8 STUDIES IN GENERAL PHYSIOLOGY 



convinced that the glow-worm living in the woods could see. 

 He made a glass window in a tree in which such worms 

 lived and noticed that the animals gave a start upon the 

 approach of a burning candle. 



Trembley made far better experiments. 1 He found that 

 "water fleas" can be driven around in a circle by a moving 

 candle : 



By the light of a wax taper I observed polyps to which during 

 the day I had given many water fleas; in the evening there were 

 left in the glass some which the polyps had not consumed. I 

 noticed that most of them had collected on the side toward the 

 candle. I changed the position of the taper, and they followed it. 

 As I had moved its position repeatedly, and each time had seen 

 that the water fleas followed it, I moved the taper slowly around 

 the glass without stopping. They followed, and thus made several 

 trips around it. I have had the opportunity of repeating this ex- 

 periment several times. 



Trembley's observations on the effect of light on Hydra 

 were made with great care. After he had repeatedly observed 

 that the polyps moved to the "brightest" side of the glass, 

 he placed "a glass containing many green polyps in a case 

 which had an opening on one side about opposite the middle of 

 the glass." He reports as follows concerning their behavior: 



When I placed the glass so that the opening in the case was 

 turned to the light, the polyps always migrated toward that side 

 of the glass which was opposite this opening, in such a way that 

 together they made the figure of a gable. I often turned the 

 glass around, and after several days I observed the polyps again at 

 the opening arranged as before (in the form of a gable). To vary 

 the experiment still further, I fixed the dark case so that the open- 

 ing was at times straight, at other times inverted, and again the 

 polyps arranged themselves together. 



After he had discovered that polyps which had been cut in 

 two could "move, eat, and multiply," he tried to see "whether 



1 TREMBLEY, Abhandlungen zur Geschichte einer Polypenart, transl. by GOTZE 

 (Quedlinburg, 1791). 



