i86 THE LIFE OF CRUSTACEA 



belonging to various genera have been discovered in 

 caves in America and Europe. 



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 belong- 

 ing to typically marine families like the Cirolanidae 

 and Anthuridse, 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 debris 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 



