Page:Natural History, Mollusca.djvu/84

72 very fragile; nearly in the form of a triangular pyramid; with an aperture at its base, from which proceeds a long and slender glassy spine; and a similar spine projects from each side of the middle of the shell. The hinder part of the animal is globular and pellucid, and in the dark vividly luminous, presenting a singularly striking appearance, as it shines through its perfectly transparent lantern. Both of these are found floating in great numbers on the surface of the tropical sea.

Others are entirely destitute of a shelly covering, as is that little species which occurs in enormous profusion in the Arctic Seas, and which we now proceed to describe.

These little creatures have an oblong membranous body, without a mantle; a head formed of two rounded lobes, each of which is furnished with three long tentacles, capable of being withdrawn into a fold of skin, or protruded at pleasure. The mouth, which is terminal, has two small fleshy lips; and two eyes, of elaborate structure, are placed at the back of the neck.

The species best known is that which is commonly called by our northern voyagers, Whale-food (Clio borealis). Though not more than an inch in length, it occurs in such countless millions as to form the principal part of the nourishment required by the most gigantic of living creatures. The Clio bears some slight resemblance to a butterfly just emerged from the chrysalis, before the wings are