Now hold a perfect mussel-shell in your 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 the person, or beyond the ligament, and the valve on the left hand is the one which covers the left side of the animal, while the valve on the right hand covers the right side of the animal. The forward end will be the end away from the person, and the hinder end will, of course, be the end toward the person. (See Fig. 1.)
Let us now endeavor to collect some fresh-water mussels alive. These may be found partly buried in the sand or mud of rivers and lakes. As they crawl along partly buried in this way, they plough up the sand, leaving a well-marked furrow or groove behind them. Every boy that goes in bathing is familiar with the peculiar furrow left by the fresh-water mussel. By following such a furrow, the mussel that made it will soon be found.
Fig. 8 represents the appearance of a common fresh-water mussel in the act of crawling.
Fig. 8.—Showing Position of Mussel when crawling.
f, Foot buried below the Surface of the Sand s. Above the line s is supposed to be water, the line representing the bottom of a lake or river.
Having collected a few in this way, they may be placed in a large, shallow pan of water, and allowed to remain quiet for a while. Gradually the shells will open a little, and from the hinder end a curious fringed border appears; on examining this border, it will soon be found that it forms two openings which lead into the shell.
Great care must be taken not to jar the dish, or the table upon which it rests. The slightest jar will cause the shells to instantly close. If some indigo, or small particles of dirt, be dropped near these openings, currents of water will be revealed; one current pouring out of the opening nearest the back, and another current as steadily pouring in at the other opening. The opening into which the current of water is passing is called the incurrent orifice, while the orifice from which a current of water is passing is called the excurrent orifice. The incurrent orifice is sometimes called the respiratory orifice, because the water is taken in to supply the gills which are the breathing or
