330 



ANIMALS IN INLAND WATERS 



into greatly branched stems often up to 30 cm. high (Fig. 96). 

 Numerous flatworms hide under the stones {Dendrocoelum, Planaria, 

 Polycelis) ; flatly compressed leeches (Glossiphonia, Herpobdella) 

 attach themselves by means of their suckers. May-fly nymphs are 

 also much compressed ; many caddis-fly larvae weigh down their cases 

 with heavier stones. In lakes, the snail Goniobasis, with its wide, 

 sucker-like foot, is widely distributed; also occasionally the shield- 

 shaped snail, Ancylus fiuviatilis (Fig. 87). The mussels of the plantless 



Fig. 96. — Euspongilla laciistris from quiet water and from moving water, attached 



to a floating thread. After Weltner. 



shores, Anodonta (Fig. be) and Unio alike, are small and convex, with 

 relatively thick shells often considerably corroded at the umbo, in 

 contrast with the large, thin-shelled, beautifully colored forms found 

 in the quiet waters of bays. The Anodonta of Lake Constance, for 

 instance, become 9 cm. long; those in the pond of the Waldsee in upper 

 Wurttemberg, up to 19.6 cm. 19 The snails of the genus Limnaea change 

 form in the moving waters of the lake shore because of the constant 

 pull to which their shells are exposed, which in turn produces a reac- 

 tion on the softer parts of the body (Fig. 97). Steep shores are poor 

 in life. 



The deep pedon. — The rest of the lake bottom makes up the deep 

 pedonic region. It is not sharply marked off by biotic characteristics 

 from the communities of the littoral region. Those parts lying nearest 



