4 ELIZABETH LOCKWOOD THOMPSON 



When the tank containing recently captured snails was jarred 

 or disturbed in any way, they would instantly expel the air 

 from the lung, retract into their shells, and drop from the surface 

 film upon which they were crawling, to the bottom of the tank. 

 Many of them frequently remained motionless on the bottom 

 for an hour or more before they again sought the surface and 

 refilled their lungs. It became necessary therefore, to " tame " 

 the snails which were to be used in the experiments, to so accustom 

 them to handling that they would remain extended and retain 

 the air in the lung while they were being worked upon. 



About twenty specimens of approximately the same size were 

 selected for the tests. These were placed in two bacteria dishes. 

 In each dish was about a liter of water with water plants and 

 as much lettuce as the snails would eat. The snails were taken 

 in the hand at intervals and moved about under water. Each 

 was held in the hand beneath the water until it emerged from 

 the shell and suspended itself from the surface film. They 

 gradually became accustomed to this handling to such an extent 

 that they could be moved from dish to dish, at the will of the 

 operator, without retracting or ex]3elling the air from the lung. 



These " tamed " snails were then divided into two groups. 

 Each individual was numbered by means of white water-proof 

 paint applied to the shell after it had been thoroughly dried. 

 These groups were worked on alternate days. Each group was 

 starved for a period of twenty-four hours before tests were made. 

 The snails belonging to the group were thus believed to be in 

 approximately the same state of hunger at the time of experi- 

 mentation. After a group had been worked on it was allowed 

 to feed for twenty-four hours and was again starved, before it 

 was used. In this way an attempt was made to obtain physio- 

 logical uniformity among the snails directly under observation. 

 The physical conditions were also kept as uniform as possible. 

 The whole series of daily experiments was carried on at approxi- 

 mately the same hour. Each snail in turn was placed in a clean 

 dish of fresh, filtered tap water. The water was of the same depth 

 and at the same (room) temperature as that in which the snails 

 had been living and the lighting was at all times uniform with 

 that to which they were accustomed. Each individual was 

 put through a series of ten tests per day with one exception 

 (check series, Table I, p. 6) and a separate record made of each. 



