BIOLOGY OF PHYSA 85 



Physa can sease its food only at a comparatively short 

 range. The distance depends in part at least upon the rate of. 

 diffusion and odor of the food. In spite of the fact that Physa 

 cannot sense its food keenly, the snail probably is rarely without 

 food, since the great variety and abundance of food in most 

 habitats together with the animal's constant activity cause it 

 to meet with nourishment continually. 



The eyes of Physa are rudimentary and probably play no 

 part in the collection of its food. Experiments show that 

 the head and first few mm. of the ventral foot of the snail are 

 sensitive to a food stimulus. If food is touched or brought 

 into proximity to these parts, the animal gives the food reaction. 

 This reaction consists of a lowering and raising of the head, 

 accompanied by a rhythmical movement of the jaws. Ex- 

 periment also shows that the parts sensitive to contact with 

 food are also sensitive to some foods, if they are waved gently 

 above the surface of the water as the snail crawls upon its film. 

 Physa is positive in its reactions to some chemicals and nega- 

 tive to others. The snail turns toward a chemical which in- 

 duces a positive response and gives the food reaction. In a 

 marked positive response the snail, if it is upon the surface of 

 the water, may raise its head above the film. Other chemicals 

 cause the snail to turn away and fail to give the food 

 reaction and, in marked cases, even to curl its head back into 

 its shell. 



Physa alternately approaches and withdraws its head from 

 a food particle that it has sensed until the particle is brought 

 over its moving jaws and ingested. If the food passes back- 

 ward beyond the region that is sensitive to chemical stimuli 

 the chances are that it is lost to the snail. The ventral part 

 of the head and foot often form a cup-like depression about 

 the food, serving to hold it while the snail feeds. 



Table II sums up briefly the behavior of a fresh, well fed snail 

 under a mechanical and chemical stimulus simultaneously 

 received. It is a notable fact that the mechanical stimulus 

 seems to be sensed and obeyed first, followed after an interval 

 of time by the chemical. If the mechanical stimulus is very 

 strong, the snail seems to fail altogether to recognize the chem- 

 ical stimulus. The physiological state of the snail, due to the 

 amount of food taken, affects its reaction to food. Conflicting 



