156 MODIFICATIONS OF BEHAVIOR 



Convoluta roscoffensis, a small green turbellarian worm 

 which lives in sandy beeches overflowed by the tide, makes 

 periodical depth migrations in the sand. At low tide the 

 worms come to the surface where they form a green coat 

 upon the shore. When the tide comes in the worms de- 

 scend and thus avoid the shock of the waves. Bohn found 

 that the same periodic migrations occurred in specimens 

 which were taken from the shore and kept in an aquarium. 

 If the worms were placed in a tube of sand a green ring could 

 be seen to rise and descend synchronously with the ebb 

 and rise of the tide. These rhythms persisted for several 

 days after the worms were removed from the beach. Keeble 

 has compared the vertical movements of Convolutas kept 

 in the laboratory with the movements of specimens on the 

 beach, and states that "for eight successive tides the 

 animals in the laboratory maintain their rhythm, synchron- 

 ous with the ebb and flow of the waters over the roscoffensis 

 zone: then, though the rhythmic movement up and down 

 may continue, its temporal periodicity loses precision, and, 

 finally, the rhythm is worn down." 



A parallel phenomenon was discovered by .Bohn in the 

 diatom Pleurosigma. When the sea retired these diatoms 

 were observed to form a brown scum over the sand. When 

 the tide came in the diatoms descended. Placed in an 

 aquarium they performed regular migrations for several 

 days in accordance with the tidal rhythms. This periodic- 

 ity failed to manifest itself in darkness and, therefore, 

 according to Bohn, depends upon variations in phototaxis 

 instead of the response to gravity. Bohn has reported 

 that a curious tidal rhythm occurs in the small gastropod, 

 Littorina rudis, which lives upon the rocks where it is only 

 occasionally wet by the waves. At low tide these molluscs 

 withdraw into their shells and remain inactive. When 



