The Pteropoda, or wing-footed mollusca, are very small; they are incapable of crawling or fixing themselves to solid objects, but they are furnished with two fins like the wings of a butterfly, with which they float or row themselves about in the ocean, far from land in vast multitudes. The shell of the typical species Hyalæa (A, fig. 177), which resembles the thinnest transparent glass, consists of two valves, one, which is placed on the front of the animal, is long, flat, and ends in three points; the other valve, which is applied to the back, is short and convex, and in the lateral fissure between the two, the mantle is protruded. The head and fins project from an opening at the top of the shell. The fins, which are formed of muscular fibre, are fixed on a short thick neck, with the mouth lying between them, containing a tongue crossed by rows of long reversed teeth. The head has no tentacles, and the animal appears to be blind, but it has an auditory vesicle lined with cilia, which keeps a few otolites in motion. This little animal is highly organized; it has a gullet, a kind of crop and gizzard, a liver, a respiratory tube, a heart, a circulating and nervous system, which enables it to swim with a flapping motion of its fins. [Illustration: Fig. 177. A, Hyalæa; B, Clio.] The Clio pyramidata (B, fig. 177) is an elegant animal belonging to the same class. Its fragile transparent shell has the form of a triangular pyramid; and from its base proceeds a slender spine, and a similar spine extends from each side of the middle of the shell. The posterior part of the body is globular and pellucid, and in the dark it is vividly luminous, shining through the glassy shell. The fins of the Hyalæa and Clio or Cleodora are of a bright yellow, with a deep purple spot near the base. Both are inhabitants of the ocean. [Illustration: Fig. 178. Clione borealis.] The Clione borealis (fig. 178), which exists in millions in the Arctic Seas, is the most remarkable instance of the Naked Pteropods. It has neither shell nor mantle; its membranous body is not more than half an inch long, its head is formed of two round lobes, on each side of the neck there is a large muscular wing or fin; in swimming the animal brings the tips of the fins almost in contact, first on one side of the neck and then on the other. In calm weather, they come to the surface in myriads, and quickly descend again. There is a pair of slender tentacles close to the head, which are organs of feeling, a pair of eyes are placed on the back of the neck, and acoustic vesicles lined with cilia keep otolites in motion. Besides these organs of sense, the Clione has respiratory, digestive, and nervous systems. The latter consists of a nerve-collar round the gullet, with two nerve masses in its upper part, so the Clione is well supplied with nerves.