All natural rhythms can develop an acquired periodicity in organisms. We know the suggestive observations of Bohn on the marine worms of the genus Convoluta which are found in masses on certain sandy shores on our coast. These minute planarians are colored green by the unicellular algi which live symbioti-cally in their tissues. When the tide goes out, they emerge on the surface of the strand and make on it a green fringe. When the tide comes back, they dig into the sand and completely disappear. But, says Bohn, these alternating movements, synchronous with those of the tide, persist in the aquarium when these animals feel slight movements: in a glass tube containing moist sand and some of these Convolutas, the green ring rises and descends, occupying the high position at the moment of low tide and the low position at the moment of high tide. And-a still more curious thing,-the irregularities of the sea are followed by the Convolutas in the aquarium. The movements are slower in still water and more rapid in running water, and this continues for some days after they have been put into the aquarium.
Similar oscillating movements have been noticed by Bohn and Fauvel with Pleurosigma cestu-ari, which is a littoral diatomaceous alga. According to Bohn, the gasteropods of the genus Littorina have a double series of oscillations which draw them toward shady surfaces or away from them: daily oscillations of thirteen hours, which correspond to the flux and to the reflux, and oscillations of fifteen hours, according to the running water and the still water. These two kinds of rhythms persist in the aquarium and consequently have been acquired.