Fam. 5. Serolidae.[[102]]—The genus Serolis comprises flattened forms bearing a curious resemblance to Trilobites, which Milne Edwards considered more than superficial. The genus is confined to the littoral and deep waters of the southern hemisphere.
Fam. 6. Sphaeromidae.[[103]]—These are flattened, broad-bodied forms, most commonly met with in the Mediterranean and warmer seas. Without being actually parasitic, they are frequently found as scavengers in decaying material, and they show some relationship to the parasitic Cymothoidae. In some of the genera, e.g. Cymodoce, the ovigerous female shows a degenerate condition of the mouth-parts, while the maxillipedes undergo an enlargement, and are used for causing a current through the brood-chamber.
Sub-Order 3. Valvifera.
Fig. [83].—Munnopsis typica (Munnopsidae), ♂, × 2. A, 2nd antenna; Ab, abdomen; T, 5th thoracic appendage or 4th leg. (After Sars.)
The Valvifera, illustrated by the Idotheidae and Arcturidae, are characterised by the uropods being turned back and expanded to form folding doors covering up the delicate pleopods, which are mostly respiratory in function, though the anterior pairs may serve as swimming organs. Arcturus is a typically deep sea genus, many species, remarkably furnished with spiny processes, having been taken by the Challenger in the southern hemisphere. The Idotheidae are more littoral forms, several species of Idothea being commonly met with off the British coasts, occasionally penetrating into brackish or even fresh water.
Sub-Order 4. Asellota.
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.