208 PROCEEDINGS OF THE AMERICAN ACADEMY. 



of response vary correlatively with the intensity ? (3) Within what 

 limits of intensity is the animal responsive ? (4) Does the kind of 

 response vary at different intensities ? (5) Is there a difference in the 

 sensitiveness to light of the two sides of the animal's body ? (6) In what 

 part, or parts, of the animal's body does the sensitiveness reside? (7) 

 How does the animal move when in the dark and deprived of all stimu- 

 lating influences ? These various problems came up gradually as the 

 work progressed and were considered in turn. Other interesting studies 

 have suggested themselves in the course of the investigation, but there 

 has not been time to go much beyond a consideration of the questions 

 above proposed. The experiments performed were all phototactic ; that 

 is, they were studies of the response of the slug to the direct rays of 

 light. 



Methods. — The methods used were simple. For light, the standard 

 candle and the ordinary small Christmas candle, of a one fourth candle 

 power, were employed. The candle was placed in a box 50 cm. (20 

 inches) high and having a bottom 12.5 cm. (5 inches) wide and 20 cm. 

 (8 inches) long. It could be raised or lowered to any desired position by 

 means of an adjustable stage inside the box. A circular opening in the 

 middle of one of the broad sides of the box 2 cm. (£ inches) in diameter 

 permitted the light to pass out. This opening was covered by a piece of 

 oiled paper, so as to give a well-defined uniform source of light. During 

 the experiment the box was closed by a lid. The intensity of the light 

 was varied by altering the distance between the box and the animal. 

 Additional thicknesses of paraffined paper were also employed when it 

 was desired to greatly diminish the intensity of the light. The animal 

 was put on a circular glass plate which rested horizontally on a support, 

 and the box was raised so that the centre of the light opening was in the 

 same horizontal plane as the body of the animal. The movement of the 

 slug from its original position was measured in degrees in the following 

 manner. A circle of the same size as the glass plate was described on a 

 sheet of thin paper and divided by radii into 72 sectors of 5° each. This 

 sheet was pasted to the under side of a second circular glass plate (of the 

 same size as the first), on which also a heavy base line was drawn, corre- 

 sponding with a diameter of the circle. This second plate was so placed that 

 the centre of the source of light was on a line perpendicular to the base 

 line at its middle point. The slug was put on the first glass plate, which 

 could be rotated so as to bring the animal into any desired position with 

 reference to the base line. The experiments were carried on in a dark 

 room provided at one end with a hinged window which could be easily 



