36 REPORT — 1855. 



The coxa in Brachyura is universally fused with the segment of the bodyv 

 so that its normal form cannot be distinguished ; in the Macroura it is free : 

 it is here we are enabled to make out that the normal number of joints in 

 the legs of Crustacea is seven, which only vary by suppression of the last or 

 fusion of the first with the body of the animal. 



In the A)nphipocla, except the aberrant tribe of Lcemodipoda, the coxa is 

 always developed into a scale-like process, and has been always considered 

 as side-pieces complementary to the segment of the body to which the legs 

 belonged, and received the name of epimerals or side-pieces by M. Milne- 

 Edwards. 



These so-called epimerals, we think, we shall here be able to demonstrate^ 

 are homologically the coxae of the legs, and represent the first joint in the 

 typical condition of Crustacea. But this is so contrary in its description to 

 the opinions of all the highest authorities, that it is necessary we should 

 produce good evidence of the reason why we are induced to affirm that the 

 scale-like form belongs to the first joint of the leg, rather than to the segment^ 

 of which the leg is an appendage. 



The normal number of joints is most conspicuous in Nephrops and HomU' 

 rus, where the coxa is an articulating joint, but appears to have no very great 

 extent of movement. In the Brachyura and the Lcemodipoda, that is the 

 Aherrantia of the table accompanying this Report, the coxa is fused with the 

 body ; but in the Amphipoda it \^ fixed to, but not fused ivith, the segment. 



There is a peculiar tendency among the Amphipoda to a development of 

 a scale-like form to the joints of the legs in general, a fact which is recog- 

 nized as a constant feature in the basis joint of the three posterior peram- 

 bulating legs. 



This is occasionally the case with the same joint in other legs, as in Poda- 

 cerus, but appears to reach a culminating point in the genus Stdcator, where 

 there is a peculiar tendency to this kind of development in almost every part 

 of the visible members. 



The object of this peculiar development seems to be for the protection of 

 the branchial organs, which are suspended from the inner surface of the legs, 

 and would otherwise be liable to accidents, particularly to such animals as 

 Sulcator arenarius, whose habitat is in the damp sand. 



But the chief object which here we have to demonstrate is, that this scale- 

 like development belongs to the leg and homologically is the first joint (or 

 coxa), and that it is not a lateral or separate portion of the annular segments 

 of the body of the animal, and, in fact, that no side-pieces or epimerals exist; 

 to this end we think we are justified by the following arguments, which we 

 shall endeavour to substantiate: — 



1st. That seven joints are the normal number in the legs of all the Mala- 

 costracous Crustacea. 



2ud. That the branchia is normally an appendage of the leg and attacbed 

 to the coxa. 



3rd. That the moveable power of the leg is always between the coxa and 

 the leg, and never between the coxa and the body. 



4th. Tliat llie coxa (the so-called rpinuM'al) in Amphipoda overlaps the 

 segment to whicli it is attached, and except by a small portion only, is not 

 united by the whole of the margin in jiixlaj)osition with the segment. 

 5th. Tiiat there are no cpinierals where tliere are no legs. 

 6th. That epimerals ;ire toinid in no other type, except the Iidriophtlialma 

 among Crustacea. 



1st. Thai seven is the normal number of joints to a leg, we think we 

 have already disposed of, in comparing the leg of the Macroura type with 



