Such shells are called canaliculated shells. The aperture of the shell is said to be entire, when it does not possess this notch, or canal. Let the pupil separate the shells having the aperture entire, from those shells having a canal. These differences in the shell, as slight as they appear, are accompanied by corresponding differences in the character and habits of the animal. Those snails having the aperture of the shell entire are with few exceptions vegetable-feeders, while those having a canal to the shell are flesh-feeders. The mouth-parts, and opercula, too, are different in the two groups. 24. Other shells will be found differing greatly in appearance from those thus far studied. One of these is represented in the lower left-hand corner of Fig. 26. 30): Another species, called the limpet, looks like this (Fig. FIG. 30.-SHELL OF LIMPET. These shells will be found sticking with great tenacity to the rocks, and some skill and force will be required to remove them. This can be done by using the large blade of a pocket-knife, and suddenly scraping them from the rock. If they are then placed in a saucer of water, with the shell downward, the animal within will be found to have the broad, creeping disk, head, tentacles, and other parts, peculiar to the snails already studied. In others they are united in a long string Fig. 32). 1 a Portion of a String of Egg-Capsules, from a Florida Species of Whelk similar ki 47 #1, # single Capsule separated, showing Outlet, o, through which the Young T The common cockle sticks its separate egg-capsules side by old upon the rock (Fig. 33). The beach-cockle deposits its eggs in a broad ribbon of sand cemented together, looking very much like a deep saucer, with the bottom broken out, and the side separated (Fig. 34). FIG. 33.-EGG-CAPSULES OF COMMON COCKLE.-(A shell of the animal which produces these capsules is shown in the upper central figure of Fig. 26.) This ribbon is elastic when wet, and, if it is held up to the light, the little transparent spaces for the eggs will be plainly seen. FIG. 84.-EGG-RIBBON OF BEACH-COCKLE (the shell of which is shown in the lower CHAPTER V. FRESH-WATER MUSSELS. 26. LOOKING over our fresh-water shells again, we find many that are known as mussels, or clams. These shells are common everywhere along the margins of brooks, rivers, and lakes. The musk-rats feed upon the soft parts of the mus sels, and the remains of their feasts may be found in piles of mussel-shells, all along the shores of certain lakes. The shell is composed of two pieces, or valves, as they are called. The two valves are often found united, and the margin along which they are connected is called the hingemargin, because the shells hinge at this part, and will open and shut as a door swings upon its hinges. Let the pupil now examine a perfect fresh-water mussel, that is, a mussel in which the valves are united in this way, and he will observe that they are connected by a brownish substance, which is quite elastic when the shell is alive, but becomes brittle when dried. The shells are held together as the covers of a book are held together by the back. This substance is called the ligament, and the position of this ligament will indicate the back, or dorsal region of the animal. 27. On the outside of the shell will be seen fine lines, which run nearly parallel to the outside margin of the shell. These lines are the lines of growth, and indicate the successive stages of growth, or increase of the shell, as in the lines of growth in the snail-shell already studied, and, as in the snails, the growth takes place at the margin of the shells. The pupil may trace these concentric lines back, as they grow smaller and smaller, till they are found to start from one point at the back of the shell, and this point is called the beak or umbone. It represents the starting-point in the growth of the shell. In fresh-water mussels, the umbones are eaten away by some corrosive action of the water, and the Forward or anterior end. early stages in the growth of the shell are usually destroyed. In very young shells, however, the early stages can be plainly seen. Back or dorsal region. น FIG. 35.-A FRESH-WATER MUSSEL.-, Ligament; u. Umbone; f, Foot; ex., Excurrent 28. The ligament is always behind the beak, or umbone, in fresh-water mussels, and in nearly all bivalve shells (so called, because they have two valves or pieces, while the snail-shells are sometimes called univalve shells, because they have but one valve or piece). Let the pupil now hold a perfect mussel-shell in his hand (that is, a mussel in which both valves are together, and united across the back), with the ligament uppermost, and the umbone away from him, or beyond the ligament, and the valve on his left hand is the one which covers the left side of the animal, while the valve on his right hand covers the |