< Page:Popular Science Monthly Volume 7.djvu
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THE POPULAR SCIENCE MONTHLY.

operculum. "When the snail has retired within the shell, the operculum 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 margin.

Fig. 4.—Jar of Water, in which is contained a Number of Species of Mollusks. Some of them are near the surface breathing air:
A and C are taking in air; B is just expelling a bubble of air from the lung; D is crawling on the surface of the water: E, G, and I, 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 observe 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

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