42 ELIZABETH LOCKWOOD THOMPSON 



In order to conduct the experiments under controlled lighting 

 conditions and thus avoid any possible phototropic influence it 

 was deemed advisable to create special conditions governing 

 the light and keep them uniforrri throughout the experiments 

 (fig. 15, Plate V). An experimental tank consisting of a battery 

 jar 6 inches high and 4 inches in diameter was painted black 

 on the outside and a level floor was obtained by means of a 

 false bottom made of weighted paraffin. The center of the bot- 

 tom was marked in the paraffin. This tank, during experimenta- 

 tion, was placed in the center of a " house " constructed from 

 gray beaver-board fastened together with strips of gummed 

 paper and painted dead black on the inside. The walls were 

 26 cm. by vS8 cm. with a strip 19 cm. by 5 cm. cut from the lower 

 edge on three sides for ventilation. For this purpose also small 

 holes uniformly spaced were punched 2 cm. from the upper edge 

 of the four sides. The fourth wall, the one next the operator, 

 had a horizontal slit 2 cm. wide cut in it at 10 cm. from the 

 top. The part of this wall below the slit was made removable. 

 With the lower part of the fourth wall removed the operator 

 could prepare experiments without taking the tank from the 

 " house." When the movable part of the fourth wall was re- 

 placed there remained a slit 2 cm. in width, through which 

 observations could be made. The top of the house was ground 

 glass above which at a distance of 4 cm. was an 8 candle power 

 electric light. This was centered over the house and held in a 

 stationary position l)y means of a wooden frame. Two thick- 

 nesses of ordinary medium weight white paper were placed on 

 top of the ground glass to further weaken and diffuse the light. 

 The room in which the apparatus was located was darkened 

 during experimentation. This apparatus was used without 

 change throughout the experiments. The labyrinth itself was 

 varied to conform with the requirements of each set of experi- 

 ments. All snails used were tamed, individually numbered and 

 kept under the same conditions as in the experiments with 

 simultaneous stimuli . 



3. Experiments on Preference 



Two different kinds of preference tests were carried out. The 

 first was to find whether a tendency to turn to the right or left 

 existed. The asymmetry of the animal suggested the possibility. 



