THE GEOTROPISM OF FRESHWATER SNAILS. 97 



according to Cooke (i, p. 196), Limncea, Planorbis and Physa have 

 statocysts, which are situated near the pedal ganglion, and are 

 probably connected with the cerebral. The statocyst also con- 

 tains statoliths. The number of the statoliths "varies in dif- 

 ferent genera and species." There are a hundred in Limncea 

 stagnalis on which the writer has experimented, but about fifty 

 in Planorbis contratus and Physa fontinalis. Therefore, Planorbis 

 trivolvis and Physa gyrina Say very probably have statoliths. 

 These statocysts with statoliths maybe the organs for geotropicori- 

 entation. At any rate, the most probable factors of geotropic ori- 

 entation, positive or negative, seem to be internal, that is, physi- 

 ological, and not external. 



BIBLIOGRAPHY. 



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After writing this paper the author found three important articles by W. Baun- 

 acke, but was not able to take these into consideration in this present paper. 



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