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5. If the surface of the shell be examined closely, delicate lines running from one suture to another will be seen, as in the figures already given; and, if the shell be looked at from the side of the aperture, these lines will be found running parallel to the edge of the aperture, or lip, as it is called.

These delicate lines are called lines of growth.

FIG. 7.-SHOWING LINES OF GROWTH RUNNING PARALLEL TO THE EDGE OF THE Aperture.

The shell is increased in size by successive layers of shelly matter added to the borders of the aperture. In this way the shell grows.

A clearer idea of the growth of a shell may be obtained by studying the next figure (Fig. 8): A representing in outline a young shell; B representing the full-grown shell in outline; and C representing the same outline as B, with a number of lines of growth represented upon it.

If the shell were now to continue its growth a single halfturn, or whorl, the dotted lines would indicate the increased stages it would assume: a representing the first increase in size, the next stage, and c the appearance of the shell when the additional half-whorl has been completed.

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FIG. 8.-ILLUSTRATING THE INCREASE IN SIZE OF A SHELL.

Among the lot of shells collected by the pupil, differentsized ones will be found. Now, if a number of these, of different sizes, can be picked out, provided they belong to the same kind, or species, it will be noticed that the apex of all of them will be the same; but that the shells have increased in size at the aperture, and the aperture will be larger, and the larger shells will have more whorls than the smaller shells. The following figures illustrate four different ages of the same species of shell:

FIG. 9.-ILLUSTRATING DIFFERENT AGES OF THE SAME SHELL-THE LOWER FIGURES REPRE

SENTING A VIEW OF THE SHELL FROM THE APEX.

6. The axis around which the whorls revolve is called the columella. This axis is generally solid, though in many shells it is hollow, as if the whorls had turned around a shaft which had afterward been withdrawn. This hollow axis looks like an opening in the base of the shell, as in the following figure :

FIG. 10.-A SNAIL-SHELL SEEN FROM BELOW.

This opening is called the umbilicus. The apex of the shell is sometimes called the nucleus, because the shell commences to grow from this point.

CHAPTER II.

FRESH-WATER SNAILS.

7. THE pupils will now be required to bring in some live snails. Let them examine bits of bark, chips, or branches, found in ditches, or muddy brooks. Under lily-pads and on the stems and leaves of other aquatic plants, and on stones in rivers, snails of various kinds will be found. A dipper with the bottom perforated, or made into a sieve, and attached to a wooden handle four or five feet in length, will be found useful in scooping up the sand or mud from the

bottom of rivers and ditches. The dirt having been sifted out, the shells and other objects will be left behind. The dipper may be made as in the figure.

FIG. 11.-DIPPER ATTACHED TO A WOODEN HANDLE FOR COLLECTING SNAILS.

Shells collected with the snails inside, and cleaned for the cabinet, are called live shells. They are always more fresh and perfect than dead shells.

Having made the collection, the snails should be kept alive in a wide-mouthed jar, or bottle, care being taken not to have more than fifteen or twenty in a jar holding a quart of water.

8. The pupils will have secured some of the following forms:

FIG. 12.-FRESH-WATER SNAILS.

The broad, creeping disk upon which the snail rests, and by which it retains its hold to the glass, is called the foot. The snail moves about, and crawls or glides slowly along, by means of the foot.

The two little horns or feelers, in front, are called tentacles, and, as the snail moves, the tentacles are seen stretched out in front, and occasionally bending, as if the creature were feeling its way along. The eyes are seen at the base of the tentacles, as two minute black dots. The mouth is between the tentacles, and below. The part from which the tentacles spring is called the head, and the opposite end of the body is called the tail. The surface upon which the snail rests is called the ventral or lower surface, and consequently that portion of the body which is above is called the dorsal surface, or back.

9. The pupil, in watching the habits of the snails he has collected, will notice some of them crawling to the surface of the water to breathe air. The snail accomplishes this by raising the outer edge of the aperture to the water's edge, and then opening a little orifice in the side, through which the air enters to the simple lung within.

This orifice is on the right side in those snails having dextral shells, and on the left side in those snails having sinistral shells.

Many kinds of snails which live in fresh water are called air-breathers, because they are forced to come to the surface of the water to breathe air. In doing so they first expel a

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