90 



Herbert Eugene Walter 



to the force of gravity. The moment the worm begins to ghde up 

 the sides of an aquarium, however, the relation of the axes of its 

 body to the center of the earth changes and directive geotaxis 

 resuks. 



No systematic attempt was made to analyze compound stimuli, 

 for such a study would overstep the boundaries set for the present 

 inquiry. Nevertheless certain facts bearing on this point were 

 incidentally noted and these may properly be detailed here. 



Geotaxis. In a majority of cases, Planaria gonocephala seems, 

 after several hours of exposure to the dark, to be positively geo- 

 tactic, and after several hours of exposure to light, negatively geo- 

 tactic, as shown in the following series of observations. 



A cylindrical aquarium jar 20 cm. in diameter and 40 cm. high 

 was placed before a moderately lighted window and stocked with 

 a freshly obtained supply of about 300 worms. No stones, sand, 

 or water-weeds, which would afford places of concealment, were 

 introduced. At intervals during the next 10 days the distribution 

 af the worms was recorded and these records are brought together 

 in Table XVIII. 



TABLE XVin 



The distribution of about ^00 planarians {Planaria gonocephala) in an aquarium, as observed forenoons 

 and afternoons during 10 days. The figures express percentages 



Place in the Aquarium 



Time of day 



April 26. 

 April 27 . 

 April 28 . 

 May I . . . 

 May 2. .. 

 May 3 . . , 

 May 4. . . 

 May 5... 



Top 



Sides 



61 



74 

 7i 

 63 



43 

 50 



p. m. 



p. m. 



Bottom 



56 



Average. 



60.5 



30.6 



13.2 



27.2 



56.2 



The forenoon census was taken about 8 o'clock, when the worms 

 were re-arranging themselves after the darkness of the night, while 

 the afternoon records were made about 4 o'clock, when the worms 



