466 G. W. KIRKALDY, NOTE ON THE PHOTOTROPISM OF DAPHN . 



exposed to the action of light and able to choose between different 

 intensities, is at first positively phototropic — that is to say, it goes 

 towards the zone of greater light ; then — as regards the adults — it 

 gains the zone of less illumination. As to the young individuals, 

 they appear to have tendencies less precise ; nevertheless, the 

 positive reactionis maintained in a very great number of cases. 



Schouteden finds in previous literature confirmations of these 

 facts. Zschokke, Bruyant, and France note that during the day 

 the plankton, which consists, of course, very largely of Ento- 

 mostraca such as Daphnia, etc., retires a certain distance from 

 the surface, to return at night, and the circumstance that the 

 captures made at the surface during the day are mostly young 

 individuals is also confirmed. Tliese authors, however, seem to 

 attribute such variations, at least in great part, to thermic 

 phenomena ; this is denied, as an ultimate factor, by Schouteden. 

 The Belgian zoologist has aj)parently overlooked the researches of 

 Lubbock, summarised by him in 1891,* upon which considerable 

 light is now thrown. 



Lubbock placed a number of Daphnias in a shallow wooden 

 trough, divided by cross-partitions of glass, so that the parts 

 illumined by difi'erent coloured rays of a spectrum could be 

 isolated ; the partitions were then removed so that the Daphnias 

 could circulate freely. After this they were exposed to the light 

 for ten minutes, and then the glass partitions were inserted 

 and the Daphnias in each division counted. Lubbock concludes 

 that, while Daphnias prefer light to darkness, there is a certain 

 maximum of brilliancy beyond which the light becomes incon- 

 veniently bright to them, and that they can distinguish between 

 light of diflerent wave-lengths. He supposes that it would be 

 impossible to prove that they actually perceive colours ; but to 

 suggest that the rays of various wave-lengths produce on their 

 eyes a different impression from that of colour, would be to propose 

 an entirely novel hypothesis. He thinks that he has shown that 

 they do distinguish between rays of different wave-lengths, and 

 prefer those which to our eyes appear green and yellow. 



Schouteden promises further experiments, which will be 

 awaited with interest. 



* " On the Senses, Instincts, and Intelligence of Animals, with special 

 reference to Insects." Edition 3. [Internatlcnal Science Series, Vol. LXV., 

 pp. 211-31). These experiments were made on BajfhJiia, confirming 

 previous observations upon ants. 



Jouni. Qv.ekitt MiO-oscopical Club, Ser. 2, Vol. VIIL, No. 52, April 1903. 



