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NATURE-STUDY REVIEW [10:8— Nov., 1914 



left a clear path behind it where the scum had been cleaned off. 

 We could plainly see the mouth open and close as the animal grazed 

 along. Every time the mouth opened, the tongue was thrust 

 out, and the whole operation reminded us of a cat lapping milk. 

 The brown jaw was also plainly seen. 



As we were watching the animals on the side of the aquarium, 

 one of them rose suddenly from the bottom of the tank to the top 

 of the water: there it floated, shell downward, and with the foot 

 applied to the under surface of the top of the water. Sometimes 

 a faint, cHcking sound could be heard when one of the pond 



r 



Pond Snail Crawl- 

 ing on Glass 



Tadpole Snail 



Egg Mass of Physegyma 



snails made this ascent. This, Professor Parker said, was caused 

 by the escaping of the imprisoned air from the lung. 



George inquired how the snail was apparently able to crawl on 

 the under side of the surface of the water and also how the little 

 insects called water-striders could run over the surface as though 

 it were perfectly solid. 



"This apparently impossible feat," answered the Professor, is 

 easily explained when we understand some of the laws of physics, 

 and those of you who are studying the subject at the University are 

 probably well acquainted with the fact. It is now a well-establish- 

 ed fact that the surface of the water and other liquids, is covered 

 with a very thin film, and the insect is able to walk upon the upper 

 side of this film and the muUusk on the lower side. One proof of 

 the presence of this film is found by the oft-repeated experiment 

 with the needle. If we carefully place a fine needle on the surface 

 of the water, it does not sink, but will float, although seven times 



