12 ELIZABETH LOCKWOOD THOMPSON 



made to use a " conditioned " or double stimulus by the simul- 

 taneous application of pressure and food. 



A. Special apparatus and methods. — This required the con- 

 struction of a special apparatus which would make possible the 

 application of a uniform pressure at a fixed distance from the 

 food stimulus when the latter was applied to the mouth of the 

 snail. Out of the several appliances tried, the one here pictured 



(fig. 1, Plate I) gave the best results. 



(A) was a wire nail which served as a handle. A small strip of 

 sheet metal (B) 5 mm. in width, and bent in the form of a s: uare 

 cornered U, was soldered to the end of A. B contained two 

 smooth-edged holes, one directly above the other and both as 

 large as would permit the free movement through them of a 

 smooth steel rod (C) which was made from a No. 5 sewing needle. 

 C was kept from dropping through the holes, by a fine wire (D) 

 which was passed through the eye of the needle. To the lower 

 end of the rod C was soldered a U-shaped piece made of No. 30 

 platinum wire. One limb of the U (E) 16 mm. in length, ended 

 in a small hook. The other limb (F) 18mm. in length, ended 

 in a three-pronged, dull-pointed platinum fork 3 mm. wide and 

 turned at right angles to the plane of the U-shaped piece. A 

 was held in the hand of the operator and the fork on the lower 

 end of F was allowed to rest on the exposed ventral surface of 

 the snail. At the same time food (a small piece of lettuce 

 rolled between the fingers to form an inverted cone) suspended 

 from the hook at the lower end of E was applied to the mouth 

 of the snail. The free movement of the lower part of the appara- 

 tus permitted by the smooth-edged holes in B, made it possible 

 for the operator to manipulate the apparatus in such a way that 

 it remained practically stationary upon the surface of the snail 

 during the period of stimulation in spite of the constant motion 

 of the animal. 



B. Ex perhncuts wJiicJi show lack of response of untrained snails 

 to pressure alone. — Before using the apparatus in experiments 

 with the conditioned stimulus, a series of twenty trials was made 

 on each of six snails in order to learn whether the application 

 to the foot of the pressure fork alone induced response. The 

 snails were starved for twenty-four hours previous to the experi- 

 ments and each was tested in filtered tap water. The above 



