THE ISOPODA 207 



forming, in the I;itter fumily, a large operculum which completely 

 covers the following pairs. In the male sex the Ascllidae have the 

 first pair not greatly ditterent from those of the female ; the second 

 pair are small, with a short protopodite and two rami, each of two 

 segments, the endopodite having a cavity in the distal segment. In 

 the Stenetriidae the first pair are partly fused together and the second 

 pair are to some extent intermediate in structure between those of 

 the other two families. In the Parasellidae the first pair are fused 

 together, their enlarged j)rotoi)odites each with a small immovable 

 terminal ramus forming the middle plate of a tripartite operculum, 

 of which the lateral parts are the enlarged protopodites of the 

 second pair. The rami of the latter pair are borne on the inner 

 margin of the protopodite ; the exopodite of two segments is hook- 

 shaped and serves to fasten the lateral to the middle plate of the 

 operciilum ; the endopodite forms a geniculate copulatory organ of 

 two, sometimes of three segments, with a cavity in its distal part 

 communicating with the exterior by a narrow canal. 



In most Isopoda the \iropods differ widely in form and position 

 from the pleopods. In the adult females of some Epicaridea, 

 however, the uropods do not differ, except in size, from the 

 appendages in front of them, and this is also the case in the 

 Cymothoid genera Anuropus and Branchuropiis, where they resemble 

 the pleopods in structure and position and appear to share their 

 branchial function. Although the uropods are usually biramous, 

 one or other of the rami may disappear in many Asellota, some 

 Sphaeromidae, Valvifera, and Oniscoidea. The uropods are entirely 

 wanting in the Sphaeromid Vireia and in some E2:)icaridea. The 

 rami are never composed of more than one segment except in the 

 Asellotan Acanthocope, where the uniramoiis uropods present three 

 or five .segments, including the peduncle. In' the Valvifera 

 the uropods are curiously modified in form and position (Fig. 125). 

 They are attached far forwards at the sides of the greatly enlarged 

 telsonic segment, and are folded inwards so as to cover completely 

 the branchial pleopods. Each consists of a large plate formed liy 

 the expanded protopodite with the small endopodite at its tip, 

 while the exopodite ^ is vestigial or absent. In many Anthuridae 

 the exopodite is attached near the base and the endopodite at the 

 tip of the elongated peduncle (leading to the erroneous statement 

 that the endopodite has two segments), and the exopodite is 

 usually folded backwards over the dorsal surface of the telson. 



Alimentarij S//stem. — The stomodaeum forms, in the majority of 

 Isopoda, a masticatory stomach, which is comparatively sim})le in 



^ What is here called the exopodite is usually regarded as the endopodite, and 

 vice versa. The interpretation given above depends on the assumption that the 

 uropods have reached their present position by a movement ol' rotation, not of simple 

 translation. 



