226 PROCEEDINGS OF THE AMERICAN ACADEMY. 



(3) To strong light, slugs, on the average, give a strong negative 

 response. 



(4) The degree of response gradually diminishes with the reduction 

 in the strength of the stimulus. 



(5) There is a certain strength of light which appears neither to repel 

 nor attract the slug. This may be said to be a neutral stimulus. 



(6) Reduction of the intensity of the light beyond the neutral point 

 changes the phototaxis from negative to positive. 



(7) The positive response becomes stronger up to a certain degree of 

 intensity. 



(8) It then gradually diminishes with decreasing intensity until abso- 

 lute darkness accompanied by no response is reached. 



(9) Slugs are responsive to light stimuli covering a wide range of 

 intensities. 



(10) The principal organ of response is probably the eye. 



(11) The response is unsymmetrical on the part of the right and left 

 sides of the animal's body. The right side is not as sensitive to stimuli 

 as is the left. On the whole the right side moves through a slightly greater 

 arc in a period of 45 seconds than does the left. 



(12) In the dark, other directive stimuli being eliminated, the slug 

 tends to travel in a spiral of gradually increasing radius, though almost 

 invariably producing one or more loops. Some slugs make right-hand 

 loops, others left-hand ones ; there is a slightly greater tendency to 

 right-hand circling. 



These responses of the slug to touch, gravity, and light-stimuli empha- 

 size the fact that it is an animal's normal environmental conditions which 

 chiefly determine its general response to artificial stimuli. The variations 

 in precision and character of this general response are mainly dependent 

 on certain internal factors, such as the food conditions of the animal, its 

 fear of an enemy, and desire to escape captivity. 



Bibliography. 

 Aderhold, R. 



"88. Beitrag zur Kenntnis rich tender Krafte bei der Bewegung niederer Or- 

 ganismen. Jena. Zeit. Bd. 22, pp. 310-342. 

 Davenport, C. B. 



'97. Experimental Morphology. Part I. pp. xiv. + 280. New York. 

 Davenport. C. B., and Perkins, Helen. 



'97. A Contribution to the Study of Geotaxis in the Higher Animals. Jour. 

 of Physiol. Vol. 22, pp. 99-110. 



