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.
A number of species of Cyclopidae and Cypridae, many of which are blind and colourless, have been brought up in well-water. The Amphipod Niphargus puteanus has long been known from a similar source in England[[170]] and all over Europe, and several other blind Gammarids inhabit the subterranean waters and caves in various parts of the world. Among Isopods, Asellus cavaticus is recorded from wells and caves in various parts of Europe, Caecidotea stygia and C. nickajackensis from the Mammoth and Nickajack Caves in America, and two very remarkable blind Isopods are described by Chilton from the subterranean waters of New Zealand, viz. Cruregens fontanus, whose nearest allies are the marine Anthuridae, and the Isopods Phreatoicus typicus and P. assimilis, which bear an extraordinary resemblance superficially to Amphipods. Besides these, a small number of subterranean Decapoda are known which retain the eye-stalks but are without functional ommatidia. These are Troglocaris schmidtii, in Hungary, related to the fresh-water Atyid Xiphocaris of East Indian and East Asiatic fresh waters rather than to the South European Atyephyra; Palaemonetes antrorum, from artesian wells in Texas; and several species of Cambarus from the Eastern United States. A blind species of Cambarus, C. stygius, has been described from the caves of Carniola, and if this determination is correct, is the sole Cambarus occurring outside America.
It will be seen from the above account that the subterranean Crustacea are an exceedingly interesting and, in many respects, archaic group, many of which have survived in these isolated and probably uncompetitive districts, while many secular changes were going on in the quick world overhead.
The remaining fresh water Malacostraca may be mentioned under the headings of the groups to which they belong.
Only one “Schizopod,” apart from Paranaspides, is known from fresh-water lakes, viz. Mysis relicta, which was discovered in 1861 by Lovén in the Scandinavian lakes, and has since been found in the Finnish lakes, the Caspian Sea, Lake Michigan, and other localities in N. America, and Lough Erne in Ireland. This species is closely related to Mysis oculata of Greenlandic seas.