574 A. R. MOORE 



The material used in the following experiments was Daphnia 

 pulex, kindly identified for me by Professor Kofoid of the Depart- 

 ment of Zoology. In all of the experiments the animals were 

 put into a finger bowl with 20 cc. of tap water, and set at a dis- 

 tance of 1 foot from the mercury arc. Under such conditions 

 all of the animals form a complete negative collection in twenty 

 to thirty seconds. The Daphnia never move toward the light 

 but always away even in the first second of exposure. 



If the finger bowl containing the Daphnia be covered by a 

 plate of ordinary window-glass, thus cutting off the light of 

 wave lengt-h shorter than 3341 A. u., the results are not so uniform. 

 In some cases there is an irregular wandering away from the 

 fight, but never a negative collection. Usually even after one 

 and one-half hours' continuous exposure, the Daphnia are still 

 scattered about the dish. Sometimes they even form a positive 

 collection. 



That the action of the fight of wave length shorter than 3341* 

 A. u. is specific for negative phototropism of Daphnia may be 

 shown by a simple experiment. First cause the animals to form 

 a negative collection in an open dish, then cover with a glass 

 plate without otherwise interrupting the light. The negative 

 collection then begins to break up and the animals tend to scatter 

 evenly about the dish. In some cases the greater part of them 

 wander about on the side away from the light, in other cases 

 they form a weak positive collection. 



From such experiments we must conclude (1) that light of 

 wave length 3341 A. u. or longer is not effective in causing negative 

 phototropism of Daphnia pulex, while fight of shorter wave 

 length causes them to become negative instantly; (2) that the 

 effect of ultra-violet light of wave length shorter than 3341 A. u. 

 is apparent only during the time of action of such light, and 

 ceases to exert an effect almost instantly upon this light being 

 cut off. 



Furthermore, I found that the negative effect of ultra-violet 

 fight disappears when acids, especially CO2, are ad'ded in small 

 quantities to the water containing the Daphnia while the latter 

 are undergoing exposure to the ultra-violet light. If the negative 



