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. Soine- 



. 

 times a faint, clicking sound could he heard when X. 



one of the pond snails made this ascent. This, Profes- 

 sor 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- 

 striclers could run over the surface as though it were 



e ,- t v i Pond snail. Limnaja 



periectly SOim. niighelsi, crawling up 



"This apparently impossible feat," answered the ^quafium Side f a 

 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-established fact that the surface of 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 mollusk on the lower side. 

 One proof of the presence of this surface 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 heavier than its own bulk of water. If the needle is wet, or if it 

 is very large, it will not float, which shows that this film is very deli- 

 cate. If we place the glass holding the needle so that we can look 

 through the glass at the surface of the water, we will then see that 

 the needle rests in a little hollow, as if the water were covered with 

 a membrane." 



Among the pond snails were a number of orb snails carrying their 

 shells in a perpendicular manner, and waving their slender tentacles about. 

 Several of them were crawling along the bottom of the tank, with 

 a peculiar stepping motion. The animal pushed its foot deep into 

 the sand, the shell being drawn well down toward the head. It was 

 then pushed forward and upward, making a little furrow, which pre- 

 pared the way for another step. In this w r ay it " stepped ' along at 

 a lively gait. We learned from Professor Parker that this " stepping " 

 was more for procuring food than for locomotion. One of the large 

 orb shells was crawling up the side of the aquarium, eating everything 

 in its path. Several times a morsel was taken which proved dis- 

 tasteful to the animal, and it was immediately " spit out." 



35 



