In this group the abdominal segments are fused dorsally to form a shield-like caudal region; the pleopods are respiratory in function and reduced in numbers, the first pair being often expanded and produced backwards to form an operculum covering the rest. Several of the Asellota are fresh-water, _Asellus aquaticus_ (=Asellidae=) being extremely abundant all over Europe in weed-grown ditches, the mud of slowly-moving streams, and even on the shores of large lakes. They are mostly sluggish in habit, but the marine =Munnopsidae= (Fig. 83, _Munnopsis_) are expert swimmers, the swimming organs being fashioned by the expansion and elongation of the thoracic legs. =Sub-Order 5. Oniscoida.= The Oniscoida[104] are terrestrial forms in which the abdomen is fully segmented, the pleopods are respiratory, their endopodites being delicate branchiae, while their exopodites are plate-like and form protective opercula for the gills, and the uropods are biramous and not expanded. The epimera of the segments are greatly produced. The terrestrial Isopods, although air-breathers,[105] are dependent on moisture, and are only found in damp situations. It seems probable that they have been derived from marine Isopods, since the more generalised of them, _e.g._, _Ligia_ (Fig. 84), common on the English coasts, are only found in damp caves and crannies in the rocks. [Illustration: FIG. 84.—_Ligia oceanica_, ventral and dorsal views, × 1. (From original drawings prepared for Professor Weldon.) ] The related _Ligidium_ is found far inland, but always in the neighbourhood of water. These two genera may be distinguished by the numerous joints in the flagellum of the second antennae, the flagellum being in all cases the portion of the antenna succeeding the long fifth joint. _Philoscia muscorum_ occurs usually near the coast, but it is also found inland in England under trees in damp moss. This genus and the common _Oniscus_, found in woods, are distinguished by the presence of three joints in the flagellum of the second antenna. _Philoscia_ can be distinguished from _Oniscus_ by its narrower body and the pretty marbled appearance of its back. The genus _Trichoniscus_ has four joints in the flagellum; various species are found in woods. In _Porcellio_ and _Armadillidium_ there are only two joints in the flagellum, while _Armadillidium_, the common garden wood-louse, can be distinguished from all others by the flattened shape of the uropods, and the habit of rolling up into a ball like an Armadillo. There is also a very peculiar species, _Platyarthrus hoffmannseggii_, which occurs in England and Northern Europe, and always lives in ants’ nests. It is supposed that they serve as scavengers for the ants, which tend them carefully, and evidently treat them as domestic animals of some kind. The small creature is quite white and blind, and has exceedingly short antennae. =Sub-Order 6. Epicarida.=