MUSSELS, bivalved mollusks that dwell in lakes and rivers; river mussels, or river-clams. They belong to the family Unionicke, allied to the cockles, which has a large and thick foot, no byssus, siphon short (when present), and a parasitic em bryonic growth. The shells are equivalve, varying according from thin and smooth to very thick, rugose and knobbed; the hinge variable (in Anodonta having no hinge-teeth); and the interior always thickly nacreous, mak ing it useful in the arts as "mother of pearl,) and often producing fine pearls. The family is world-wide in its distribution and includes about a dozen genera, two of which (Unio and Anodonta) occur in most parts of the world. These mollusks dwell in rivers and ponds, and vary greatly according to the character of their home, whereby a great number of supposed species have been named that are now known to be merely varieties of the same stock result ing from different environment. They stand upright in the sand on the blades of the shell, so that the heavy hinge margin receives any blow from drifting stones, or other harm; and slowly move about, sucking in the minute ani mal and vegetable organisms upon which they feed. (See PELECYPODA). The development of their young is most unusual. The eggs when ejected from the ovaries are cauglit in the gills of the mother and are sustained by a nutritive mucus-like secretion, until they reach a certain degree of age, when they become They then have a larval shell, provided with strong hooks, and possess a long filament. After a period they are expelled through the exhalent siphon into the water, and this ejection may be timed to the passing of a small fish, to whose body if they touch it the glochidia at once cling by means of the, hooks.
Should they miss against a fish when thrown out the embryos sink and lie upon the bottom with their shells gaping and the filament floating upward. There they remain until a whose' comes within reach; but this must soon happen or they will perish. The glochidia of Unio usually become attached to the gills; those of Anodonta to the skin or the fins. In this position they become overgrown by the skin or mucous membrane of their host, and are nourished by his juices. This goes on for about 10 weeks, during which time the glochid ium has been metamorphosed into a young normal mussel, drops off and begins the ordi nary course of life. Their life is probably long.
Mussels abound in all the rivers of the United States and were extremely numerous and varied in those of the Mississippi; and they entered very largely into the fare of the native red men, as is attested by the large refuse heaps of their shells to be found in all the river courses. It was • long ago discovered, however, that these shells yielded pearls of great beauty and price (see PEARL.), while the mother-of-pearl of many species was market able for the manufacture of buttons and similar articles. The result has been a serious deple tion of the mussels of many parts of the Middle West, and nearly an extinction of some species. Consult Lefevre and Curtis, 'Studies in the Production and Artificial Propagation of Fresh Water Mussels' in the United States Fisheries Bureau Bulletin (Vol. XXX, Washington 1912), and other publications of the department.