Conchifera (Lat. concha, Gr. a shell, and fero, to bear), a class of mollusks including all the bivalve shells, as oysters, clams, scallops, etc.; they were called lamelli-brancMata by De Blainville. They are next to the univalves in variety and importance, and are invariably aquatic. They are all inhabitants of the sea, excepting a few widely scattered genera, live on every coast, and are found in every climate. The shells of the bivalves are united at the back by an elastic ligament and articulated by a hinge, which is sometimes furnished with teeth shutting by the side of each other and acting much like the common butt hinge. The valves are closed by strong muscles which pass from one valve to the other, and when these are relaxed the shells open spontaneously by the contraction of the cartilage. After the death of the animal, when the muscles lose their power, the elasticity of the ligament causes the valves to gape wide, more so than during the life of the mollusk. (See Conchology.) The mantle, which is a conspicuous part of the animal, is a broad membrane which lines all the interior of the shell and encloses the whole body.
Its edges are more or less fringed, and are either free, partly united, or entirely so, excepting a passage for the foot before and for the siphons behind. The foot is a muscular mass which may be protruded from the shell, and serves as the organ of motion. In nucula and some others the foot is deeply cleft, and capable of expanding into a disk like that on which the snail glides; while in the mussel, pearl oyster, and others, which habitually spin a byssus or thread'of fibres by which they attach themselves permanently to substances, the foot is fingerlike and grooved, and serves only to mould and fix the threads of which the byssus is composed. The branchiae or gills are arranged somewhat like ruffles behind the foot, enveloping the abdominal mass. These are the respiratory organs, but they serve also another and very important purpose. Firmly fastened to rocks and other substances, or at best moving slowly and awkwardly in their muddy or sandy beds, these animals have not the power of following their prey, nor are they furnished with the means of seizing it, but the branchiae convey to the mouth whatever particles the current brings, whether organic or inorganic, animal or vegetable; upon these the mollusk lives.
The water is furnished to the branchiae by one siphon, while another serves as a passage for the excrement. The branchial siphon has its orifice surrounded by a double fringe. When unmolested, a current flows steadily into the opening of this siphon, while another current rises up from the exha-lant tube. The burrowing species have a strong foot, with which they bore into the sand and clay upon the shore so as to entirely conceal themselves. They never leave these abodes, and often become fossilized in them. The teredo or ship worm, and some other borers, which were formerly included among the univalves and multivalves, are now arranged in this class. The interior of the shell is marked with characters derived immediately from the shellfish, and affording a surer clue to its affinities than those which the exterior presents. The structure of the hinge characterizes both families and genera, while the condition of the respiratory and locomotive organs may be to some extent inferred from the muscular markings.
1. Mussel. 2. Nucula. 3. Venus verrucosa. 4. Anodon anatinus.