The shells which have been noticed in the preceding chapter belonged literally to the stay-at-homes of the family. They rarely wander far, and many, as we have seen, never leave the place which the young shell first selected as its home. What are known as the univalves, the mollusks with one shell, or perhaps no shell at all, are the reverse of this, being in many instances travelers, wandering here and there. This suggests that they have more highly organized locomotive organs. Those shown on the upper part of Figure 94 are univalves, and if we make a section of a univalve (Fig. 95), we see that the shell is much more complicated than in the previous forms. The univalve has a shell-secreting mantle and organs resembling those of the bivalves, only differently placed. This marvelous mantle performs some singular feats, judging from the spines found on many shells. To make these, the mantle must have been thrown outward and upward, forming a tube in which the spine was secreted. [Illustration: FIG. 95.--Section of a univalve.] In the univalves a distinct head is seen (Fig. 98) with tentacles and prominent eyes. The foot is now elaborated into a huge sucking, clinging, disklike organ. In the whelk it is as long as the shell, the latter being perched high above it, presenting a remarkable spectacle as it moves along the sandy floor of the ocean. On the head are two tentacles, feelers or sense organs, and sometimes the eyes are mounted on tall stalks, that the shell may have a wide range of vision. A siphon, such as we have seen in the clam, is present and extended upward and forward. It protrudes from a canal formed in the shell for the purpose, and is often very long. If the whelk (Fig. 99) is disturbed, it suddenly withdraws its body, including the enormous colored foot; and if the shell is picked up, the entrance is found securely closed by a horny door called the operculum, which is attached to the foot (Fig. 99). This door takes many shapes. In the beautiful conch it is saber-shaped, and is used to dig into the sand, or, as a lever, to force the conch along by a series of jerks. In other shells it is apparently made of porcelain, hard and highly polished. It is well known as the "eye stone" of popular fancy. [Illustration: FIG. 96.--Tongue and teeth of a univalve: _A_, portion of tongue of Velutina; _B_, portion of tongue of whelk; _C_, head and tongue of limpet; _D_, portion of same enlarged.]