Popular Science Monthly/Volume 7/September 1875/Fresh-Water Mollusks
UNDER lily-pads and on the stems and leaves of other aquatic vJ 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 seen in Fig. 1.
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.
Some of the following forms will have been secured:
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.
In watching the habits of the snails he has collected, the reader 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 or right-handed shells, and on the left side in those snails having sinistral or left-handed 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 bubble of air, which may be seen escaping from the breathing-orifice, as in Fig. 4, B.
These fresh-water air-breathing snails may be kept under water for many hours before life is extinct.
Among the snails collected, there will probably be found some which have a peculiar scale on the hinder part of the body. When the snail crawls, this scale will be seen just behind the shell, as in
Fig 5, o. This scale is called the operculum, and when the snail has contracted, or drawn within the shell, the operculum is seen to fit the aperture of the shell, closing the shell as a stopper closes the mouth of a bottle.
Nearly all sea-snails, that is, snails which live in salt-water, and many species of fresh-water snails, and also many snails which live in damp places on the land, and which are called land-snails, have an 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.
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 proboscis or rostrum. [See Fig. 5.) The shells, too, are, as a general thing, more solid.
Thus far we have examined those snails which live in fresh water. Some of these were air-breathers, and came to the surface of the water
o, Operculnm; e, Eye; r, Rostrum; g, Entrance to Gill-Cavity.
at intervals to breathe air. We have studied other fresh-water snails which did not breathe air directly, but performed this function by means of 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.
We have 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.
Looking over our fresh-water shells again, we find many that are known as muscles, or clams. These shells are common everywhere along the margins of brooks, rivers, and lakes. The muskrats feed upon the soft parts of the muscles, 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 hinge-margin, 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.
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.
We 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 early stages in the growth of the shell are usually destroyed. In very young shells, however, the early stages can be plainly seen.
l, Ligament; u, Umbone; f. Foot; ex, Excurrent Orifice; in, Incurrent Orifice.
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 uni-valve shells, because they have but one valve or piece).
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.
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 respiratory organs of the mussel, and this orifice corresponds to the siphon in the sea-snails already studied. This current of water, besides bathing the gills, also carries in minute particles which are floating in the water, and these particles are conducted to the mouth of the creature, and swallowed as food. At the opposite end of the shell from these openings, or the forward end, a whitish, fleshy mass will be seen protruding. This is called the foot, and corresponds to the foot or creeping disk in the snails. By means of this foot the mussel crawls through the sand.
The mouth is above the foot, and always concealed within the shell. In Fig. 7 the foot is shown, and also the excurrent and incurrent orifices, with arrows drawn to indicate the direction of the currents of water.
In some small species of fresh-water bivalves, the excurrent and incurrent orifices are prolonged into tubes, and then they are called siphons. Fig. 9 represents a common species which the pupils may find in muddy brooks and ditches. By using the long-handled dipper already described, some specimens will probably be found. They are quite small, from the size of a pea to that of a nickel cent. The siphonal tubes are prominent, and the foot is long and tongue-shaped, and the animal is very active in crawling about; also in Fig. 4 K and L represent two of these small animals with bivalve shells.
The foot of these creatures resembles in appearance and action the foot of a fresh-water snail, only there is no mouth nor tentacles in sight. These parts are present, but are never protruded beyond the edges of the shell.
When the fresh-water mussels are partly open, a fleshy border will be seen just within the edges of the shell, and this is the border of the mantle, and corresponds to the same parts described in the snails; the fringed membrane which formed the openings at the hinder part of the mussel is simply a continuation of the mantle.
When the shells are removed from the animal, the mantle will be found lining the shells, just as the blank pages line the inside of a book-cover. While the edge of the mantle deposits the successive layers, which increase the size of the shell, the entire surface of the mantle deposits the pearly substance which lines the inner surface of the shells, and which is so characteristic of the fresh-water mussels.
Grains of sand, or other particles, getting in between the mantle and the shell, are soon covered by layers of pearly substance poured out, or secreted by the mantle. In this way pearls are formed. If pearls are broken open, a centre, or nucleus, will be found, consisting of some particle of dirt or sand, or some substance which had found its way by accident between the mantle and the shell, and around which the pearly matter has been formed in successive layers.
In shells having a brilliant, pearly lining, or nacre, the pearls obtained are oftentimes very beautiful, and from certain Oriental species living in the sea, called Avicula, the most brilliant pearls of commerce are obtained. If, on the other hand, the nacre lining the shell is dull white, as in the common oyster, the pearls are dull colored. These kinds of pearls are often found in oysters.
The Chinese have long been familiar with the art of making artificial pearls. By partly opening the shells of certain fresh-water mussels, and inserting little lead images, or other objects, between the mantle and the shell, the objects soon become covered with a natural layer of pearl.
Let us now study the markings on the inner surface of the shells of river-mussels. The shells of these creatures are called valves, and are spoken of as right or left valves, according to whether they are on the right or left side of the animal.
Certain ridges and prominences will be seen at the hinge, and, when the valves are carefully joined, the ridges in one valve will correspond to grooves in the other valve. These ridges are called teeth. The short ones, near the beak, are called cardinal teeth, and the long ones lateral teeth. The margin upon which they occur is called the hinge-margin, for it is upon this margin that the valves turn. (See Fig. 11.) Certain scars, or impressions, will be found marking the inside of the valves, and these indicate the point of the attachment of certain muscles to move the valves, and to enable the animal to protrude its foot, and crawl along. These marks are hence called muscular marks, or muscular impressions, and will be found to correspond in the right and left valves.
An irregular, round impression will be found at each end of the valve, near the hinge-margin. These show where the muscles are attached to move and close the valves, and hold them firmly together. The muscles run directly across from one valve to the other; and, to open a live mussel, it is necessary to pass a sharp blade between the valves, and cut through the muscles, before the valves will open. These muscles are called the adductor muscles and the scars or impressions on the valves are called the adductor muscular impressions. Very close to the adductor muscular impressions are seen smaller impressions, and these indicate where the muscles are attached which move the foot. These muscles are called the pedal muscles, and the impressions are called the pedal muscular impressions. One occurs just behind the anterior adductor impression; the other will be found just above, and in front of the posterior adductor impression.
Besides these marks, we see a delicate and slightly irregular line running from the anterior to the posterior muscular impression, just inside, and nearly parallel with the lower margin of the shell. This line is called the pallial line, and indicates where the mantle is attached to the shell. It will be observed that, when the soft parts are removed from the shell, the mantle adheres along this line.
When the mussel is opened by separating the adductor muscles with a knife, the valves slowly open, and after the animal is removed the valves still remain partly open, and, to preserve them closed, a string has to be tied around them, and in this condition, if the ligament is allowed to dry, the valves will then remain closed. From this it is evident that the ligament acts upon the valves to draw them apart. To keep them closed, then, the animal must continually exert itself by contracting the adductor muscles; and it will be found that, when these creatures are left in the water, undisturbed for a while, the muscles relax, and the valves partly open. The ligament is elastic, and is stretched as it were from one valve to the other, over the back. A possible imitation of the action might be represented by partly opening the lids of a book, and then gluing across the back, from one lid to the other, a sheet of elastic rubber. If, now, the lids are tightly closed, the rubber is drawn out, or stretched across the back, and, if allowed to regain its elasticity, the lids are pulled apart. This experiment illustrates the way in which the ligament acts in those shells which have the ligament external.
- From "First Book of Zoology," now in press of D. Appleton & Co.