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THE POPULAR SCIENCE MONTHLY. 



operculum. "When the snail has retired within the shell, the opercu- 

 lum will look like this in the aperture of the shell (Fig. 6) : A series 

 of concentric lines will be seen marking the operculum, and these are 

 the lines of growth, the operculum growing around the outer edge by 

 successive additions, just as the shell grows by successive additions 



to Its outer margm. 



Fig. 4. Jak of Water, in which is contained a Number of Species of Mollusks. Some op 

 them abe near the surface breathing alri 



A and Care taking in air ; B is just expelling a bubble of air from the lung ; I) is crawling on the 

 surface of the water: E, G, and /, are in the act of crawling up, to get a fresh supply of air; 

 and J" is a water-breather, having gills, but no lung. 



The Western rivers teem with species of snails having opercula. 



If the pupil has any of these operculated snails alive, he will ob- 

 serve that they do not come to the surface to breathe air. Instead 

 of a lung, they have a cavity containing an organ, or part, called the 

 gill, by means of which the snail is capable of getting from the water 

 what the air-breathing snail gets from the air, namely, oxygen. It 

 will be seen that the head of the snail is shaped differently in the 

 snails having an operculum, the mouth being at the end of a sort of 



