ISOPODA. 107 



Stenosoma, Leach. 



The Stenosomae only differ from the Idotese in the linear form of 

 their body, and the length of their antennae which is more than half 

 that of the body(l). 



Arcturus, Lat. 



The Arcturi are very remarkable for the form of the second and 

 third feet, which incline forwards and terminate by a long, bearded 

 and unarmed or feebly unguiculated joint; the two anterior are laid 

 on the mouth and are unguiculated; the last six are strong, ambu- 

 latory, thrown behind, and bidentated at the extremity. In the 

 length of the antennae and form of the body they approach the Ste- 

 nosomae. 



I have never seen but a single species, the Arct. tuherculatus^ 

 which was brought to Europe, from the Arctic seas, in one of 

 the last expeditions to those regions. 



The fifth section — Asellota, Lat. — comprises Isopoda with four 

 very apparent setaceous antennae, arranged on two lines, and termi- 

 nated by a pluriarticulated stem; two mandibles; four jaws covered, 

 as usual, by a kind of lip formed by the first foot-jaws; vesicular 

 branchiae, in pairs, covered by two longitudinal and biarticulated, 

 but free leaflets; a tail composed of a single segment, without late- 

 ral fins, but with two bifid stylets, or two very short tubercular ap- 

 pendages on the middle of its posterior edge. Other lamelliform 

 appendages, situated at its inferior base, which are now numerous 

 in the males, distinguish the sexes. 



AsELLus, Geoff. 



Two bifid stylets at the posterior extremity of the body ; eyes 

 separated ; the superior antennae at least as long as the peduncle of 

 the inferior ; the hooks at the end of the feet entire. 



The only species of this subgenus that is known — the Aselle 

 d'eau douce, Geoff., Ins. II, xxii, 2; Squille aselle, Deg., Insect., 

 VII, xxi, 1; Desmar., Consid., XLIX, 1,2; Idotea aquatica^ 

 Fab., — is very abundant in fresh and stagnant waters as well as 

 in the marshes, in the vicinity of Paris. Its gait, unless alarm- 

 ed, is very slow. In the spring it issues from the mud in which 



(1) Stenosoma lineare, Leach; Desmar. op.cit. Ib.xlvi, 12; — Stenosoma hedicum, 

 lb.; — Idotea viridissima, Risso, Crust., Ill, 8. For the other species, see Desmar. 

 op. cit. 



