196 THE WONDER OF LIFE 



becomes appreciable, and the animal arranges itself so 

 as to secure chemical equilibrium, which doubtless spells 

 comfort. In short, the addition of the acid has quantita- 

 tively increased the chemical effects of the hght, so that 

 it ceases to be negligible. 



Rhythmic Movements. — The story of the Uttle green 

 Planarian worm, Convoluta, illustrates the combined effect 

 of periodic external changes on the one hand and the 

 internal rhythms of the body on the other. On the flat, 

 sandy beach of some parts of Brittany, the small worms 

 come up in crowds when the tide is out, and form green 

 splashes on the surface. When the tide comes in they 

 retire into the shelter of the sand. Their movements are 

 synchronous with those of the tide. But Bohn has shown 

 that the Convolutas in a quiet aquarium or in a glass tube 

 behave in the same way ; they ascend when the tide goes 

 down, they descend when the tide comes in — though they 

 are, of course, quite away from all influence of the tides. 

 What is still more remarkable is that they keep time with the 

 irregularities of the tide. Bohn beheves that the alteration 

 of the geotropism from plus to minus may be associated 

 with the alternation of relative desiccation and relative 

 hydration during the periods of low tide and high tide. 

 In the case of hermit-crabs, however, Anna Drzewina 

 observed that there was a rhythmic change from going 

 towards the light at high tide and from the light at low tide, 

 and that this occurred in an aquarium where they were 

 covered with water all the time. It is very interesting 

 to find that a rhythm established ia relation to external 

 periodicities persists in an aquarium where there is uni- 

 formity of conditions. A beautiful corroboration of the 

 original dependence of the rhythm on the tides is found in the 



