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.’ 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 
1TREMBLEY, Abhandlungen zur Geschichte einer Polypenart, transl. by GOTZE 
(Quedlinburg, 1791). 
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