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Show 3 1 2 DR. H. J. HANSEN ON THE [N o v . 2 9 . more, the third pair in the male is shaped exactly as that pair which in the female follows the operculum; the penultimate pair in the male is exactly like the penultimate in the female, but differs from the preceding and from the last pair. We must therefore conclude that the three posterior pairs in both sexes are homologous. That the undivided operculum found in all genera, Asellus excepted, in the female is homologous with the first pair in the male, must be concluded from the fact that in all these genera the two appendages constituting this pair in the male have their sympods coalesced or, as in Stenetrium, completely fused. The second pair, which in the male bears the copulatory organs, is therefore wanting in the female. Next, the interpretation of the parts constituting the two anterior pairs in the male must be considered. Asellus presents the best starting-point. That the two joints of the first pair in this animal are respectively the distal joint of the sympod-its two proximal joints having disappeared-and one of the rami must, I think, be admitted, and is easily seen from comparison with Cirolana, jEga,&c., but it is impossible to decide whether the distal joint, the ramus preserved, is the endopod or the exopod. The second pair in Asellus is easy to interpret: each appendage consists of the sympod with the two two-jointed rami proceeding from its distal end; no other interpretation is possible, but the result is that it is the endopod itself which is transformed as a kind of copulatory organ, with a cavity in the interior of its distal joint. Let us, then, look at the first pair of the male in other Asellota. In Stenetrium (PI. XX. fig. 2 g) the sympods are fused, and the plate thus formed bears two unjointed rami, but, as in Asellus, it is impossible to decide whether they are the endopods or the exopods. Comparing this structure with that in Ianira (PI. XXI. fig. 5), and especially in the undescribed genus (PI. XXI. fig. 4), it must be admitted that the distal pair of lobes marked off by oblique lines from the long proximal plate must be the rami found in Asellus and Stenetrium. Finally, we must consider the second pair of the male in Stenetrium and other Asellota. As in Asellus, we find a sympod with two rami, the essential difference being that these rami proceed not from the end but from the inner margin of the sympod. The most distal ramus, which in all genera, Stenetrium excepted, is shaped as and performs the function of a hook, is therefore the reduced exopod ; as in Asellus it is always two-jointed, Stenetrium excepted, but even in a species of this genus a vestige of a division into two joints is discernible. The copulatory organ is the endopod ; as in Asellus it is two-jointed-in Eurycope I found the basal joint divided again into two joints (PI. X XI. fig. 6)-and the distal joint has an internal cavity, Stenetrium. excepted. (Beddard, in his ‘ Challenger ' Isopoda, has already correctly interpreted the rami as endopod and exopod in Stenetrium and Ischnosoma.) It can be added that we have now found the key to the interpretation of the endopod of the second pair of pleopoda. |