The Pteropoda are defined by being free and pelagic, swimming by means of two wing-like appendages (epipodia), developed from each side of the anterior extremity of the body. The flexure of the intestine is neural.
As to the position of the Pteropoda in the Molluscan scale, they must be looked upon as inferior in organisation to any of the Gasteropoda, of which class they are often regarded as the lowest division. They permanently represent, from certain aspects, the transient larval stage of the Sea-snails.
The Pteropods are all of small size, and are found swimming at the surface of the open ocean, often in enormous numbers. Locomotion is effected by two wing-like fins, developed from the sides of the head, and composed of the greatly-developed "epipodia." The true "foot" is rudimentary and rarely distinct, but the "metapodium" is sometimes provided with an operculum (Limarinidae). There is usually a symmetrical, glassy, sometimes chitinous, shell (fig. 222), either consisting of a dorsal and ventral plate united, or forming a spiral (fig.215, B), but in some cases the body is naked, the mantle being absent or rudimentary. The head is rudimentary, and bears the mouth, which is occasionally tentaculate, and which is furnished with an odontophore. There is a muscular stomach and a well-developed liver; and the flexure of the intestine is neural, so that the anus is situated on the lateral or ventral surface of the body.
Fig. 222. - Pteropoda. a Cleodora pyramidata ; b Cuvieria columnella. (After Woodward.)
Fig. 223. - Hyalea tridentata, showing the shell and the lateral fins attached to the sides of the head.
The heart consists of an auricle and ventricle. The respiratory organ is very rudimentary, and consists of a ciliated surface, which is either entirely unprotected, or may be contained in a branchial chamber.
The ganglia of the nervous system "are concentrated into a mass below the oesophagus " (Woodward), united by a commissure above the gullet; and the eyes are rudimentary.
The sexes are united in all the Pteropods, and the young pass through a metamorphosis, having at first a bilobed ciliated veil attached to the sides of the head.
The Pteropoda are divided into two orders, termed Thecoso-mata and Gymnosomata ; the former characterised by possessing an external shell and an indistinct head; the latter by being devoid of a shell, and by having a distinct head, with fins attached to the neck.
The Pteropoda, as already said, are found swimming near the surface in the open ocean, and they are found in all seas from the tropics to within the arctic circle, sometimes in such numbers as to discolour the water for many miles. They are nocturnal in their habits, and, minute as they are, they constitute in high latitudes one of the staple articles of diet of the whale. They themselves are, in turn, carnivorous, feeding upon small Crustaceans and other diminutive animals. Though all the living forms are small, geology leads us to believe that there formerly existed comparatively gigantic representatives of this class of the Mollusca.