional importance. 



This efferent passage from the branchial cavity, in a large part of the 

 Brachyura, covers each half of the prtelabial plate, or else the outer 

 portions of each half; and in many species, when the latter is the case, 

 there is a small longitudinal ridge on the praelabial plate, separating the 

 efferent passage from the rest of the plate. The importance of such a 

 ridge as a means of distinction, is hence obvious. It is of higher 

 value than the greater or less breadth of the inner maxilliped branch 

 that covers the passage, or the fact that this branch has a straight or 

 sinuous margin, or some other like modification. 



16 



