LAKE FAUNAS. 131 



Beep 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. 63 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 amceboe, 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 worms are indicated nematoids, cestoids, 

 and turbellarians and two of annelids proper, the hirudines and 

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

 naria, Mesostornum, 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 caecus), isopods (Asellus csecus), 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 

 tipulif orm insects. The limited number of rnollusks 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 Limnsea 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, 



