COMMUNITIES IN STANDING WATERS 333 



(see p. 255). The fresh-water polyp Hydra and the sponge Spongilla 

 are found in rather deep water, in the Teufelsee (in the Bohemian 

 Forest) down to 25 m. ? in Lake Madue in Pomerania down to 40 m. 

 Of the flatworms a number of Rhabdocoela are generally characteristic 

 of the deeper parts of lakes, e.g., Plagipstomum lemani (as in Lake 

 Constance and other Alpine border lakes, summit lakes of the higher 

 mountains). Where triclad flatworms occur in the deep regions, as, for 

 instance, the ubiquitous Dendrocoelum lacteum and Planaria alpina, 

 which is adapted to low temperature, they appear in reduced sizes. 

 Slimeworms {Tubifex and its relatives) are common, and may be the 

 prevailing forms; 5750 individuals of various species have been re- 

 corded from 10 liters of mud from 32 m. A few Bryozoa also grow in 

 deep water, e.g., the widely distributed Fredericella sultana. Crustacea 

 are common; ostracods, e.g., are represented by a large number of 

 genera; of the copepods, there are species of Canthocamptus especially, 

 and several species of Cyclops. There are a number of interesting cold- 

 water isopods and amphipods in addition to ubiquitous species in the 

 border lakes of the Alps; the occurrence of blind forms such as 

 Asellus cavaticus, and the cave amphipod, Niphargus puteanus, espe- 

 cially in the deep pedon of Alpine border lakes, is important. Next to 

 the slimeworms (Tubificidae), according to numbers, the larvae of the 

 biting gnats (Tendipedidae) form the largest part of the bottom fauna; 

 a total of 8000 individuals of both forms may be found on 1 sq. m. 



Only those mollusks can live in deep water which are not dependent 

 upon the atmosphere for their supply of oxygen, i.e., mussels and gill- 

 breathing snails. In Lake Ratzeburg, for instance, only Limnaea ovata 

 and Planorbis albus of the pulmonates go deeper than 3 m. in contrast 

 with 7 gill-breathing snails, among which Bythinia tentaculata goes 

 down to 12 m., Valvata piscinalis var. antiqua to 18 m. 22 In the Alpine 

 border lakes, Valvata goes down even to 64 m., and Bythinia occurs 

 as deep as 60 m. Pisidium, on the other hand, lives in depths greater 

 than 200 m. and exhibits a great variability which has brought about 

 a separation into a large number of species. All mollusks of the deep 

 pedon of lakes, however, are stunted forms, even the plentiful pisidia. 



In a number of alpine border lakes, whose water contains a large 

 supply of oxygen in the deeper parts, some pulmonates of the genus 

 Limnaea have become adapted to life in the depths, as for instance in 

 Lake Geneva. 23 They do not rise to the surface to breathe as their 

 littoral relatives do, but their lung space is filled with water from 

 which they take the necessary oxygen. In Lake Geneva three forms of 

 such Limnaea occur: L. profunda, L. forelia, and L. abyssicola. It has 

 been proved that the first two are varieties of L. ovata, and L. abyssi- 



