72 JEAN DAWSON 



do so was probably due to the tough, resistant capsule in which 

 the eggs are incased. 



III. The effects of drought and temperature upon food taking. 



The snail takes no food for months during its winter sleep 

 and during desiccation. In the laboratory, however, active 

 snails may die of starvation in four or five days at room tem- 

 perature, the time varying greatly with the individual snail. 

 Some of these have been revived by feeding even after they 

 had fallen on their sides through hunger. 



The snail loses visibly in size from day to day during this 

 period of fasting, the foot being most conspicuous in this shrink- 

 age. In this condition the general activity of the animal is 

 greatly lessened, the snail remaining quiet for long periods 

 at a time even in bright sunlight. The temperature of the 

 surrounding water seems to have a very marked effect upon 

 the snail in its food relations. Eagerness for food is greatly 

 lessened if the temperature of the water is lowered, and in- 

 creased if it is raised. This is demonstrated by the following 

 experiments: Snails that were kept at room temperature 

 and accustomed to being fed cracker and apple would not 

 respond in the morning to this food if the water in the aquarium 

 had become about ten degrees cooler during the night. If, 

 however, the temperature of the water was raised by the addi- 

 tion of warm water, they would respond to the food imme- 

 diately and become much more active. Snails that have fasted 

 for some time and have in consequence become very weak and 

 inactive will also go about with greatly increased activity of 

 movement, and the reflex action of the jaws will be greatly ac- 

 celerated if the temperature of the water is raised. 



IV. The sensitivity of Physa to food. 



As Physa feeds, whether upon the surface film, bottom or 

 sides of a pool or containing vessel, the oral surface of the head 

 is alternately applied to the surface and withdrawn from it. 

 As the head is brought in contact with its food, the radula^ close 

 upon its surface and the head is then withdrawn about i mm. 

 By this means small pieces of food are torn away so that they 

 may be ingested. Thus the food reaction of the snail consists 

 of a double set of activities; the alternate application and 



