The genuine Isopoda are divided among the _Flabellifera_, in which the terminal segment and uropods form a flabellum or swimming fan; the _Epicaridea_, parasitic on Crustaceans; the _Valvifera_, in which the uropods fold valve-like over the branchial pleopods; the _Asellota_, in which the first pair of pleopods of the female are usually transformed into a single opercular plate; the _Phreatoicidea_, a fresh-water tribe, known as yet only from subterranean waters in New Zealand and an Australian swamp nearly 6000 ft. above sea-level; and lastly, the _Oniscidea_, which are terrestrial. Only the last of these, under the contemptuous designation of wood-lice, has established a feeble claim to popular recognition. Few persons hear without surprise that England itself possesses more than a score of species in this air-breathing tribe. Those known from the world at large number hundreds of species, distributed among dozens of genera in six families. That a wood-louse and a land-crab are alike Malacostracans, and that they have by different paths alike become adapted to terrestrial life, are facts which even a philosopher might condescend to notice. Of the other tribes which are aquatic there is not space to give even the barest outline. Their swarming multitudes are of enormous importance in the economy of the sea. If in their relation to fish it must be admitted that many of them plague the living and devour the dead, in return the fish feed rapaciously upon them. Among the most curious of recent discoveries is that relating to some of the parasitic _Cymothoidae_, as to which Bullar has shown that the same individual can be developed first as a male and then as a female. Of lately discovered species the most striking is one of the deep-sea Cirolanidae, _Bathynomus giganteus_, A. M. Edwards (1879), which is unique in having supplementary ramified branchiae developed at the bases of the pleopods. Its eyes are said to contain nearly 4000 facets. The animal attains what in this order is the monstrous size of 9 in. by 4. A general uniformity of the trunk-limbs in Isopoda justifies the ordinal name, but the valviferous Astacillidae, and among the Asellota the Munnopsidae, offer some remarkable exceptions to this characteristic. Among many essential works on this group may be named the _Monogr. Cymothoarum_ of Schiödte and Meinert (1879-1883); "_Challenger_" _Report_, Beddard (1884-1886); _Cirolanidae_, H. J. Hansen (1890); _Isopoda Terrestria_, Budde-Lund (1885); _Bopyridae_, Bonnier (1900); _Crustacea of Norway_, vol. ii. (Isopoda), Sars (1896-1899), while their multitude precludes specification of important contributions by Benedict, Bovallius, Chilton, Dohrn, Dollfus, Fraisse, Giard and Bonnier, Harger, Haswell, Kossmann, Miers, M'Murrich, Norman, Harriet Richardson, Ohlin, Studer, G. M. Thomson, A. O. Walker, Max Weber and many others.