298 



ANIMALS IN INLAND WATERS 



rotifers and of a few cladocerans are found; the property of adhesion 

 to the surface of the water, which these small bodies possess, gives 

 them the advantage of being driven by the wind into the littoral region 

 where they find a higher temperature which promotes development. 22 

 The glochidian larvae of the lake and river mussels (Anodonta, Unio) 

 and the lake-floor larvae of the midge (Tendipedidae) are occasionally 



Fig. 79 Fig. 80 



Fig. 79. — Holopedium gibberum. X 14. After Fric and Vavra. 

 Fig. 80. — M astigocerca setifera in its gelatinous case. X 130. After Lauterborn. 



mixed with the plankton by means of the wave action of shallow 

 waters. 



The character of the fresh-water plankton is further considerably 

 influenced by the fact that the size of its animals is much less than in 

 the ocean. Such large forms as siphonophores, ctenophores, heteropods, 

 pteropods, and arrowworms are not known in fresh waters. The forms 

 just mentioned are mostly jelly-like; their bodies contain an extraor- 

 dinary amount of water, which considerably increases their size. The 

 absorption of large amounts of fresh water into the body of an animal 



