100 JEAN DAWSON 



considered: (i) Light, (2) temperature, (3) mechanical dis- 

 turbance of the water, (4) a greater oxygen content of the 

 indropping water. These various possibiHties were tested as 

 follows: The water was allowed to drop into different parts 

 of the tank by changing the hose from place to place. In every 

 case in the course of a few hours the snails would collect as be- 

 fore ; the time required depending upon the general activity 

 of the snail. Light as a possible factor was thus eliminated. 

 No difference in temperature could be detected in different 

 parts of the tank. The fact that the water dripped into the 

 tank slowly from small pipes exposed to room temperature, 

 accounts for the uniformity of the temperature. There are two 

 possible factors which may account for the gathering of the 

 snails, disturbance of the water and the relative amount of 

 oxygen. 



After the water in the tank had become again equally aerated 

 throughout, air w as forced through it in different parts of the tank 

 in succession, so as to produce areas of greater oxygen content. 

 After a time the snails collected in the better aerated waters. 

 There were about two hun,dred and fifty snails in the tank and 

 at one time there were fifty gathered on the pipe where the 

 water, agitated by bubbling air would break over them. Other 

 like tubes dipping into the water but having no air or water 

 leaving them had few or no snails upon them. About one 

 hundred and fifty snails were found in the fourth of the tank 

 in which the air was bubbling, but not in the most disturbed 

 waters. The thermometer again showed no difference in tem- 

 perature. The following experiments were performed to see 

 whether the snails were attracted merely by the mechanical 

 disturbance of the water rather than by aeration. A very 

 thin rubber bag was placed over the end of the tube through 

 which the air was passing in such a way as to transmit the 

 motion of the air to the surrounding water but without aerating 

 it; i. e., the air was allowed to escape through the mouth of 

 the bag without touching the water outside. The snails did not 

 collect in the region or on the bag. A stirrer, such as chemists 

 use, was propelled by a water motor and made to stir the water 

 in the tank. The paddle of the stirrer revolved in a horizontal 

 plane at the bottom of the tank, so that the surface of the water 

 was not disturbed and in consequence was not mixed with air. 



