BIOLOGY OF PHYSA 73 



withdrawal of the head from the food accompanied by a rhyth- 

 mical action of the radulae. If a snail senses food it turns toward 

 it and gives this food reaction . A piece of fresh meat was placed 

 in the center of a bacteria dish and Physa were watched to see 

 how far away they could detect its presence. Maps were made 

 of the snail's paths as they wandered up and down, now on the 

 film and now on the substratum. They would pass within 

 a cm. or less of the meat and not detect its presence; and in- 

 deed, might even touch a tentacle against it while moving 

 rapidly, and yet pay no more attention to it than if it were stone. 

 Meat that was soaked in water for about twenty-four hours 

 and then placed in fresh water was seemingly more readily 

 detected than fresh meat used in a similar experiment. Pos- 

 sibly this is due to a difference in the rate of diffusion of the 

 juices in the two pieces of meat. A tube filled with crushed 

 apple, whose juices could be seen to radiate through the fine 

 wire gauze fitted over each end was placed in the aquarium. 

 The snails seemed to detect this for a greater distance than a 

 piece of solid apple. This was due perhaps to a greater con- 

 centration of juice in a given area. They would start to cross, 

 the path of the radiating juice and then would pause and turn 

 toward the end of the tube. 



Different kinds of food were placed in the aquarium to find, 

 if possible, which would be the more readily detected by the 

 snail. Pieces of apple and pear were more easily found than 

 meat, and when crushed more snails collected upon them in 

 a shorter space of time. This was due no doubt, as Pearl (1903) 

 found for planarians, to a greater diffusion of the juices of the 

 food in the surrounding water. It was also found that a food 

 that had an odor was more quickly detected than one that 

 was odorless. Snails seemed to be able to find stale, ill-smelling 

 meat more easily than fresh meat; this is probably the reason 

 that more collect on it than on the fresh meat. 



The distance at which the snail, moving at a slow rate, can 

 detect a given food depends largely upon the odor of the food 

 and the rate of diffusion of its juices. The distance is short 

 at best, being on an average not much over icm. Lymnaea 

 stagnalis as experiment showed, is much more sensitive and can 

 detect food at a greater distance than Physa; responding to 

 both strong and weak odors more keenly. Yet this large snail 



