228 MARY O. MCGINNIS 



tions. It was desired (1) to learn whether, under the influence of 

 lights of different intensities, the response remained positive, and 

 (2) to find whether exposure to darkness had any effect on the 

 character of the response. 



For the first experiment, a dark box, 4 feet wide, 3 feet high, 

 and 2 feet deep was constructed, and a 12 c.p.- incandescent lamp 

 was suspended from the top of the box. A rectangular black pan 

 was placed on the floor of the box, about 2 feet from the lamp. 

 This pan was provided with a black cardboard cover having a 

 central opening, 2 inches square, directly under the light. The 

 pan was filled with water to a depth of about 2 inches, and eight 

 specimens of Branchipus were dropped in at one corner. When 

 the cover was put on the pan, all the animals came very quickly 

 into the light space. 



When once trapped here, they swam about at random until 

 they came in contact with the shadow, to which they immedi- 

 ately responded negatively, and so remained in the lighted area. 

 The shadow was as effective a barrier as a stone wall would have 

 been. The experiment was repeated many times, always with the 

 same result. When the opening in the center was closed, and an 

 opening made in the corner of the cover, the light remaining 

 above the center, the animals went to the lighted comer. 



The specimens were next placed in a rectangular glass aquarium 

 in the dark box. A 12 c.p. incandescent lamp, which was movable, 

 was placed at the end of the aquarium. The animals inunediately 

 swarmed into the end next the light. When the light was moved 

 to the opposite end they turned and again swam toward it. As 

 often as the light was changed, the animals changed their course 

 accordingly. With the specimens in a tall glass cylinder, and the 

 light above or below, the same positive reactions were obtained. 



These preliminary experiments show that Branchipus is posi- 

 tively phototropic to a 12 c.p. light. 



* The candle powers given in this paper are from actual photometric measure- 

 ments of the electric lights used; but since the strength of the current varied con- 

 siderably from time to time the figures are at least only approximate. Generally 

 speaking c.p. as used in the text corresponds to candle meters but the conditions 

 of the experiment do not permit an exact statement in regard to light values. 



