The common Cockle digs into the sand, and uses its foot both for digging and leaping; it is cylindrical, and when the Cockle is going to leap, it puts out its foot and bends it into an elbow; then having fixed the hooked point firmly in the sand, by a sudden contraction of the muscles it springs to a considerable height and distance, and leaps actively along the surface of the sand. The lowest part of fig. 166 is a magnified section of the foot, showing the muscular system which gives the animal that power. It consists of many rows of longitudinal muscles, interlaced at regular distances by transverse fibres. When the foot is extended, the Cockle has the power of distending it by filling a network of capillary tubes with water till it is almost transparent. The water is also distributed through the body and into the gill-chamber, which opens and shuts every ten minutes or oftener, in order to maintain the supply; and it has egress through the pores in the mantle and foot, for some burrowing mollusks squirt it out through the foot when disturbed. This water-system is unconnected with the circulation of the blood. [Illustration: Fig. 166. Foot of Cockle.] Each bivalve mollusk is both male and female; and the fertilized eggs pass into the gills of the parent, where they undergo a kind of incubation. At a certain time the yellow yolk of the egg is divided into a granular mass, which separates from the liquid albumen and produces cilia. The cilia cause the albumen to revolve round the interior of the egg; at last the granular mass revolves with it, while at the same time it rotates about its axis in a contrary direction at the rate of six or eight times in a minute. When still in the egg, all the organs of the little embryos are formed in succession, even the little valves of the shells are seen to open and shut, but the embryos are hatched before they leave the parent, and swim about in the cavity of the external gill. _Shells of the Mollusca._ When these mollusks come into the water, they soon find their transparent white shell too small, and begin to increase its size by means of the mantle, which is an exquisitely sensible fleshy envelope applied to the back of the animal, extending round its sides like a cloak, only meeting in front, and it is for the most part in close contact with the whole interior of the shell. Its edges are fringed with rows of slender tentacles, and studded with glands, which secrete the colours afterwards seen in the shell; the glands in the rest of the mantle secrete only colourless matter. When the animal begins to enlarge its shell, it attaches the borders of the mantle to the margin of the valves, secretes a film of animal matter, and lines it with a layer of mucus containing carbonate of lime and colour in a soft state, which soon becomes hard, and is then coated internally by the other glands of the mantle with colourless carbonate of lime.