76 JEAN DAWSON 



or a piece of dry cracker and a snail's integument, which are not 

 wet with water. As soon as the cracker crumb becomes wet 

 it is not pulled toward the snail. A cracker crumb that comes 

 thus in contact with the snail's head gives a sufficiently strong 

 stimulus to cause it to withdraw its head momentarily. The 

 crumb remains usually in contact with the head and causes 

 the animal to give the food reaction, i.e., the movement of the 

 jaws and the depressing and raising of the head. The distance 

 to which the snail lowers its head below the film is in direct 

 proportion to the depth at w^hich the crumb rides in the water. 

 The deeper the crumb floats the further the snail depresses its 

 head. After repeated trials the crumb will, perhaps, be brought 

 over the mouth and ingested. If however, the particle passes 

 back over the mouth, the head of the snail rises up out of the 

 water and bends toward the food, the jaws moving the while. 

 An indentation is formed on the surface of the head around the 

 food so that it is held in a cup-like depression. If the crumb 

 is of considerable size, the foot bends toward the head and its 

 edges curl in so that the whole snail is wrapped about the food. 

 This phenomenon is well shown if a piece of meat is given to 

 Lymnaea stagnalis when it is on the film. The snail has been 

 seen to hold its food thus wrapped in its foot for hours together 

 while it feeds. If the food once passes from the sensitive region, 

 i.e., the head and the first mm. or so of the foot, the chances 

 are that it is lost, at least, for the time being. It may hoAvever, 

 be regained if it chances to be caught in the slime that has 

 hardened on the foot of the snail in the way previously described. 

 The snail bearing a heap of hardened mucus on its foot, turns 

 toward the mass of mucus almost immediately after it has 

 ingested or lost its food, still giving the food reaction. This 

 turning is due, possibly, to the fact that when the animal starts to 

 move forward a pull is felt on the foot from the hardened mucus. 

 In this way it may recover the lost food almost immediately. 



VI. The relation of mechanical and chemical stimuli to food taking. 



The food of Physa is solid, and contact with it necessitates 

 the snails receiving a chemical and mechanical stimulus together. 

 In order, therefore, to interpret the snail's behavior towards 

 food the observer must bear in mind that different food sub- 

 stances, as meat and onion, evoke different responses, some 



