163 



one to be used where necessaty inst(^ad of " first " or " second 

 pereiopoda." 



The first pair <>/ pleopoda (Plate xxvi., fig. 1) like the following 

 pairs appears to be branchial in function and is not specialized to 

 form an operculum as in Anthura, Jaiiira, &c. The basal portion, 

 probably representing the coxos and basos, is subrectangular and 

 has the outer edge fringed with long setfe, while the inner edge 

 bears a few long setae chiefly towards the distal end. The endopodite 

 is shorter than the exopodite, irregularly ellijjtical in form, margins 

 quite free from setre and with a shallow emargination at the 

 extremity. The exopoditp. has the inner margin nearly straight 

 and the outer one slightly cui"ved and converging distally towards 

 the inner so that the joint is widest towards the base and narrows 

 towards the extremity. The whole margin of the exopodite is 

 bordered with seta?, those near the end longest and plumose, the 

 others simple. 



The second pJeopoda (Plate xxvi., fig. 2). In the male these 

 appendages differ from those of the female in having a part of the 

 endopodite specially modified to form a " penial filament." The 

 whole appendage is considerably larger than the first pleopod. The 

 basal portion is subrectangular and is almost completely filled with 

 the powerful muscles which move the endopodite and exopodite. 

 The fndopoditp is .somewhat swollen near the base and gives off on 

 the inner side a long narrow semi-cylindrical process (the "penial 

 filament ") which is articulated to tlie other portion of the endo- 

 podite, and is moved by powerful muscles situated in the swollen 

 portion of the base. This process extends beyond the end of the 

 endopodite as far as the extremity of the first joint of the exopo- 

 dite ; it curves slightly outwards and appears to be hollowed out 

 along the outer side so as to form a half tube ; at the end it bears 

 a few short stout setje. Most probably this process acts as an 

 accessory copulatory organ. A somewhat similar organ has been 

 described and figured liy Bovallius in lauthe speciosa* Beddard 

 has described organs named by him " penial lilaments " in Jchno- 

 soina bacilloidfis and in Acanthomimna protois, which appear to 

 be similar in function to the one found in Phrenf.oicus australisj 

 In Asellus aqvaticvs, as described by 8ars, the whole of the second 

 pleopod seems to be modified in a very different way for repro- 

 ductive purposes. + In some genera, sach as latdlie, J(era, and 

 apparently also in Ichnosoma and Acaiithoinunna the first j^air of 

 pleopoda (as well as the second) is modified to form a male organ, 

 but this is not the case with Phreatoicus. 



* Bihanff. Till. K. Svenska Vet. Akad. Handlingar Band 6, No. 4. p. 10, 

 plate iii., figs. 30 and 31. 



t Eeport of the " Challeuger " Isopoda, Fart II., p. 4G, plate vi., fig. 10. 

 and p. 49, plate xii., fig. 13. 



X Crustaces d'eau douce de Norvege, p. 101, pi. x., figs. 6 and 7- 



