306 DR. H. J. HANSEN ON THE [Nov. 29, 



backwards ; it has been mentioned and well drawn by Sars ; its 

 function is cei'tainly to produce a current in the water of the 

 marsupium. 



The pleopoda show, however, some more interesting features. 

 As in Stenetrmm, the three posterior pairs in the male do not 

 differ fx-om those in the female ; the very large two- jointed 

 exopods cover, as in Stenet7^mm, not only the small respiratory 

 endopods but almost the whole lower surface of the abdomen, 

 and are freely exposed with the exception of a rather small basal 

 portion. The fourth and fifth pairs are essentially alike, both con- 

 sisting of a short sympod and two rami, viz. a two- jointed exopod, 

 somewhat larger than the unjointed endopod ; consequently we 

 have here a well-marked difference in the fifth pair between Stene- 

 trium and Asellus, as in the former genus the exopod — according 

 to my interpretation above — is unjointed and the endopod want- 

 ing. In the female the pleopoda of the first pair are not fused 

 as in Stenetrium, but independent and originate rather distant 

 from each other ; each pleopod consists of a nearly rudimentary 

 sympod and a moderately large circular plate distally edged with 

 plumose set^. In the male each appendage of the first pair consists, 

 as in Stenetrium, of a short sympod and a much longer suboval, 

 movable ramus ; but while in the latter genus the two sympods 

 are completely fused, they are free in Aselhcs, but yet furnished 

 with some hooks * along their inner margin, so that they can be 

 coupled together nearly as the second joint of the maxillipeds. The 

 second pleopoda in the male are interesting ; the distal half of the 

 left sympod with its rami is shown from below in PI. XXI. fig. 3. 

 The sympod is shortly oval, with both rami proceeding from its 

 end and containing strong muscles for their movement. The 

 exopod is oblong, nearly lamellar, only a little shorter than the 

 endopod, two-jointed ; the distal joint is somewhat larger than 

 the basal one, obliquely oval, with marginal seta3 and containing a 

 good- sized muscle. The endopod consists of two movable joints ; 

 the proximal joint is short, but produced into a long, slender, 

 curved process, turning inwards and forwards along the inner 

 margin of the sympod ; it contains a small muscle to the second 

 joint. This is obliquely oval, its distal end rather rounded, but 

 near the end a few minute teeth and irregular incisions and 

 depressions are seen ; the joint is besides inflated, and the major 

 portion of its interior is occupied by a large pear-shaped sac, which 

 opens at a short distance from the end of the joint ; the wall of 

 this sac is well chitinised. Having removed by dissection the 

 major part of the wall of the joint itself, I was able to examine 

 the wall of the inner sac. In the female second pleopoda ai'e 

 wanting. 



Before attempting to decide as to the systematic import- 

 ance of the differences between Stenetrmm and Asellus, it may 



* Each hook is a very thick and rather short spine, the end of which is broadly 

 rounded, curved very slightly upwards, and the upper surface of its terminal portion 

 is set with from seven to ten tiny, sharp, oblong teeth. 



