very minute eyes--true eyes--dotted about on it), and always divided transversely to its length (not right and left) into eight separate pieces, which, indeed, seem to be really separate, independent little shells, corresponding to eight segments like the segments of a shrimp or an earth-worm. Let us now compare the soft animal of one of the bivalves--say the common cockle--with the soft animal to which a univalve shell belongs--say the limpet. They can be kept alive and watched in a finger-glass of sea-water, and can be removed from their shells and examined more closely--by killing them by dipping them for half a minute into very hot (not boiling) water. Both these molluscs--like all others--adhere tightly at one place to the shell. They cannot be removed from it alive, and make a new shell or creep back into the old one, as can some worms (_e.g._ the serpula) and other creatures which form a hard shell to live in. Certain muscles of the soft mollusc are so closely fixed to the shell that they must be torn in order to separate it. These muscles draw the two valves of the bivalve together, and shut it tight. You can verify this whenever the oyster-man "opens" an oyster for you. When at rest the shells gape, being kept open by the horny, elastic hinge-piece. Some bivalves (for instance, the common scallop, or pilgrim's shell, which can often be dredged in shallow water, and of which a large kind is sold in the London fish shops) actually swim in the sea-water by aid of this mechanism, the shells opening by elasticity and being closed by the muscle joining one to the other, at rapid intervals, flapping like the wings of a butterfly.