LAKE FAUNAS. 131 



Deep Faunas of Lakes. — The most systematic and thorough 

 investigations that have been made into the nature of deep lacustrine 

 faunas are those of Forel upon the fauna of Lake Geneva.''^ As the 

 result of the observations of this naturalist it would appear that 

 the abundant fauna of the floor of this lake comprises representa- 

 tives of nearly all the primary divisions of fresh-water — inhabiting 

 Invertebrata, and that even a fair proportion of the secondary 

 groups are also represented, although by a very limited number of 

 species in nearly all cases. Included in the lowest forms are sev- 

 eral amojbse, and Epistylis, Opercularia, and Acineta among infu- 

 sorians. The hydroids are represented by the common brown hydra 

 (Hydra rubra — to one hundred metres), and the rotifers by Flos- 

 cularia. Three orders of w orms are indicated — nematoids, cestoids, 

 and turbellarians — and two of annelids proper, the hirudines and 

 chiEtopods (Lombriculus, Tubifex, &c.). The turbellarians (Pla- 

 naria, Mesostomum, Dendroccelum) have no less than eleven species, 

 one of which, Vortex Lemani, is found at all depths between fifteen 

 and three hundred metres. A cestoid was dredged from a depth 

 of two hundred and fifty-eight metres. The crustaceans are repre- 

 sented by a limited number of species belonging to the amphipods 

 (Gammarus csecus), isopods (Asellus ctecus), cladoceres (Lynceus), 

 ostracods (Cypris, Candona), and copepods (Cyclops, Canthocamp- 

 tus). Other articulates are four or five species, and as many genera, 

 of arachnids (Arctiscus, Hydrachnella, &c.) and the larvae of some 

 tipuliform insects. The limited number of mollusks inhabiting the 

 depth is not a little remarkable ; of the lamellibranchs there is the 

 single genus Pisidium, with about three species, and of the gastero- 

 pods only the genera Limnaea and Valvata. Although the Unioni- 

 dse (Anodon) are very abundant in the littoral fauna, they are com- 

 pletely absent below. One species of Limnsea (L. abyssicola) was 

 found to be sufficiently abundant at a depth of two hundred and 

 fifty metres, a circumstance to which Forel calls attention as indi- 

 cating the readiness with which an air-breathing mollusk can ac- 

 commodate itself to conditions largely at variance with those which 

 are considered necessary to conform to certain structural peculiari- 

 ties. * 



* When brought to what might be considered its proper position, the sur- 

 face of the water, the mollusk almost immediately adapted itself to the new 

 conditions of existence, apparently without undergoing any inconvenience. 



