PLATE XXV

THE BLIND CRAYFISH OF THE MAMMOTH CAVE OF KENTUCKY, Cambarus pellucidus. (NATURAL SIZE)

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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.

At all events, we do not find in any truly subterranean species large and peculiarly modified eyes like those of many deep-sea animals, and this may be associated with the complete darkness of their habitat, not lighted by phosphorescent organisms as the deep sea is. In another respect these animals differ from those of the deep sea, for they are all colourless or nearly so; while many of the inhabitants of the deep sea, as we have already seen, are brilliantly coloured.