Upon the upper surface of the foot, in many Gastropods, a flat hard structure termed the _operculum_ is situated, which, when the animal is retracted, partly or entirely closes the aperture of the shell. In some cases, as in the _Turbos_, it is very strong and of a stony nature, but in most instances it is horny. It is differently constructed in distinct families: it may be annular and multispiral, annular and paucispiral, subannular and ovate, or subannular and unguiculate. In the _Nerites_ it is shelly, somewhat semicircular, closes the aperture of the shell, and is furnished with a stout projection on the straight edge, fitting like a hinge under the inner lip of the shell. A series of _opercula_ is exhibited in side table-case C. [Sidenote: The breathing-organs.] Thread-like processes on either side of the body, the so-called gill-filaments, often unite with those in front of and behind them, and so give rise to plates; these, when well developed, are best seen in the division to which the Oyster and the Mussel belong, and which, therefore, has been called the division of the plate-gilled Molluscs, or _Lamellibranchia_. Where the body is coiled or twisted on itself, as so often happens, the gills of one side may be altogether lost. Sometimes, as in _Phyllirhoë_, when the body is small and its wall thin, the gills (_ctenidia_) disappear altogether, and there is no special breathing-organ; in others the loss of the gill is compensated by the formation by the mantle of a lung; this is most often seen in the forms that live on land. But these so-called gills may have other functions: in the Lamellibranchs, where there is no head and no special means by which the creature can obtain food, the delicate waving filaments or _cilia_ with which they are covered cause currents in the surrounding water, by means of which minute organisms are brought to the mouth. [Sidenote: The radula.] All Molluscs, except the Lamellibranchs, have a very remarkable structure developed in the floor of their mouth-cavities; on a basis of cartilage, which may be moved backwards and forwards by muscles, there is developed a horny plate, which may be of considerable length, and which has its upper surface covered with a number of more or less fine, flattened, or spiny outgrowths, which are known as teeth. This is the _odontophore_, _tongue_, _radula_, or lingual ribbon (see fig. 3).[1] [Sidenote: The eyes.]