It is no less wonderful how perfectly the size and texture of the fins corresponds with the habits and necessities of the different species of fishes. Those which traverse vast portions of the ocean, or have frequently to struggle against swelling waves, are furnished with large and strong fins, while these organs are soft in the species which confine themselves to greater depths, where the winds cease to disturb the waters. From the great variety which is met with both in the number and position of the fins, they are also of the greatest use in the classification of fishes, and afford the naturalist many of the chief characters which serve to distinguish the several orders, families, genera, and species of these aquatic vertebrates.[M] [Footnote M: Cuvier divides the fishes into: I. Chondropterygii--Skeleton cartilaginous; fins supported by cartilaginous rays; and II. Osteopterygii--Skeleton composed of true bone. The Chondropterygii are subdivided into three orders: (_a_) Sturionidæ (sturgeons), with free gills. (_b_) Selacii (rays, sharks), with gills fixed and a mouth formed for mastication. (_c_) Cyclostomata (lamprey, myxine), with gills fixed and a mouth formed for suction. The osseous fishes, which are far more numerous, are subdivided into six orders: (_a_) Acanthopterygii; distinguished by the stiff spines which constitute the first fin-rays of the dorsal fin, or which support the anterior fin of the back in case there are two dorsals. In some cases the anterior dorsal fin is only represented by detached spines. The first rays of the anal fin are likewise spinous, as well as the first ray of the ventral fin. To this extensive order, which comprises about three-fourths of the osseous fishes, belong, among others, the families of the perches, gurnards, mackerels, mullets, breams, gobies, blennies, &c. The three following orders of the osseous fishes have the rays that support the fins soft and composed of numerous pieces articulated with each other, with the exception in some cases of the first ray of the dorsal, or of the pectoral. Their leading character is afforded by the situation or absence of the ventral fin, which in the (_b_) Malacopterygii abdominales are suspended beneath the abdomen, and behind the pectorals; in the (_c_) Malacopterygii subbrachiales beneath the pectorals; and in the (_d_) Malacopterygii apodes are totally wanting. To the abdominal soft-rayed fishes belong the herring, salmon, pike, sly, and carp families; to the subbrachial, the cod family, the side-swimmers, and the lump fishes; and, finally, to the apodal malacopterygians, the single family of the anguilliform fishes. The small order of the (_e_) Lophobranchi comprises the pipe-fishes, sea-horses, in whom the gills are not pectinated, as in the preceding subdivisions, but consist of little round tufts; and, finally, the