A not unapt comparison has been made of these Mollusks to a tortoise, the mantle representing the back shield, and the foot the breastplate. They are more or less oval in outline, flattened, with the back convex; the mantle is fleshy, and projects considerably on all sides; the head is placed beneath its front margin, and carries two superior tentacles with a pair of minute eyes, sunk beneath the skin at the outer part of their bases.
GILL-PLUME OF PLEUROBRANCHUS. The upper part of the head is developed into a broad triangular veil, the lateral edges of which carry another pair of tentacles. These organs, as well as the superior pair, are imperfectly tubular, and are formed by the bending together of their sides, which meet and overlap, but do not unite. The mouth is a short, fleshy proboscis, provided with horny jaws and a long ribbon-like tongue, armed with teeth, as in the Nudibranchs. The branchial plume projects from between the mantle and foot; in crawling it is transverse, and appears, when viewed from above, to be composed of many triangular laminæ, overlapping and pointing backwards. Each lamina shows transverse wrinkles. Viewed laterally, it is seen to consist of a central stem, with about eighteen pinnæ on each side, each pinna being again pinnated on each side. The stem,