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 
