There is reason to suppose that each spe cies of Pteropod remains during the whole year in the same regions of the ocean. These regions are of different degrees of extent, and currents doubtless tend to enlarge their boundaries ; probably to this cause must be attributed the extensive diffusion of certain species met with in all climates ; whilst others of larger size are only found in the torrid zone, and others again of equal dimensions are peculiar to cold climates.
A table appended to the Memoir of 111. d'Orbigny assigns the limits between which each species has been found, and its nocturnal or crepuscular habits. From this table it appears, that of twenty-nine species of Ptero pods known to the author, fourteen are met with both in the Atlantic and Pacific oceans, whilst eleven are proper to the Atlantic and four to the Pacific ; of these seventeen are altogether nocturnal in their habits, and only eleven crepuscular.
The Pteropoda swim in a very peculiar manner. Their cephalic fins are only able to support them by a constant repetition of rapid movements, resembling those of the wings of a butterfly. These fins are kept in motion continually ; and, according to the di rection of their stroke, the animal advances horizontally, or mounts or descends, the body remaining all the time either in a vertical position or slightly inclined. Sometimes they keep spinning round without changing their place, or even keep at a certain height in the water without any apparent exertion ; but this power of remaining motionless has only been observed in a small number of species, the butterfly-movement of the wings being most commonly resorted to. If while they are thus in motion, the appearance of any strange body or even a sudden shock given to the vessel in which they are contained, causes them alarm ; their wings fold upon their bodies, or in some species are entirely withdrawn into their shell, and the animal sinks rapidly to the bottom of the vessel. Most probably, when at liberty, as soon as the creature has sunk to a sufficient depth to ensure safety, it again unfolds its wings, and sustains itself in the water instead of allowing itself to go quite to the bottom.
The Hyalea and Cleodora swim with the greatest rapidity, in Pneumoderma and Clio the movements are less vivacious.
The larger Pteropods seem to feed prin cipally upon smaller species of their own class, as well as upon the minute crustaceans that swarm in the seas they frequent.
Cuo. Integument.— The skin of the Clio is not smooth, but studded with numerous little wart-like eminences, causing a roughness, which is in direct relation with the red colour of the integument, and is consequently most conspicuous near the extremity of the tail. Both the roughness and the red colour in deed are produced by the presence of a mul titude of little cavities or sacculi filled with an oily red pigment, the pointed ducts of which project externally. These pigment-sacs are not only most abundant near the extremity of the tail, but in that part of the body are of larger size than elsewhere : they are all flask-shaped, opening upon the surface of the body by a narrow neck, while their larger ex tremity is imbedded in the subcutaneous cel lular tissue. Beneath these larger pigment sacs smaller ones of a similar description are perceptible, much smaller in their dimensions than the preceding, and in many places where the larger ones are deficient, the smaller pig ment cells are proportionately more nume rous : both kinds are filled with the same oil like colouring material, and are apparently comparable to simple mucous follicles, only their secretion is of a more oily character.
With the exception of the pigment cells, the integument of the living Clio is quite trans parent, but after being kept in spirits of wine, its transparency is cerisiderably diminished ; in its substance, muscuiar fasciculi are perceptible, the direction of which is principally towards the crucial muscles of the fins. Upon the dorsal region of the body, these tegumentary muscles first become distinct at the trans verse constrictions above referred to. These constrictions disappear as soon as the skin is cut through, and the inner layers of the dorsal region then appear quite lax. In this way, indeed, the existence of transverse fas ciculi of cutaneous muscles is rendered evi dent, even when their presence cannot be proved by direct observation.