312 DR. H. J. HANSEN ON THE [NoV. 29. 



more, the third pair in the male is shaped exactly as that pair 

 which in the female follows the operculum ; the penultimate paii' 

 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 homo- 

 logous. That the undivided ojDerculum 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 StenetriuoJi, 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 paii-s 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, ^ga, &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. 2gr) 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 lanira (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 ofi" 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 Stene- 

 trium and other Asellota. As in Aselhts, we find a sympod with 

 two rami, the essential difierence 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, Stenetriitm 

 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. XXI. 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 

 IschnosoTYia.) It can be added that we have now found the key 

 to the interpi-etation of the endopod of the second pair of pleopoda 



