The Western rivers teem with species of snails having opercula.

11. If the pupil has any of these operculated snails alive,

FIG. 16.-APERTURE OF SHELL CLOSED BY OPERCULUM, 0.

he will observe that they do not come to the surface to breathe air.

Instead of a lung, the snail has a cavity containing an organ, or part, called the gill, by means of which it is capable of getting from the water what the air-breathing snail gets from the air, namely, oxygen.1

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 proboscis or rostrum. (See Fig. 15.) The shells, too, are, as a general thing, more solid.

12. Thus far the pupils have examined those snails which live in fresh water. Some of these were air-breathers, and came to the surface of the water at intervals to breathe air. He has studied other fresh-water snails which did not breathe air directly, but performed this function by means of

If the class is sufficiently advanced, the teacher may here explain about oxygen and what the blood requires, and gets by respiration.

an organ called the gill. And these snails were operculated, that is, they all possessed a little scale called the operculum, which closed the aperture tightly when the snail contracted within the shell.

He has also learned that the shells grow in size by successive additions of limy matter deposited around the free border of the aperture, and that the delicate lines which mark the surface of the shell, and which run parallel to the outer edge of the aperture, are lines of accretion, or lines of growth.

CHAPTER III.

LAND SNAILS.

13. THERE are many other species of snails which live out of the water altogether, though they are generally found in damp places; and these are called land snails, because they live on the land.

Let the pupils now endeavor to collect some land snails. By going to some hard-wood grove of maple, beech, or oak, and turning over the layers of dead leaves, old rotten logs, or pieces of bark, they will be sure to find some specimens of land snails. Some of them do not grow larger than a pin's-head, others have shells as large as a walnut.

They are generally light brown in color, and the smaller species often have highly-polished or shiny shells.

The spire is generally depressed or flattened. In many,

the border of the aperture has a thickened white rim, or lip, as it is called. Such a collection having been made, the pupil will find among them some of the following kinds :

FIG. 17.-THE SHELLS OF LAND SNAILS.

14. The snail, as it crawls along, leaves a slimy trail after it. This trail consists of a fluid, which flows not only from the creeping disk, but also from the surface of the body. If the back of the snail is irritated by a sharppointed stick, a little whitish mass of this fluid, or mucus,

will adhere to the end of the stick.

By placing the snail on a piece of glass, and allowing it to adhere and crawl on it, a good view may be obtained of the peculiar movements of this creeping disk, by looking through the glass from the other side.

The breathing orifice may be found just within the aperture of the shell, on the right side of the snail.

This orifice will be seen opening and closing at intervals. (See Fig. 18, b.)

15. In the fresh-water snails there are but two tentacles upon the head. In the land snails with few exceptions, the

tentacles are four in number, a larger and a smaller pair. The larger tentacles are called the superior tentacles; the

FIG. 18.-LAND-SNAILS CRAWLING.-b, Breathing Orifice; & t, Superior Tentacles; it, Inferior Tentacles; m, Mouth.

smaller ones, often appearing as mere tubercles, are called the inferior tentacles. (See Fig. 18, s t and i t.) As the snail crawls, the superior tentacles are seen in constant motion, as if the creature were feeling its way about with them.

The eyes, instead of being at the base of the tentacles, as in the fresh-water snails, are found at the tip of the superior tentacles.

In the land snails, with few exceptions, the tentacles can be drawn within the head, and for this reason they are also called retractile tentacles.

While the snail is crawling, if the pupil touch the end of the tentacle with his finger, or, even if he alarm the snail by a sudden jar, he will see the tentacles quickly withdraw within the head. The pupil will observe that the bulbous

end containing the eye disappears first, as the end of a glovefinger disappears, when the hand is withdrawn from the glove, the glove turning wrong-side out.

FIG. 19.-SHOWING SNAIL WITH TENTACLES RETRACTED, A; AND TENTACLES PROTRUDED, B.

16. Something may now be learned as to the way in which land snails eat.

By placing before the snail the tender leaves of lettuce or cabbage, the head will be seen to move, as little mouthfuls of the leaf are bitten off. The upper lip of the mouth is furnished with a hardened piece called the buccal plate. It is crescent-shaped, and, in some species, the cutting edge is notched, so that it acts like an upper set of teeth, by which it bites off little bits of the leaf. The floor of the mouth is lined with a membrane having upon it rows of little points which enable the snail to rasp and grind its food. These parts

FIG 20.-JAW, OR BUCCAL PLATE OF A LAND SNAIL, HIGHLY MAGNIFIED.-(It can just be discerned without a microscope.)

are so minute that they can be studied only by the aid of a microscope. If the pupil will watch his fresh-water snail

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