This group has numerous fresh-water representatives, e.g. Gammarus of several species, the blind well-shrimp Niphargus, and the S. American Hyalella; but the vast majority of the species are marine, and are found especially in the littoral zone wherever the rocks are covered with a rich growth of algae, Polyzoa, etc. The Talitridae or “Sand-hoppers” have deserted the waters and live entirely in the sand and under rocks on the shore, and one common European species, Orchestia gammarellus, penetrates far inland, and may be found in gardens where the soil is moist many miles from the sea.
The Rev. T. R. R. Stebbing, in his standard work[[112]] on this group, recognises forty-one families, and more than 1000 species, so that we can only mention a few of the families, many of which, indeed, differ from one another in small characters.
Fam. Lysianassidae.—The first joint of the first antenna is short, with an accessory flagellum. Mandible with a palp, and with an almost smooth cutting edge. The third joint of the second gnathopod is elongated. This family is entirely marine, comprising forty-eight genera, with species distributed in all seas. One genus, Pseudalibrotus, inhabits the brackish water of the Caspian Sea. Lysianassa has several common British and Mediterranean species.
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.
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.