Page:Natural History, Mollusca.djvu/111

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(Naked-gilled Mollusks.)

Scarcely any of the animals which the marine naturalist meets with in his dredging voyages, or his sea-side excursions, are so attractive as those of this Order. They are remarkably elegant in their forms, which present great variety. Their motions are graceful and lively, their colours peculiarly brilliant, and their history and economy marked by points of great interest. Nor is it rarely that the zoologist is gratified with an opportunity of observing these very attractive Mollusks, for the species are numerous, the British shores alone yielding more than a hundred.

These are not air-breathers, like the members of the preceding Order; they are all marine, and respire by means of gills, which are not enclosed within the body, but (at least during action) exposed freely to the surrounding medium. Sometimes these organs are capable of being partially withdrawn into a cavity, situated on the medial line of the back; but more commonly they take the form of simple or branched warts, arranged along the sides. The foot is large and broad. The mantle is ample, and projects above the foot on all sides. None of the species are protected by a shell, except in early infancy. The young, on emerging from the egg, is enclosed in a shell, which closely resembles that of a nautilus, and bears very little resemblance to the parent in its form; it has,