74 JEAN DAWSON 



cannot be compared to the graceful Physa in general activity. 

 It would seem that natural selection had operated to sharpen 

 the senses in the slowly moving L. stagnalis and had left them 

 less speciaHzed in Physa, since it had selected in this little 

 creature a sufficiently compensating motor activity. 



a. Sensitive regions of the body. Experiments were made to 

 locate the regions that were sensitive to chemical stimuli in the 

 exposed parts of the snail's body. It was found necessary, 

 for reasons which will appear later, to use snails fresh from the 

 field, and to select for this experiment only those that were not 

 giving the food reaction at the time. It was early learned that, 

 if the snails were once stimulated with food, their jaws would 

 begin to work and would continue to do so for perhaps an hour 

 or more, even if the animal was placed in fresh clear water. 

 The starting of the snail's jaws into activity and an accom- 

 panying lowering and raising of the head are the only means 

 one has of knowing whether or not the food has been sensed 

 by the snail. Thus after the food has stimulated the jaws into 

 activity the same snail cannot be depended upon (at least for 

 some time) to give reliable data as to the sensing of food by 

 other parts of the body. In this experiment such foods as sugar, 

 bread, pear, apple and meat were used at different times, with 

 the same result in each case. The food substance was very 

 lightly touched to the exposed parts of the snail's body, or held 

 in close proximity to them. All parts of the head and the first 

 few mms. of the ventral surface of the foot were found to be 

 sensitive to food. A snail was found to give the food reaction 

 when foods with strong odors such as apple or pear were gently 

 waved several mms. above the snail's extended foot as it moved 

 upon the surface film. The region of the foot that is sensitive 

 to food thus held in the air is the same that is sensitive to food 

 when applied or approximated under water. It could not be 

 determined whether this whole area is sensitive to food stimuli 

 or only some localized parts of it. 



It would seem from the above experiments that Physa and 

 Lymnaea have a sense similar at least, to the olfactory sense, 

 in some of the land snails. The organ of smell in Physa, accord- 

 ing to Lacaze-Duthiers (1872), is a large ganglion, the osphra- 

 dium, situated in intimate connection with the lung cavity. 

 So far as can be ascertained this region of the lung is not sensi- 



