Fishes naturally divide themselves into two series, according to the composition of their internal skeleton. This is usually osseous; nevertheless, a whole group of them constantly retain the cartilaginous or fibro-cartilaginous state. With some this frame presents even less power of resistance, and remains membranous.
It is precisely upon this peculiarity of structure that we found the great division of the class of fishes into two great groups of Cartilaginous and Osseous fishes, the first being again subdivided into three orders: namely, I. Cyclostomata, II. Selachia, III. Sturiona. The second into four orders: I. Plectognathii, II. Lophobranchii, III. Malacopterygii, IV. Acanthopterygii. Agassiz's system of classification of fish, founded on the form of the scales, is perhaps better suited than this to the palæontologist, but the one given above, founded as it is principally on the internal parts of the animal, is better suited to the zoologist. Agassiz's orders are the Ctenoid, type, Chromis, Placoid, Ganoid, and Cycloid.
CARTILAGINOUS FISHES.
Cartilaginous fishes are generally animals of considerable size, their structure ranging from ordinary fishes to eels. They are chiefly sea-fishes, only a few species being river-fishes. Naturalists divide them into two orders; namely, those having the gills free on the outer edge (the gilled Chondropterygeans), and those having these organs fixed on both edges. The first order comprehend three families: I. Cyclostomata, or Eels, Lampreys, &c., in which the mouth forms a sucker; II. Selachians, including Raias and Sharks, in which the mouth is furnished with jaws; III. Sturiona, or Sturgeons, which have the gills free.
I. Cyclostomata.
The first are characterised by the singular conformation of the mouth, which is formed for suction. The body is elongated, naked, and viscous, reminding us of serpents in their external form; they have neither pectoral nor ventral fins; their vertebræ are reduced to simple cartilaginous rings, scarcely perceptible one from the other, traversed by tendons, and covered by a second and more solid series of rings, which surround the soft cartilaginous spine. Their gills, in place of presenting the comb-like appearance of other fishes, have something of the form of a purse. The lampreys may be considered as the type of this family.
Fig. 348. The Lamprey (Petromyzon marinus).
The Lampreys (Petromyzon) are cylindrical, with seven gill-openings on each side of the neck, forming two longitudinal lines; mouth round, armed with many teeth. The Sea Lamprey, P. marinus (Fig. 348), belongs to the Mediterranean; it is also found in the German Ocean, and the friend who supplies this note has caught it with cockle bait in the South Esk, Forfarshire. In the spring it ascends the rivers, where it is sometimes caught in abundance. Full-grown it is about three feet long, marbled brown upon yellow; the dorsal fins are separated by long intervals; its mouth is circular and surrounded by a fleshy lip, furnished with cirri, having a cartilaginous plate for support; it is provided on its internal surface with many circular rows of strong teeth, some single, the others double.
The Lamprey feeds on worms, molluscs, and small fishes; its mouth is a powerful sucker, by the aid of which it attaches itself to fishes often of great size, and sucks them like a leech. It is taken by hook and line, and speared by a sort of barbed harpoon, like the trident of the mythological Neptune, which is thrown, javelin fashion, at the animal when seen at the bottom of the water; the flesh is fat and delicate. In the twelfth century one of our kings, Henry I., surfeited himself at Elbeuf by partaking too largely of the Lamprey. The river-lamprey resembles the above in its general conformation, but is much smaller, and differs in the armature of the mouth, having only a single circular row of teeth. It is blackish above, silvery beneath, and is common in the markets of London and Paris, being frequently taken in the Seine. A smaller species, about ten inches in length, never leaves the fresh waters. It resembles the last species in colour, but its two dorsal fins are continuous; it is found in most European rivers and brooks. In some of the English rivers they are frequently taken in the eel-pots, weighing two and three pounds. They frequent stony bottoms, where they find small animals on which they feed. In its larva state it was long considered to be a distinct species of Ammocætes; it is now, however, ascertained that it only acquires its perfect form at the end of its second or third year.