=Fam. Haustoriidae.=—The members of this family are specially adapted for burrowing, the joints of the hinder thoracic limbs being expanded, and furnished with spines for digging. Some of the species are common on the British coasts, _e.g._ _Haustorius arenarius_. _Pontoporeia_ has an interesting distribution, one species, _P. femorata_, being entirely marine, in the Arctic and North Atlantic, _P. affinis_ inhabiting the Atlantic, and also fresh-water lakes in Europe and North America, _P. microphthalma_ being confined to the Caspian Sea, and _P. loyi_ to Lakes Superior and Michigan. =Fam. Gammaridae.=—Includes fifty-two genera. The first antennae are slender, with the accessory flagellum very variable. The mandibles have a dentate cutting edge, spine-row, and molar surface, and a three-jointed palp. The first two thoracic limbs are subchelate. This family includes a few marine, but mostly brackish and fresh-water species. _Crangonyx_ is entirely subterranean in habitat, as is _Niphargus_, _N. forelii_ occurring, however, in the deep waters of Lake Geneva. Both these genera are blind. _Gammarus_ has thirty species, _G. locusta_ being the common species on the North Atlantic coasts, and _G. pulex_ the common fresh-water species of streams and lakes in Europe. A number of Gammaridae inhabit the Caspian Sea, _e.g._ _Boeckia_, _Gmelina_, _Niphargoides_, etc., while the enormous Gammarid fauna of Lake Baikal, constituting numerous genera, showing a great variety of structure, some of them being blind, belong to this family, _e.g._ _Macrohectopus_ (_Constantia_), _Acanthogammarus_, _Heterogammarus_, etc. [Illustration: FIG. 95.—_Gammarus locusta_, ♂ (above) and ♀ (below), × 4. _Abd.1_, First abdominal segment; _T_, telson; _Th_, seventh free thoracic segment (= 8th thoracic segment); _U_, third uropod. (After Della Valle.) ] =Fam. Talitridae.=—This family may be distinguished by the absence of a palp on the mandible, and by one ramus of the uropods being very small or wanting. The various kinds of “Sand-hoppers” belong here, familiar creatures on every sandy coast between tide-marks. The genera _Talitrus_ and _Talorchestia_ always frequent sand, while _Orchestia_ is generally found under stones and among weed. Some species of _Orchestia_, e.g. _O. gammarellus_, live inland in moist places at some distance from the sea; one species of _Talitrus_ (_T. sylvaticus_) occurs at great elevations in forests in Southern Australia. _Hyale_ is a coastal genus, and is also found on floating objects in the Sargasso Sea. _Hyalella_ is confined to Lake Titicaca and the fresh waters of South America. _Chiltonia_ from S. Australasia. =Fam. Corophiidae.=—The members of this family have a rather flattened body and small abdomen, and the side-plates on the thorax are small. The uropods are also small and weak. Some species of the genus _Corophium_ are characteristic of the Caspian Sea. =Sub-Order 2. Laemodipoda.=