TANAIDACEA AND ISOPODA— TATTERSALL. 



193 



Antarctic Expeditions, but only two species, Gnathia antarctica (Studer) and Glyptonotus 

 antcurcticus , Eights, var. acutus, Richardson, are common to the three lists. 



The most interesting morphological point revealed by the examination of this 

 collection is the modification of the exopodites of the first pleopod in the males of 

 certain species of the family Arcturidae, as accessory sexual organs. It may be recalled 

 that Ohlin described such a modification in Pseudidothea honnieri from the Magellan 

 region, and was so impressed with the importance of this feature that he created a new 

 family for the reception of that species entirely on the characters of the first pleopod of 

 the male. I have found two distinct types of modified first pleopods in the males of 

 certain Arcturidae. 



The first type is characteristic of the genus Antarcturus, and is illustrated 

 diagrammatically in text-fig. 1 , A, which shows the relations of the various parts of the 



A. 



B. 



Fig. 1. — First pair of pleopods of male. A, Antarcturus. B, Pseudardurella. 



h. basipodite ; en. endopodite ; ex. exopodite ; g. groove ; I. lobe on inner margin of exopodite ; 



ji.f. penial filament. 



first pair of pleopods in the males of that genus as they appear when the animal 

 is laid on its dorsal surface and the doors of the operculum opened. In the median 

 line lies the penial filament (formed by fusion of the two filaments present in most 

 Isopods), which is about as long as the basipodite of the pleopods. The exopodite of 

 each pair of pleopods lies on top of the endopodite, and on its under sm-face there 

 is a rather deep oblique groove which commences at the inner proximal corner, 

 immediately at the posterior end of the penial filament, and traverses the exopod to 

 the outer distal corner. It becomes more or less completely roofed over distally, and 

 at the distal end there is a protuberance on the margin of the exopod roofing over the 

 distal opening of the groove. This type of first male pleopod I have found in all the 

 Antarctic species of Antarcturus and the allied genus Dolichiscus. It has been noticed, 

 but not sufiiciently emphasised, by Barnard in Antarcturus Jcladophorus, Stebbing, and 



2 F 2 



