CLASS I. LAMELLIBRANCRIATA (1,v.), (Lat. lamella, a plate; Or. birave•Y, gills), called by Lamarck chonchifera. These animals have no distinct head; body protected by a bivalve shell, as in the brachiopods, but the shells differ as much as the animals. In the brachiopods one shell is generally considerably larger than the other, while in the lamellibrauchiates the two shells are generally of equal size. Again, in the brachiopods either one of the shells is symmetrical, or equilateral; that is, a may divide it into two equal and relatively similar halves, while the valve of a lamellibranchiate is never quite equilateral. See SHELL. The respiratory organs are two latnelliform gills on each side of the body, whence the name of the class. Sometimes there is only one gill on each side. These gills, or plates, or branchiae, are composed of tubular rods and a net work of capillary vessels. Externally they are furnished with vibratory cilia for the circulation of water over the surface. In some the margins of the mantle (the integu mentary covering in all mollusca, and which secretes the shell, see MOLLUSC.%) are
united to form a closed brauchial or respiratory chamber into which water is admitted and expelled by tubes called siphons. In others the margins of the mantle are free. The valves of the shell are bronglit together by one or two muscles, called adductors. Those having but one are called monomyaria, those having two, diniyaria. Their habits are various; some lie on the bottom, as the oyster and scallop; others are fixed to objects, as mussels; others are sunk several inches deep in the sand on the sea-shore; others bore holes in rocks or wood, while many are free and locomotive. The lamelli branchiates are divided into two sections, with respect to the respiratory- organs; those having the margins of the mantle free, without siphons, are with two exceptions called asiphonida, the other siphonida. Loth sections comprise 21 families.