The mantle may form a free fold on either side of the body, as in the Bivalves, or it may become largely attached to the body-wall, as in the Snail or the Slug, and so give rise to an air-chamber, which, when its walls are richly supplied with blood, serves as a lung. The ventral surface of Molluscs is produced into the so-called “foot,” which may be very variously modified. The foot may be more or less hatchet-shaped, or curved and capable of serving as a leaping-organ, or sole-shaped and adapted for creeping; its margins may be produced into elongated processes, as the so-called arms of the Octopus, eight in number and provided with suckers, or of the Nautilus, where the arms are much more numerous, but shorter and without suckers. In the Cephalopods, also, another part of the foot may fold over from either side and form a median funnel, through which the water of respiration is driven outwards, causing the animal to move in the opposite direction—this part of the foot having, therefore, still the function of an organ of locomotion. By means of their muscular foot the Solenidæ, or Razor-shells, burrow in the sand, the Pond-Snails (Limnæidæ) crawl on aquatic plants and swim reversed on the surface of the water, the Limpet clings to the rock, and the Cockles and Trigonias take surprising leaps.

The operculum.

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.

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.

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]]

The eyes.

Eyes may be absent, as in nearly all the headless Lamellibranchs; but in other Molluscs they are generally present, and may be more or less well developed. An instructive series of stages is exhibited by the Cephalopoda. In Nautilus the eye remains an open pit; in Ommatostrephes two chambers appear, the anterior of which is bounded posteriorly by the lens, and is open to the exterior, so that sea-water enters it; in Sepia, finally, the anterior chamber becomes closed in front. We may observe that the eyes of all Cephalopods are at first pit-like, or pass through a stage which is permanent in Nautilus, one of the geologically oldest types.