REPORT ON THE ISOPODA. 13 



prehensile organ like the preceding liml), but smaller. In the female it is simihir to the 

 succeeding appendages. 



The remaining thoracic ajij^endages are similar to each other, and are always 

 furnished with tufts of hairs and spines. The first joint, which is always the longest, is 

 generally smooth, or provided on the inner surface with fine plumose hairs like the single 

 sensory hair commonly found upon the terminal point of the filament of the first 

 antennae. The four following joints are smaller, and generally furnished on the inner and 

 outer side with tufts of sword-like and of serrated spines. In many species (Serolis 

 witarctica) these are to a great extent replaced by fine hairs, and in Serolis necera 

 all the thoracic appendages are clothed with fine plumose hairs similar to those wdiich 

 in other species are only to be found upon the abdominal appendages. A more detailed 

 description of the varieties of these hairs and spines will be found under the description 

 of the several species.' 



The last pair of thoracic appendages (see p. 17) are usually smaller, and verj^ often 

 in the males {Serolis gracilis) furnished on the inner side with tufts of delicate sensory 

 hairs like those upon the second thoracic appendage in the males of Serolis convexa. 



The abdominal appendages [see F\. I. figs. 12-14) are much specialised; the three 

 anterior pairs form "swimming feet"; each of these consists of a proximal joint, the 

 protopodite, which is generally triangular in form and attached to the segment which 

 bears it for about a quarter of the length of the base ; the projecting angle bears three 

 plumose hairs in the first of these appendages and two in each of the two following. In 

 some species (viz., Scrolls schjthei, Serolis paradoxa, and in all the Australian species), the 

 protopodite is narrower, less triangular in form, and has no hairs. The inner margin is 

 usually clothed with a dense covering of fine delicate hairs, which are also found, though 

 to a less extent, upon the outer border. About half-way between the summit and the 

 base the endopodite is attached, and the exopodite at the summit of the protopodite ; both 

 the exopodite and endopodite are delicate seraicircular lamellfe, the former being larger 

 than the latter ; the outer convex margin of both is provided with numerous plumose hairs 

 which, as shown on PI. I. figs. 15, 16, consist of a central hollow stem gradually decreasing 

 in diameter towards the extremity, and giving off' on either side a series of extremely fine 

 lamelhe ; in the interior of the stem I was able to observe, in several cases (PI. I. fig. 16), 

 a fine thread occasionally looped upon itself and possibly a nerve fibre ; the function of 

 these hairs may perhaps be to test the quality of the water coming to the gills. 



The lower border of the endopodite in the second of these appendages is prolonged in 

 the male into a penial filament (PI. VII. fig. 2', h). The next two pairs of appendages con- 

 sist of a short basal joint laterally elongated, with which are articulated two broad lamella), 

 an endopodite and exopodite, which are the gills ; the exopodite of the first appendage is 



1 I make an arbitrary distinction here and elsewhere between the finer " hairs " and stouter " spines " in order to 

 express more easily the differences between the ambulatory appendages of different species. 



