PTEROPODA (Gr. wmpov, a wing, was, a foot ; Fr. Ptcropodes ; Lat. llfollusca pin nata).—An order of Mollnscous animals es tablished by Cuvier, and named in accord ance with his arrangement of the Molluscous division of the animal kingdom, from the po sition of their organs of locomotion, which in the creatures we are about to examine is very remarkable. All the animals belonging to the order are marine, and in some regions of the ocean crowd the surface of the sea at certain seasons in immense numbers, swim ming by the aid of two muscular expansions resembling fins, which are attached to the op posite sides of the neck, and serve as paddles, although, in the language of Natural History; named feet.
Notwithstanding the multitudes of indi viduals belonging to this group, which are said to swarm both in the polar regions and in tro pical climes, the number of genera at present ascertained to exist is very limited, and such is their minute size and the delicacy of their structurk which precludes the possibility of studying them, unless in a fresh state, that, up to a very recent period, their anatomy was imperfectly understood, and, doubtless, much remains yet to be achieved by those who may be favourably situated or investigating them more closely.
The characters which they present in common, and by which they are separated by naturalists as a distinct group of Mollusca, are the following : — Their bodies are free, and organized for natation ; they are fur nished with a distinct head, but possess no locomotive organs, except a pair of lateral fins.
M. d'Orbigny, in a memoir read before the Academy of Sciences in Paris*, gives some interesting particulars relative to the organi zation and habits of this remarkable class of molluscous animals. They are met with in all seas, under the equator as well as in the vicinity of the polar circle ; and, being emi nently constructed for a pelagic life, never approach the shore. They are all, moreover, nocturnal or crepuscular, voyagers agreeing that they are never to be seen during a clear day when the sun shines brightly ; but towards five o'clock in the evening, when the weather is cloudy, two or three species begin to make their appearance at the surface of the water, generally belonging to the genus Hyalea.
As soon as twilight begins, large quantities of small Cleodorce, Hyalece, and Atlantce may be caught ; but the larger species only come to the top when night has set in ; at which time only the Pneunzodermas, the Mos, and the large Cleodorce can be procured. Certain species indeed only approach the surface on very dark nights, as, for example, the Hyalea balantiunz. Very soon all the smaller species again gradually disappear, as do the large ones a little later, and towards midnight a few stragglers only of different kinds are to be met with. At sunrise not a single Pteropod is to be seen, either at the surface, or at any depth to which the eye can penetrate. Each spe cies, in fact, seems to have its appropriate hours, or rather its appropriate degrees, of darkness.
M. d'Orbigny supposes, from these habits, that each species lives at a certain depth in the water which is proper to it, and where it is consequently exposed to a diminution of light proportionate to its distance from the surface. Every species, therefore, will only come to the top at that period of the twenty four hours when the obscurity approximates to that to which it is accustomed in its usual situation while the sun is .above the horizon, niounting gradually upwards as the light of day diminishes. If the Pteropoda remained all night at the surface of the sea, there might be reason to think, as M. Rang supposed, that they ascend at sunset for the purpose of ob taining food or fresh air in the most super ficial strata of the ocean ; but as these could be procured at all hours, it seems more pro bable that it is the light which thus regulates their movements.