MUSSEL (AS. mush., muc.r/c. from Lat, nu/s ea/us, small fish, sea-mussel, diminutive of Ill UN, word mussel is one of those pop ular names of such uncertain limits that to at tempt to define it is a difficult matter. At the present time it is applied to two totally distinct groups of bivalved mollusks, one marine and the other fresh-water. The first group ineludes the shell-tish of the family Mytilida:. especially the genera Mylilus. Modiola, and Modiolaria. This family is characterized by the presence of a well developed byssus, two adductor muscles. a fringed vestige of a siphon, and an equivalve but inequilateral shell, with a thick epidermis. 1110 With a weak and usually toothless hinge. The mussel cThilis) of all north ern coasts is in the Old World notch raised for food, but ifs greatest importance in the United States lies in its II:endues,: as bait for fisher men and as manure for farmers near the coast.
It is gregarious and occurs in masses, closely crowded together, adhering to rocks, sand, and each oilier, by the very tough byssus, the whole 'he Is' being thus practically slum together. These heds are generally in shallow water. less than thirty fathoms, and are often exposed at low tide. The shell is oblong with a terminal um bone, and generally about three inches long by an inch and a half wide: but under favorable conditions they may laTome six inches or even more in length. The outside of the shell is black, but inside it is pearl• blue. \Vhen young, nuts sets move about, but they soon anchor• themselves by the byssus, and unless artificially disturbed will remain fixed throughout life. So strong are the byssus-threads that these mollusks have been utilized as an aid in the strengthening of break waters. the young mussels being planted on the rocks in myriads and thus assisting in uniting the separate pieces into a rigid whole. The horse mussels (q.v.) belong to the genius Modiola, which does not have the m»bone terminal. They are more elongated than Mytilns and some spe des reach a much larger size. See Colored Plate of CLAMS AND EDIBLE MUSSELS.
The other group to which the name mussel is given is the family Unionithe, which are fresh water mollusks of very wide distribution. The family is eharaeterized by the possession of two adductor muscles and regular shells, with thick epidermis and thin nacreous layer, prominent external ligament, and a variable hinge. Up ward of a thousand species have been named, a very large proportion of which occur in the States, where the family is very abundant and widely distributed. It is still an open question as to what constitutes a valid species o• even a genus in this family. though several
genera, as Unio. Anodon, and Margaritana, are recognized. These fresh-water mussels occur in lakes, 'totals, and streams, generally in large num bers together, and are sometimes eight inches or more in length, by four in breadth. but they are nsually smaller and They are some times used for food, hut are tough and not very palatable. Their ehief importance lies in the fact that they frequently produce pearls, and are indeed the only basis of the fri.sh-water pearl in dustry. As high as $25,000 has been paid for a single mussel-pearl. (See PEARL). All of the Unionidas have a remarkable life-histo•y, for the eggs when laid find their way into the mother's gills, where they remain until the young are hatehed. Although these young have a bivalve shell. they are totally different from the adults and have a very simple structure. At this stage of development they are called `glochidia' larwc. After leaving the maternal gills they attach themselves to the skin of a fish, where they give rise to an excrescence which gradually swells until it is a capsule big enough to see with the naked eve. These capsules are almost black and are usually seen on or near the tins of the host fish : they are often very numerous on a single fish. Within the capsule the gloehidium lives until it finally assumes the adult form. when it breaks through the outer wall of the capsule and to the bottom as a miniature mussel. The distribution of the species is therefore mainly dependent on the activity of the fishes which serve as hosts.
Fossil nmssels are found in most geological formations from those of Ordovician age to those of recent time. They comprise the super famil• Mytilacea, containing the families lopsida., Mytilithe. and Dreissensiithe. The first, now entirely extinct, are essentially ancestral forms of the 1)lytilida‘, whieh range through De vonian to recent formations. The Paleozoic species of this family belong mostly to the genus Alodiola. which began in Devonian, was abundant in Jurassic and Tertiary time, and still persists as a common shell of the littoral zone of all seas. Mytilus began in the Triassic. Lithophagus or Lithodonms, the mussel which bores cavities in limestone rock, has been recognized in Mesozoie and Tertiary formations. The are of later origin, appearing first in the Tertiary. They are essentially fresh-water and estuarine mussels, and have probably evolved from Mytilits or a closely allied genus of the littoral zone in late Mesozoie or early Tertiary time. See Plate of ABALONE, ETC., and illustrittion under Mom. LOSE.