AN ANALYSIS OF THE LEARNING PROCESS IN THE SNAIL 41 



arms A and B were 8.5 mm. by 4.9 cm., the base of the U, C 

 was 8.5 mm. by 4.25 cm. This rested on an upright stem D, 

 8.5 mm. by 3.6 cm., the base of which was cemented into a block 

 of lead E, 2.1 cm. by 1.6 cm. by 6 mm. The celluloid part of 

 the apparatus was carefully squared to the lead base E, before 

 the cement was allowed to harden. Physa in crawling up 

 anything but a broad surface has a tendency to crawl in a spiral 

 direction, as may be readily demonstrated by allowing it to 

 crawl up a glass rod placed upright in the aquarium. Should 

 the animal ascend the stem of the labyrinth in a spiral path 

 it would have the long or correct arm sometimes on its right 

 and sometimes on its left. In order that the snail might have 

 a choice of right and left paths on the labyrinth it was necessary 

 to keep it confined to one side of the apparatus. In an attempt 

 to accomplish this, the celluloid path was backed with galvanized, 

 0.5 inch-mesh wire netting, so cut that the entire path was 

 bordered by a " picket fence " of wire, the pickets extending 

 outward in the same plane as the path rather than at right 

 angles to it. By cutting out every other " picket " the re- 

 maining wires were just far enough apart to make it mechanically 

 impossible for the snail to pull its shell between them. This 

 device proved an effective but not an absolute barrier, inas- 

 much as the animal occasionally managed to crawl over the end 

 of a " picket," but the number of crossings became so reduced 

 by its use, that this source of error was practically eliminated. 

 The wire was cemented to the path by means of celluloid dis- 

 solved in acetone. The wires themselves and the lead base 

 were also coated with this material, so that all parts of the 

 completed apparatus with which the snail could come in contact 

 were either made of celluloid or coated with it. This apparatus 

 when in use was placed in the exact center of the experimental 

 tank, thus bringing it directly under the centered Hght, so that 

 the rays falling on each branch of the path A and B (fig. 14, 

 Plate IV) were of equal intensity and thus the possibility of pho- 

 totropism directing the course of the snail was avoided. Tap 

 water at room temperature was used in the experimental tank. 

 This temperature varied within the limits of a degree or two 

 but it will appear later in the records of the experiments that 

 the variations in temperature were the cause of no irregularities 

 in the results. 



