The mollusks or shells present a striking contrast to the worms. They have no joints, are soft, seemingly without form, and are very helpless creatures. The body is enveloped in a muscular coat or mantle, as shown in the oyster (Fig. 79). They have a nervous system, and a heart (_H_) which pumps colorless blood. Some have a foot for locomotion and eyes more or less well developed. The oyster represents a large group which have two shells, called bivalves. In Figure 80 we see the animal portion of the common snail, which illustrates another group with but one shell. These are called univalves. To the bivalves belong the shells best known, the oysters, clams, scallops, pectens, pearl oyster, razor shell, and many others, of which the oyster is the most familiar. The mantle, the soft, delicate lining, is the shell maker, and not only forms it, but repairs damages to it, piling up layer after layer of pearly matter called nacre. As there is a mantle on each side, two values are secreted. The sharp portion of the oyster is called the beak. Here the growth of the shell begins, and here are the marvelous valves which fit with such accuracy. These complicated parts are easily seen in the clam (Fig. 81). The hinge is joined by teeth (_c_, _d_, _d_,) which fit into cavities on the opposite valve, while the valves are held together by a perfect hinge, a horny ligament (_h_) that tends to open the shell or throw the valves apart. [Illustration: FIG. 81.--Bivalve shells: _a_, beak; _k_, base; _b_, _b_, hinges; _c_, _d_, _d_, teeth; _n_, ligament; _e_, _e_, adductor muscles; _l_, lines of growth; _f_, pallial line.] In the interior of all shells are seen certain scars; in others a purple mark. These marks (_e_, _e_,) indicate the location of a strong muscle by which the clam or oyster closes its shells and keeps them closed with such rigidity. In opening oysters the man severs this muscle and the shell opens, forced apart by its ligament. This explains why most shells found on the beach are wide open. The curious columnar objects in rows are the gills or breathing organs of the oyster, and are covered with little oars (Fig. 82), or cilia, which move to and fro, continually sweeping the currents of water along, bearing oxygen and food. The former is taken up by the gills to purify the blood, and the latter is swept into the mouth located near the lungs. [Illustration: FIG. 82.--Cilia or oars of a mollusk, highly magnified.] There is great variety in the hearts of shells. In the oyster (Fig. 79) it is composed of one auricle and one ventricle; but in other shells the heart may be three-chambered, or there may be two distinct hearts, each having two chambers. The eyes of the shells are very minute and are situated along the mantle. Those of the pecten are very beautiful and are distinctly visible, resembling gems or emeralds. [Illustration: FIG. 83.--A clam: _f_, foot; _m_, mantle; _s_, siphon.]