ARMORED FISH (Ganoidei)
These include, among others, the Bony Pike and the Sturgeons.
Bony Pike or Garfish.—A remarkable genus of fishes inhabiting North American lakes and rivers, and one of the few living forms that now represent the order of ganoid fishes so largely developed in previous geological epochs. The body is covered with smooth, enameled scales, so hard that it is impossible to pierce them with a spear. The common garfish attains a length of five feet, and is easily distinguished by the great length of its jaws.
Sturgeon (Acipenser).—These large, sluggish fishes, some reach a length of over ten feet, and live on worms, crustacea, and mollusks. The body is long and narrow with five rows of bony shields. There are many species of sturgeon, all confined to the northern hemisphere. They live in the sea and great lakes, and ascend the great rivers. All supply valuable commodities, for which they are regularly captured on a large scale. These commodities are their flesh, which is palatable and wholesome, their roe (caviare), and their air-bladders, from which isinglass is made.
The most important sturgeon-fishery in Europe is that of the Volga and the Caspian Sea. The flesh of the fish is salted, and caviare and isinglass made on a large scale from the roes and air-bladder.
The Sterlet (A. ruthenus) is a much smaller species, which is common in the Black and Caspian Seas, and ascends the Danube as far as Vienna. It is one of the principal objects of the sturgeon fishery on the Volga.
In America sturgeon flesh is eaten fresh, and caviare is made both in Georgia and in San Francisco; but there is no great fishery in any particular district, and the manufacture of isinglass does not receive much attention. The sturgeon of the great lakes (A. rubicundus) and the Shovel-nose of the Mississippi valley are the chief American species.
LUNG-FISHES OR DOUBLE-BREATHERS (Dipnoi)
are at present represented by three fresh-water types, the insignificant remnant of a group that was once dominant in the sea, and would have become entirely extinct if some of its members had not taken to live in the waters of the land. These types are the eel-shaped mud-fishes of West Africa (Protopterus) and South America (Lepidosiren), and a Queensland form (Ceratodus). In all these the swim-bladder has been converted into a regular lung, which returns purified blood to the heart. The African form lives in streams which are liable to dry up, and [229] were it not for the possession of a kind of lung capable of breathing air, it would perish during the dry season, whereas it remains embedded in the mud in a torpid state till the rains return.
The Queensland lung-fish lives under somewhat different conditions, for its native rivers do not entirely dry up, but are reduced to a series of deep holes connected by mere trickles of water. These holes become so foul from decaying vegetation and dead fish that the possession of a lung is a vital matter, and if the ceratodus were not able to come to the surface and breathe air it would probably succumb.