163 
one to be used where necessary instead of “first” or fC second 
pereiopoda.” 
The first pair of 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 Antliura , Janira , <fec. The basal portion, 
probably representing the coxos and basos, is subrecta ngular and 
has the outer edge fringed with long set?e, while the inner edge 
bears a few long set;e chiefly towards the distal end. The endopodite 
is shorter than the exopodite, irregularly elliptical in form, margins 
quite free from setre and with a shallow emargination at the 
extremity. The exopodite has the inner margin nearly straight 
and the outer one slightly curved and converging distal ly 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 setae, those near the end longest and plumose, the 
others simple. 
The second pleopoda (Plate xxvi., flg. *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 endopodite 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 the 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 tiie outer side so as to form a half tube ; at the end it bears 
a few short stout setse. Most probably this process acts as an 
accessory copul atory organ. A somewhat similar organ has been 
described and figured by Bovallius in Ianthe speciosa.^ Beddard 
has described organs named by him “ penial filaments in Ickno- 
soma bacilloides and in Acanthomunna prateus , which appear to 
be similar in function to the one found in Phreatoicus australis f 
In A set Ins aquaticus , as described by Sars, the whole of the second 
pleopod seems to be modified in a very different way for repro- 
ductive purposes. \ In some genera, sachas Ianthe, Jcera, and 
apparently also in Ichnosoma and Acanthomunna the first pair of 
pleopoda (as well as the second) is modified to form a male organ, 
but this is not the case with Phreatoicus. 
* Bihang. Till. K. Svenska Yet. Akad. Handliugar Band 6, No. 4, p. 10, 
plate iii., figs. 30 and 31* 
f Report of the Challenger Isopoda, Part II., p. 46, plate vi., fig. 10. 
and p. 49, plate xii., fig. 13. 
J Orustaces d’eau douce de Norvege, p. 101, pi. x., figs. 6 and 7» 
