152 LIGHT AND THE BEHAVIOR OF ORGANISMS 



nated x. At 9 a.m. the next morning two had taken posi- 

 tions represented by 0. During the next 24 hours three 

 moved to positions marked y; all were well contracted and 

 motionless. They were now exposed to diffuse sunlight 

 without being moved or jarred. They began to expand 

 almost immediately and soon began to move about. The 

 paths taken are indicated in Fig. 23. After 11 a.m. they 

 did not again change their positions until the close of the 

 experiment 24 hours later. 



This shows that darkness inhibits movement in Hydra 

 viridis ; that they become exceptionally active when exposed 

 to light after having been in darkness for some time, and 

 that they apparently become acclimatized readily, since 

 they come to rest in the same intensity after having been 

 exposed a few hours. The inhibition of movement by 

 darkness may however not be due to absence of direct 

 stimulation by light, but to the effect of darkness on photo- 

 synthesis. 



c. Orientation and locomotion. — In the study of the 

 movements of Hydras, they were put into water from the 

 culture jar, i cm. deep, in a rectangular aquarium 2X5X8 

 cm. made by cementing slides together. The aquarium 

 was exposed in the dark room to light from a 50-candle- 

 power Nernst glower situated 2 meters from the end of it. 

 The glower w^as so arranged that the rays w^ere parallel with 

 the bottom and sides of the aquarium. The end of the 

 aquarium was covered with an opaque screen containing 

 an opening such that the sides and surface of the water 

 were in darkness, so that reflection from various surfaces 

 might be reduced as much as possible. All the light except 

 that in the beam w^hich fell on the end of the aquarium was 

 absorbed by screens. 



At 10 A.M., April II, six green Hydras were put into the 

 aquarium near the end opposite the glower. They became 

 attached very soon and stretched out in various directions, 

 some toward the source of light others away from it. 

 Twenty-four hours later they were near the middle of the 



