EXPERIMENTS ON WORMS 79 



differences. A reduction in the amount of Ca ions 

 in the sea-water allowed the centre of a Hydromedusa 

 to beat spontaneously. The case of marine Planarians 

 may be similar, and further experiments may yield the 

 result that with a change in the constitution of the 



O 



sea-water the posterior half of a Thysanozoon will 

 be able to show spontaneous locomotion. 



The behaviour of Planaria torva toward lio-ht j s o f 



o 



special interest. The animal is especially sensitive to 

 changes in the intensity of light. If brought from 

 the dark into the light suddenly, it begins to move. 

 At first the direction of the movements seems to be 

 influenced by the light, for the animals move away 

 from the source of light as if they were negatively 

 heliotropic. However, they do not collect at the point 

 farthest from the source, as do negatively heliotropic 

 animals, but they scatter in all directions and come to 

 rest at last in a place where the light is comparatively 

 weakest. From this it would seem that an increase 

 in the intensity of light causes them to move, while 

 a decrease in the intensity of light causes them to 

 rest. This would account for the fact that we find 

 them by day always under stones or in relatively dark 

 places. I suspect that they begin to move about in 

 the night, and that they come to rest when day 

 returns. I have repeatedly tried the experiment of 

 covering in the morning one-half of the dish with 

 black paper. During the day no change takes place, 

 but the next morning all the animals are found under 

 the covered portion of the dish. The only possible 



