At the same time a number of Cyclopidae, e.g. _Cyclops strenuus_, may occur in the pelagic region in considerable quantities, though they were never found by us in such numbers as _Diaptomus_. The life-cycle of the pelagic Entomostraca has been studied in both the Cladocera and the Copepoda. In some of the Cladocera Weismann at first supposed that males had altogether disappeared, and that reproduction was entirely parthenogenetic. It appears, however, that all the pelagic species have at least one sexual period, namely, in the autumn, when resting eggs are 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.[168] Having completed our short review of lacustrine Crustacea, we may deal with the =subterranean and cave Crustacea=,[169] 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 cavedwellers have been investigated especially at Carniola and in the American caves.