and on the Homologies of the Antennary Joints in Decapoda. 235 



be partially seen. In Leptograpsus this areola is more exposed, 

 encroaching somewhat upon the margin or outer surface of the 

 coxal joint, or, in other words, this joint is kept permanently a 

 little raised. In Grapsus the coxal joint (here the " perforated 

 tubercle " of Strahl) is still more evolved, and its sides are folded 

 in, giving it a globular form, and contracting the areola, which 

 is thus placed in a slit, and becomes almost wholly external. 

 The different form of the coxocerite in Grapsus is therefore the 

 result of a simple modification, not of structural importance. 

 In Dromia the coxal joint is also slit at one side, but the areola 

 is on the inner surface. This joint in Dromia is not " so 

 shrunken that only the tubercle remains." It is far larger in 

 proportion than is usual in the higher Crustacea. Dr. Strahl 

 says that "if we imagine the slit in the tubercle of Dromia 

 carried out to one side, so that here the peripheral margin is 

 completely separated, we have the operculum of the Brachyura 

 in its perfect form." But this prolongation of the slit would cut 

 the coxal joint in two, which is not the case in the " operculum;" 

 for this " operculum " is truly the homologue of the coxocerite 

 of Dromia and Homarus in its entirety, as may be seen by com- 

 parison with this part in Pilumnus, for instance, where the basi- 

 cerite is not soldered to the contiguous parts as is usual in 

 Cancroids, but is free and articulated directly with the " oper- 

 culum" in the same manner as it is with the coxal joint in the 

 other two genera named. Pilumnus, we may remark incident- 

 ally, would be classed with Parthenope by the character of its 

 antennse. 



Dr. Strahl proposes new names for the first two joints of the 

 external antennae : the first (coxocerite) he would call intercalare ; 

 the second (basicerite) armiger, while the third (ischiocerite) he 

 calls the first joint of the antennas, which is certainly liable to 

 mislead. Prof. Milne-Edwards, who has done so much towards 

 elucidating the homologies of these joints, has given to them 

 the names in brackets, which are more appropriate ; for there is 

 undoubtedly a perfect correspondence between them and the 

 joints of the maxillse or feet. I believe it possible to carry the 

 homology even further than the celebrated French zoologist has 

 done, and that the antenna in question, like a foot or maxilliped, 

 consists normally of seven joints. In the embryo of Hippolyte, 

 as figured by Kroyer*, there are five distinct joints beyond the 

 basicerite, which would make seven in all. Moreover, they can 

 be demonstrated in the adult Squilla, Axius, and Pagurus, and 



Vidensk. Selskabs Skrifter, 1856, iv. 288) that a far more complicated 

 auditory apparatus exists at the base of the internal antenna;. 



* Monog. Fremst. af Hippolyte's Nordiske Arter, &c. tab. vi. f. 121, 



16* 



