The branch Mollusca includes the fresh-water mussels, the clams, oysters, snails, and slugs, the cuttlefishes, and all that host of animals we call "shells" or shell-fish, which we know familiarly only by the shell which they make, live in, and leave at death to tell the tale of their existence. Not all the molluscs, however, form shells, that is, external shells which serve as houses. The familiar slugs do not, nor do a number of ocean forms called nudibranchs, which are somewhat like the land-slugs, only much prettier and more attractive. All the cuttlefishes and octopi are also without the hard calcareous shell. But most of the molluscs are shell-bearing animals. The shell may be bivalved, as in the mussel and clam, or univalved, that is, composed of a single piece which may be spirally twisted, as with the snail, or otherwise curiously shaped. The variety in the form, colors, and markings of the shells indicates the great diversity among molluscs. Molluscs live on land, in fresh water and in the ocean. No depths of the ocean abysses are too great for the octopi, no coast but has its many shells, hardly a pond or stream is without its mussels and pond-snails, and in all regions the land-snails and slugs abound. =Body form and structure.=--The molluscs are not to be mistaken for any other of the lower animals; they have a structure peculiarly their own. In them the body is not articulated or segmented as with the worms and arthropods, nor radiate as in the echinoderms, nor plant-like as with the sponges and polyps. (Where the typical molluscan body is well developed it is composed of four principal parts: a head, with the mouth, feelers, eyes, and other organs of special sense; a trunk containing the internal organs; a foot which is a thick muscular mass not at all foot- or leg-like in shape, but which is the organ of locomotion by means of which the mollusc crawls; and a mantle which is a fold of the skin enclosing most of the body and which produces the shell. Such a typical molluscan body is possessed by most of the snails. But in most of the other molluscs one or more of these four body-regions are so fused with some other region as to be indistinguishable. In the mussels and clams the head is not at all set off from the rest of the body, the cuttlefishes and octopi have no foot, the slugs have no shell. In the case of some of the molluscs without external shell there are inside the body the rudiments or vestiges of a shell.