A very interesting division of the fresh-water fauna is constituted by those animals which inhabit underground waters. In the South of England there is found not unfrequently in the water of wells a small colourless transparent Amphipod known as the "Well Shrimp" (_Niphargus aquilex_--Fig. 62), distinguished from the common fresh-water _Gammarus_ by the slenderness of its body, by the elongation of the last pair of tail appendages (uropods), and by the absence of eyes. The proper habitat of _Niphargus_ is not actually in the wells, but in the subterranean reservoirs and streams by which the wells are fed. These subterranean channels intercommunicate over wide areas, and are now known in many parts of the world to contain a peculiar assemblage of animals which become accessible to the naturalist in wells and in the streams and lakes of large caves. Further, the scanty "abyssal" fauna of deep lakes is partly made up of species which enter the lakes by subterranean channels, and find a suitable habitat in the deep water. Species of _Niphargus_, for example, have been dredged in Lough Mask in Ireland and in some of the Swiss lakes. Several species of blind Crayfishes have been found in caves in North America, the best known being one (_Cambarus pellucidus_--Plate XXV.) found in the Mammoth Cave in Kentucky; and blind Prawns belonging to various genera have been discovered in caves in America and Europe. [Illustration: _PLATE XXV_ THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY, _Cambarus pellucidus_. (NATURAL SIZE)] A very remarkable feature of the subterranean fauna is that a number of the animals appear to be more closely allied to marine species than to any known from fresh waters above-ground. This is especially the case with some of the Isopoda belonging to typically marine families like the Cirolanidæ and Anthuridæ, and it has been suggested that these have been derived from marine species which have entered the underground waters directly from the sea by way of submarine fissures in the crust of the earth. The environment in which these subterranean animals live resembles that of the deep-sea animals in the absence of light, and the consequent absence of plant-life. They must ultimately depend for food on animal and vegetable débris washed down from the surface, but the food-supply must be scanty, for the water in which they live is usually very clear and free from organic matter. It is not surprising to find that nearly all of them are blind, and the few species provided with visual organs which have been described, from caves, are probably only temporary or accidental immigrants. Whether the degeneration of the eyes is the direct effect of disuse, or is due to natural selection ceasing to keep the eyes up to the standard of usefulness, is a question which has been much debated, and its answer, were we sure of it, would settle some of the most fundamental problems of the evolution theory.