The Molluscs belonging to this Class have neither head, nor cephalic eyes, nor jaws or tongue like those of the other Classes, and are enclosed in a shell which consists of two plates or valves held together on one side of the margin by a horny, elastic substance, called the “_ligament_.” Bivalves do not creep about in search of food, but find their means of existence in the shape of minute particles, both animal and vegetable, which happen to be contained in the water which they breathe. Some, however, are capable of locomotion by means of a well-developed foot, and a few swim through the water by alternately opening and shutting their valves. The body is enclosed within two lobes of the mantle which line the interior of the valves, and which at their base are firmly attached to the shell, producing on the shell a scar or impression called the “pallial line.” The gills are lamellar or leaf-like, and placed on each side of the body. Each gill is called a ctenidium, and consists of an axis which is partly attached to the body of the Mollusc. This axis generally gives off two plates consisting of hollow filaments which are parallel with one another, directed downwards towards the ventral side, and in most cases long and refolded upon themselves, so that each plate becomes in reality a double lamella. In a few instances, however, the filaments are simple and not reflected. They are connected with one another by microscopic cilia, sometimes by vascular junctions, and the dependent and reflected portions (lamellæ) of each filament may be connected by “interlamellar vascular junctions.” The mouth is merely an oval aperture at the anterior end of the body, and generally furnished on each side with soft thin flaps, or labial palps, which have the function of conveying the food to the mouth. The mantle secretes the substance out of which the shell is formed. The two valves are always in contact at the _hinge_, which is generally formed by small interlocking projections or hinge-teeth, and they are closed by large adductor muscles, which are attached to impressions in the interior of the shell. When these muscles cease to act, as after death, the valves of the shell open in consequence of the elasticity of the ligament on the dorsal margin. The majority of species have two principal adductors, one at each end, like the Venus-shells, Cockles, Razor-shells, &c.; but in Oysters, Scallops, and a few others, there is but a single central muscle. All Bivalves are aquatic, and the majority marine. They are found burrowing in sand or attached to rocks. Some perforate stones and corals, others wood, and a few construct a sort of nest of fragments of shells, stones, &c.