. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. 452 ZOOLOGY. fera draw together their valves—a short interval existing, how- ever, when the animal is not alarmed, for the admission of water or air to the cavity of the shell. A very slight irritation will cause the animal to draw the shell close down on the rock, from which it is then very difficult to detach it. 786. The shell is formed, as in other Mollusca, by an exuda- tion from the mantle ; and is enlarged at intervals, in accord- ance with the increasing size of the animal. In some, the addition of an e
. Elements of zoology, or, Natural history of animals / ed. by Reese. Zoology. 452 ZOOLOGY. fera draw together their valves—a short interval existing, how- ever, when the animal is not alarmed, for the admission of water or air to the cavity of the shell. A very slight irritation will cause the animal to draw the shell close down on the rock, from which it is then very difficult to detach it. 786. The shell is formed, as in other Mollusca, by an exuda- tion from the mantle ; and is enlarged at intervals, in accord- ance with the increasing size of the animal. In some, the addition of an entire new interior layer, projecting beyond the old one, is made at every such period as in the Conchifera; but in others, the new matter is secreted only at the edge of the previous shell, and is joined on, as it were, to it; in these the line of addition is usually marked by a prominent rib on the exterior, but the interior is beautifully smoothed oft'. 787. The forms of the shell in this class vary extremely; but those which appear most widely separated may be shown to be connected by intermediate links. The open cone of the limpet may be regarded as one of the simplest forms. In an allied genus, the pileopsis, we find the point, pro- longed, and somewhat rolled upon itself; and, by various links of this kind, we are brought to the regular spiral of the snail. From this we may return to the long straight form, by the scalaria, in which the coils of the spire touch each other only by their ribs ; and by the vermetus and magilus, in which the commencement only of the shell possesses a spiral form, the remainder being pro- longed into a straight tube. When the shell is spiral, and the point and mouth are not in the same plane, a sort of central pillar is formed, like that round which a spiral staircase is constructed. This is called the columella; and it is usually grooved at its lower part for the passage of water to Murex Tenuispina. the respiratory organs which are placed
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