In the Southern Hemisphere we have a species of Anaspides, A. tasmaniae, occurring in mountain streams and tarns in Tasmania, a related form which haunts the littoral zone of the Great Lake in Tasmania, and a small species, Koonunga cursor, occurs in a little stream near Melbourne.
Of the Isopoda certain genera, viz. Asellus and Monolistra, are confined to fresh water, others, such as Sphaeroma, Idothea, Alitropus, and Cymothoa, have occasional fresh-water representatives. Packard[[171]] describes a remarkable blind Isopod, Caecidotea, from the Mammoth Cave of Kentucky, which occupies a very isolated position, and in the same work gives a very complete exposition of the cave-fauna of North America and Europe.
The Phreatoicidae are a curious family of Isopods confined to the fresh waters of Australia and New Zealand, which bear a remarkable resemblance to Amphipods, being laterally compressed and possessing a subchelate hand on the anterior thoracic leg. These Isopods are exceedingly common in small mountain pools and in streams in Tasmania, and in the Great Lake in that country I have recently found a number of species which, together with some species of Amphipods, make up the dominant feature in the Crustacean fauna. One of these species may grow to fully an inch in length. The family is confined to the temperate regions, and is usually found on mountains. A number of species are known from the mainland of Australia, one coming from a high elevation on Mount Kosciusko, and another (Phreatoicopsis) from the forests of Gippsland attaining a great size, and living among damp leaves, etc.
The fresh-water Amphipoda all belong to the families Talitridae, Gammaridae, and Haustoriidae (see p. [137]).
Among the Talitridae, or Sand-hoppers, Orchestia and Talitrus have marine as well as fresh-water and land representatives, while the American Hyalella is entirely from fresh water, most of the species being peculiar to Lake Titicaca. Many of these animals are partly emancipated from an aquatic life. Thus Orchestia gammarellus, which is common on the sea-shore of the Mediterranean, frequently penetrates far inland, and was found in large numbers by Kotschy near a spring 4000 feet up on Mount Olympus.
Talitrus sylvaticus is very common among fallen leaves and decaying timber in Tasmania and Southern Australia, many miles from the sea, and often at an elevation of several thousand feet.
Among the Gammaridae, certain genera, e.g. Macrohectopus (Constantia), from Lake Baikal, are purely fresh-water. An enormous development of Gammaridae was discovered by Dybowsky in Lake Baikal, comprising 116 species, and lately a number more have been found by Korotneff.[[172]] The majority of these were originally placed in the genus Gammarus, but Stebbing has rightly created a number of peculiar genera for them. Certain species are, however, placed in more widely distributed genera, e.g. Gammarus and Carinogammarus, which is also represented in the Caspian Sea. Korotneff found some remarkable transparent pelagic forms (Constantia) swimming in the abyssal regions at about 600 metres depth, the majority of them being blind, but some possessing rudimentary eyes, often on one side only.
Besides various species of Gammarus, a number of other Gammaridae are frequently found in brackish water. Among Haustoriidae Pontoporeia has representatives in both the oceans and inland lakes of the northern hemispheres (see p. [137]).
Of the Decapoda, seven families are typically fresh-water in habitat—the Aegleidae, containing the single species Aeglea laevis, related to the Galatheidae, which inhabits streams in temperate S. America; the Atyidae, a family of Prawns from the tropical rivers and lakes of the New and Old World, and in the Mediterranean region. A number of Palaemonidae are found in fresh water, e.g. Palaemonetes varians in Europe and N. America, while several species of Palaemon occur in lakes, streams, and estuaries of the tropical Old and New World.
The expeditions of Moore and Cunnington to Lake Tanganyika brought back an exceedingly rich collection of Prawns, comprising twelve species, all of which are peculiar to the lake,[[173]] and this is all the more surprising since Lakes Nyasa and Victoria Nyanza are only known to contain one species, Caridina nilotica, which ranges all over Africa and into Queensland and New Caledonia. The Tanganyika species, however, all belong to purely fresh-water genera, and do not afford any suggestion that they are part of a relict marine fauna. It would appear that they have been differentiated in the lake itself during a long period of isolation.