144 RHYTHM 



the brood-pouch empties itselfthe specific gravity temporarily, 

 at any rate, decreases. 



Here we have a diurnal rhythm probably depending upon 

 the variation of the food supply, and this is probably de- 

 pendent upon such factors as light and temperature. 



The little fresh- water Rhizopod Arcella also shows a certain 

 rhythm. It sinks gradually, and then, feeling the want of 

 oxygen in the depths of the water, secretes from its own 

 protoplasm a bubble of oxygen which, acting like a float, 

 brings the protozoon to the surface again. 



Light and darkness, the alternation of the seasons, produce 

 a rhythm which affects nearly all plants and animals. The 

 evening primrose opens as twilight comes on; the mosquito 

 becomes active at the same time, retiring into its base before 

 the sun rises. 



Most of the animals on the sea-shore are subject to the 

 rhythm of the tides. There is a little green worm, called 

 Convoluta, just visible to the naked eye, which occurs in such 

 numbers that when they are above ground great splashes of 

 green are observed on the sandy beaches of the coast of 

 Normandy and Brittany, near the level reached by high 

 water. As the tide laps up, these animals disappear into the 

 sand. Tv^^ice every twenty-four hours the colonies are sub- 

 merged and live in darkness underground, and tmce every 

 twenty-four hours the sand is uncovered by the sea and the 

 green animals emerge. Their burrowing away seems to be 

 due to their desire to escape the wave-shock, and their 

 returning is due to the fact that they contain many green 

 algae which require the light of the sun for their assimilation. 

 The egg-laying is similarly influenced by the spring tides, 

 the condition most favourable to egg-laying occurring when 

 the moon is full or new. A curious feature of these animals 

 is that if taken away from the sea and kept in an aquarium 

 inland they still, at any rate for a time, sink when the tide 

 is rising at their far-away home, and emerge when the tide 

 is flowing out. This habit is ingrained in them. 



Periwinkles show also the influence of the tides. When the 

 tide is down they are inert and inactive, but can be made to 

 renew their activity by shaking. If we take these periwinkles 



