100 



PROBLEMS OF LAKE BIOLOGY 



TABLE II 



Mean Vertical Distribution of Tardigrada. 



ROTATORIA 



Numerically, the rotifers are the domi- 

 nant metazoans. Although the Bdelloidea 

 are found in nearly all aquatic environ- 

 ments, comparatively few of these forms are 

 found in the sand. The ploimate genera 



are far more important. About 200 spe- 

 cies have been reported from beaches. Ac- 

 cording to Wiszniewski these may be di- 

 vided into three categories: (1) psammo- 

 biotic rotifers, almost always found in the 

 sand, elsewhere only as stragglers; (2) 

 psammophile rotifers, sandloving, which are 

 also found in littoral regions among vegeta- 

 tion; and (3) psammoxene rotifers, alien 

 plankton species usually carried into the 

 sand by the waves as adults or eggs which 

 hatch out but whose individuals soon die. 



The majority of the species found in the 

 sand are psammoxene forms, but their num- 

 bers are so small in comparison with the 

 other two groups that they are not included 

 in the results jjresented in this paper. 

 Some of the more common psammoxene gen- 

 era and species are : Synchaeta, Polyarthra, 

 Keratella, NotJiolca, Euchlanis dilatata, 



TABLE III 



Distribution of Eotatoria in Three Series of 10 cc Sand Samples. Specimens Per Sample. 



