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PROCEEDINGS OF THE AMERICAN ACADEMY. 



we not make the desire for darkness cooperate with the inherent positive 

 geotactic tendency to such an extent that the two together will over- 

 come all mechanical difficulties and cause the animal to move downward ? 

 The following table (VIII.) answers this question in the affirmative. 



TABLE VI 11. 



Geotaxis of Slug on Glass Plate at an Angle of 45° influenced (1) by 

 Gravity alone, and (2) by Gravity and Strong Light. 



These experiments were carried on in the evening. The animal was 

 first tested on a glass plate at an angle of 45° in the dark, in the ordinary 

 way. Then it was placed on a horizontal glass plate and strong lamp 

 light thrown directly upon it for a few seconds. In most cases it imme- 

 diately gave a negative response to the light. When definitely oriented, 

 the plate was again placed in the box at an angle of 45° and the box 

 covered with a black cloth. Two or three geotactic observations were 

 then taken, and the animal again exposed to strong light. The expo- 

 sure to light was repeated about three times in the course of ten observa- 

 tions. The table shows that the influence of light has been to change a 

 condition of strong negative geotaxis to one of indifference. The only 

 exception is No. 6, which seemed little affected by the light. I hope to 

 make a fuller study of the combined action of light and gravity later. 



It has been said that all slugs have an innate tendency to move toward 

 the earth. Now, this tendency is probably due to the environment and 

 habits of the animal. The slug, we know, is nocturnal in its habits. In 

 the nighttime, it is actively moving about in search of food. In the day- 

 time, it is inactive and seeks concealment, which is of course accora- 



