vii ABYSSAL AND SUBTERRANEAN FORMS 209 



produced which lie dormant during the winter. The pelagic 

 Copepods may either produce resting eggs for the winter 

 (Diaptomus), or else the winter is passed through in the 

 Nauplius stage, the larvae hibernating in the mud until the 

 spring (Cyclopidae). 



We have so far only dealt with fresh-water Entomostraca. 

 There are, in addition, a number of Malacostraca which inhabit 

 fresh water, and some of these are found in the abyssal region 

 of the great lakes, which must now be considered. 



The physical conditions of the abyssal region are still more 

 stable than those of the pelagic region, since the water is never 

 disturbed, the bottom is always composed of a fine mud, the 

 temperature is constant at 4-5 C., and there is a total absence 

 of light. It was hardly expected that animals would inhabit 

 this region, until Forel discovered Asellus aquaticus in a depth of 

 forty metres in the Lake of Geneva, and subsequently showed 

 that quite a number of animals, including a Hydra, several 

 worms, Molluscs, Crustacea, and larval Insects, may be found in 

 these or even much greater depths. 



The Crustacea of the abyssal region are two in number, and 

 these have been found in a number of European lakes ; Niphargus 

 puteanus, a blind Amphipod closely allied to Gammarus ; and 

 Asellus forelii, allied to A. aquaticus and A. cavaticus, which 

 may be either quite blind or else retain the rudiments of eyes. 



These two Crustacea, under a practically identical form, are 

 also found in the subterranean waters of Europe, and Forel 

 considers that they have arrived in the abysses of the lakes 

 from the subterranean channels, and are not derivatives of the 

 littoral fauna. 1 



Having completed our short review of lacustrine Crustacea, 

 we may deal with the subterranean and cave Crustacea, 2 which, 

 as far as light and temperature are concerned, are subjected to 

 very similar conditions to those dwelling at the bottom of deep 

 lakes. The inhabitants of the subterranean waters have been 

 chiefly brought to light in Artesian wells, etc., while the cave- 



1 Mr. C. H. Martin points out to me that in the Scottish lochs, which from their 

 geological nature are evidently not connected with subterranean waters, none of 

 them nor similar forms occur. ; nor do they in the Tasmanian lakes which are on 

 igneous diabase, so that Forel's conclusion would seem to be of wide application. 



2 See Chilton, Tra.nx. Linn. Soc. (2) vi., 1894, p. 163, with review of literature. 



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