68 JEAN DAWSON 



through the water will demonstrate that it has left no mucous 

 trail behind. Possibly the explanation of this curious feat is 

 that since there is nothing to which -a new trail may be attached, 

 none is spun out, and consequently nothing is left behind to 

 catch the brush, while the old track becomes entangled in the 

 foot, and is, in a sense, reeled up. Still again, a snail may be found 

 dangling from a thread of mucus let down from some support at 

 the surface. Obviously this is not possible except when the 

 snail has altered its specific gravity by emptying its lung until 

 it can sink in water. In all these various forms of dangling 

 the snail was always seen to carry the foot well out of the shell 

 and at the same time to expend considerable muscular effort 

 in twisting about." From this it would seem that Walter is not 

 aware that the snail can attach its thread to the surface film. 



III. FOOD OF PHYSA. 



I. Nature of food. 



II. Mucus as a food and as a means of collecting food. 



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



IV. The sensitivity of Physa to food. 



a. Sensitive regions of the snail's body. 

 V. Positive and negative response tochemical stimuli. 

 VI. Relation of mechanical and chemical stimuli to food taking. 

 VII. The influence of physiological state upon food taking. 



a. Experiments on food reactions of well fed snails fresh from the field. 



b. Experiments on the food reactions on fasting snails. 



c. Experiments on the food reactions of very well fed snails. 

 VIII. — General summary. 



I. Nature of its food. 



Physa lives upon a variety of animal and vegetable food 

 either fresh or partly decayed. Its vegetable food consists 

 of green plants and decaying plant tissue, and it will also eat 

 fruits and vegetables. The green plants that it eats are mainly 

 algae, but it devours as well the tender green shoots of other 

 water plants such as Char a and Elodea. Such leaves as grass, 

 maple and elm are eaten very commonly when watersoaked 

 and partly decayed but never when they are fresh and hard. 

 Oak leaves are never touched unless the snail is very hungry, 

 and then but little. Leaves are not wholly devoured ; the pulp 

 is taken from the vascular bundles on each side, thus leaving 



