The Fishes live in water, and are unable to live in the air; they receive oxygen from the water, but this is received much more slowly than by air-breathing animals, and as the combination of oxygen with the carbon of the system gives the elevated temperature of animals, so in Fishes, the oxygen being received slowly, their temperature rises but little above the temperature of the water they inhabit; they have thence received the name of "cold-blooded." Fishes live upon the marine animals which they capture, the various sea-weeds which grow in the water, and insects which fall into, or breed there. They have fins instead of legs, but the two pairs of fins known as "Pectoral" and "Ventral" are analogues of them, and in a pectoral fin may be found bones analogous to all the bones of the arm and hand of Man, or the fore-feet of the other Vertebrata. Fishes urge themselves through the water chiefly by the action of the tail from side to side, and in those members which have more extended forms (as Eels), by a wavey motion of the body itself; and the rapidity with which they swim is such, that Sharks have been known to go round the ship they are following many times in a very short space, although it may be sailing at a great speed before the wind. Fishes are covered with scales, and the head is united to the body without any constriction which can be considered as a neck; they are mostly of a form such as can easily pass through the water, without offering much resistance, and their specific gravity is so nearly that of water, that, by the compression of a small vessel filled with air, called the swimming bladder or "sound," they can rise and sink with little effort. Many tribes of the human family, such as the Esquimaux and Polynesians, almost live upon Fish, and even in Great Britain, in fishing districts, it forms a great part of the food of the inhabitants, and their capture gives employment to thousands.