82 J. M. D. OLMSTED 



found between the plates, and always under the black half. Of 

 the latter, one-fifth were on the under side of the upper plate 

 (ventral surface up), and four-fifths on the lower plate (dorsal 

 surface up). 



Since, as the preceding experiment showed, the influence of 

 light was so marked, it was decided to eliminate this factor 

 by conducting all the subsequent experiments in a light-proof 

 box. To eliminate thigmotropism and to provide a contin- 

 uous surface which should have all possible relations to gravity, 

 spherical balloon flasks were used. These flasks were 13 cm. in 

 diameter. They had a short neck 4 cm. long. Three regions 

 of equal area were marked off on the surface of each flask, a ring 

 about its equator and a segment at either pole. These three 

 regions were designated top, middle, and bottom. The flasks 

 were so marked that in one the neck came in the top, in another 

 in the middle, etc. In the experiments very few worms lodged 

 in the neck and the per cent of such was practically the same 

 whether it occurred in flasks with the neck in the top area, in 

 the middle, or in the bottom. In the tabulation of results 

 worms in the neck are not included. 



Twenty worms were used in each experiment. Readings of 

 their positions were made at 9 A. M., 1. P. M. and 4. 30 P. M. 

 In a few cases readings were taken at intervals of two hours, 

 but even then the animals were at rest. They were made to 

 start moving before being returned to the box, as a means of 

 redistributing them for the beginning of another trial. It was 

 found that the positions of the planarians in the flasks changed 

 greatly during the first few days after being put into the dark. 

 At first the majority were to be found in the bottom of the 

 flasks. A few days later they were equally distributed in the 

 three areas. When they were fed there was a sudden depar- 

 ture from this equal distribution and the majority would be 

 found in the top. They again distributed themselves equally 

 in the three areas three or four days after feeding. Table I 

 gives a summary of results. The numbers are the per cents 

 taken from 10 or more readings. By ' from light ' is meant 

 worms taken from the stock which had been kept in the light. 

 " From dark ' means worms which had been in the dark-box 

 for a week or longer. ' Fed ' worms are those which were 

 fed on liver on the day of the experiment or every other day 



