Mussel
a bivalve closely related to the oyster, the clam, and the cockle. The distinction between clams and mussels should be observed. The most prominent outward difference is the way in which the two valves of the shell fit together. When the valves of the mussel are clapped together, the edges meet closely all the way around. In case of the clam, a sinus or rounded opening may be seen near one end of the hinge. This opening is filled by the siphon or "neck," of the animal when living. When the shell is empty this sinus is open even though the shell be closed. The mussel may be told from the clam and the oyster, also, by its habit of traveling on a foot, like that of a snail, put out between the thin edges of the shells. By examining the sandy bottoms of clear ponds one can see the narrow furrow made by mussels as they shuffle along at a "snail's pace."
Clams are animals of the salt flats. Mussels are of two sorts--marine mussels and fresh water mussels. Fresh water mussels are familiar to every resident by a river or pond. It is considered that they are descendants of salt water species; that they have worked their way up river from the sea, and have adapted themselves to a fresh water life; or that they were retained when the land rose from the sea and the salt waters ran off to the ocean. These mussels, like clams and oysters, have soft bodies compressed into the form of an almond. The head is undeveloped; and it is without tentacles or eyes; the mouth has no horny jaws or salivary glands; the gills consist of numerous filaments grown into large plates; the heart consists of one auricle and two ventricles; the nervous system consists chiefly of three pairs of ganglia situated in the head, the foot, and the digestive tract.
It is doubtful whether the mussel can smell or hear. A fold of skin or body wall proceeding from a line along the back laps over on each side of the animal like a cloak. The edges of this fold meet along the ventral surface. While the mussel is yet a tiny creature the exposed surface of this fold secretes a limy substance and covers itself with a protection known as a shell. A portion of the fold lying along the back remains soft and forms a hinge. The inner, pearly surface of the shell is fed by the part of the fold to which we give the name of mantle. The shell grows around the edges, and increases in size to keep pace with the growth of the soft, defenseless body within. Strong muscles, called adductor muscles, draw the valves of the shell together so stoutly that a raccoon cannot open them. When these muscles relax, the valves fly apart on account of the hinge, which is elastic and is compressed--crowded--so long as the valves are held together. As the shell grows the point of attachment of these muscles shifts with the attachment of the mantle toward the edge or lip.
The ventral portion of the body is strongly muscular. When the shell opens, a wedge-like muscular projection of the body, known as the foot, reaches out and begins to plow along through the sand and mud. In boating over quiet shallows where these mussels live the furrows may be traced by the eye for many yards. Not infrequently, the position of the mussel, which bears a strong resemblance to a stone on the bottom in which it lies half buried, is betrayed by a furrow lately made. The food of the mussel consists of minute plants and animals which are wafted into the soft mouth by the ciliated gills. The stomach is provided with digestive juices. Toward autumn the mussel compacts a store of food into a crystalline substance which it stores in the stomach for winter use. Less solid material contained in the intestine is drawn upon also to tide over the winter. It is but frank to say that as yet investigators are not quite sure about this food supply.
Toward autumn eggs are squeezed out of an organ in the "foot" and are conveyed to the gills, where they are carried until each is developed into a tiny creature having, indeed, no foot, but protected by two delicate triangular valves. Each valve is provided with a hook; glutinous threads--anchor ropes--hang from the spot where the foot is to grow later. At this stage the parent ejects the young to anchor where they may. If the anchor thread touches a fish as it comes nosing by, the young creature clings on, strikes its two hooks into the surface of the fish's gill or skin, possibly at the root of a fin or near the tail. A diseased growth of the fish's skin surrounds the young creature and protects it until it is ready to drop to the bottom of the pond and begin life on its own foot. The fish thus plays an important part in the life of the mussel. In rivers it prevents the embryo mussels from being carried by the current out to sea. It carries the young from place to place and increases the range of the species. Whether mussels necessarily perish where there are no fish is a question that awaits demonstration.