MOVEMENT TOWARD LIGHT IN COELENTERATES 153 



aquarium, and the following morning (April 13) they were 

 all at or near the end facing the light. It therefore took 

 them nearly 48 hours to move a distance of 8 cm. There 

 were now nine specimens; three buds had been set free. 

 The aquarium was turned end for end so that the Hydras 

 were again at the end farthest from the glower. During 

 the following 24 hours several of the specimens traveled 

 the entire length of the aquarium, 8 cm. They therefore 

 moved as far during the 24 hours as they had during the 

 preceding 48 hours. This shows that they became much 

 more active after they had been exposed in a given light 

 intensity 48 hours than they were at first. There was at 

 no time any indication of orientation. The specimens 

 appeared to face in all directions equally, but they appeared 

 to move quite directly toward the source of light. 



In the following experiment more detailed observations 

 on movement were made. Several specimens were put into 

 the aquarium near the end farthest from the light at 9 a.m., 

 April 16. They became attached almost at once. At 12.30 

 all were removed but the five which had been most active 

 during the preceding hours. The courses taken by these five 

 specimens are recorded in Fig. 24. Two of the specimens, 

 A and B, were attached to the surface film. One remained 

 there during the entire experiment, but the other, B, went 

 to the bottom after moving a short distance. The surface 

 of the water was in darkness, but the Hydras hanging from 

 it extended into the light. The animals moved from place 

 to place by stretching out the body, attaching the tentacles 

 to the substratum, and then pulling up the foot and fasten- 

 ing it again near the end bearing the tentacles, sometimes 

 on the same side it had been and sometimes on the oppo- 

 site side. They progress by what may be called the loop- 

 ing method. From 3 o'clock to the close of the experiment 

 every progressive change in position of the specimens A 

 and B, and most of those of the other three specimens, were 

 recorded. During the periods between the looping move- 

 ments the specimens contracted and expanded from time 



