236 ■ "TERRA NOVA" EXPEDITION. 



Family ARCTURIDAE. 



Genus ANTARCTURUS, zur Strassen. 



This genus seems to be separated from the genus Arctiirus by the following 

 characters: — (l) The coxal plates of the first free thoracic segment are not produced 

 downwards and forwards to cover the mouth-parts and the bases of the first two 

 thoracic limbs ; and (2) the dactylus of the second to the fifth thoracic limb is long and 

 well developed, whereas in Arcturas it is quite small. 



In the light of Hansen's recent work on the Northern forms belonging to this 

 family, I may state that I have found four pairs of incubatory lamellae in A. lyolaris, 

 Hodgson, A. franklini, Hodgson, A. hiemalis, Hodgson, and A. fitrcatus, Studer. 



Of the other species present in the collection there was not sufiicient material to 

 make an investigation on this point. It may be noted, however, that Miss Richardson 

 found four pairs in the genus Dolichiscus. Hansen's surmise, that four pairs of 

 incubatory lamellae will be found to be a very general character of the group, therefore 

 receives considerable support from the Antarctic species. I have also found the sexual 

 differences, noted by Hansen in the maxillipedes of Astacilla, occurring in each of the 

 four species I have mentioned above. 



The curious structure of the exopods of the first pleopods of the male in this genus 

 does not appear to have been adequately noticed. In all the species I have examined 

 the exopod of the first pleopod in the male (pi. VIII, figs. 1, 2) has, on its inner or 

 posterior surface, an oblique groove or channel, running from the inner proximal corner 

 to the outer distal corner ; the channel tends to become closed in distally to form a 

 distinct tube, and the aperture or outlet is roofed over by a peculiar process on the 

 outer distal corner of the outer or anterior surface. The pair of penial filaments usually 

 found on the sternum of the last thoracic somite of the male in Isopods are in this 

 family fused into a single tapering process showing faint traces of its two component 

 parts distally and situated on the first abdominal somite*. In this genus the process 

 extends to the distal end of the basal joint of the first pleopods. The oblique 

 groove on the inner face of the exopods is, I take it, merely a channel for the 

 passage of spermatozoa from the penial filament ; and the appendage is comparable 

 in this respect to the first pleopod of the male in crayfishes, which is an appendage 

 modified into tubular form to act as a passage for the male sexual elements. In the 

 crayfishes, however, the first pleopod of the male is a simple styliform process 

 consisting of the fused protopodite and endopodite. Here it is the exopod which is 

 modified in connection with the sexual apparatus. In Dolichiscus the exopod of the 

 first pleopod of the male is rather more specialised. The inner face is very concave and 

 the distal portion bent inwards, so that the whole exopod looks like a spatula or 



* Barnard (1920) has recently pointed out that in the Valvifera the penial processes are situated 

 on the first abdominal somite and not on the last thoracic somite as in all other Isopoda. 



