The Vertebrate animals are those which have a brain and spinal cord inclosed within a bony case, an internal skeleton, to which are affixed the muscles, and which is moved by them; they all possess four extremities, more or less modified in form, with the exception of the Serpents. They are divided into four classes, Fishes, Reptiles, Birds, and Mammals.
FISHES.
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.
Fish inhabit all kinds of water, the sea, rivers, and ponds, and many which live habitually in the sea come into rivers at certain seasons; and with respect to those which dwell always in the sea, some inhabit certain tracts only, and confine themselves to them, just as quadrupeds do to certain localities on the land. It has been a matter of wonder how the Fish get into ponds formed by the drainage of lands; but most ponds are supplied by springs which run underground from adjacent lakes or rivers, or by rills running on the surface, these bring the ova or eggs, which in favourable situations become hatched. Fish are exceedingly prolific, the eggs in the roe of a Codfish were estimated by Lewenhoec, to be upwards of 9,000,000, but not one in a thousand comes to maturity; there are enemies on all sides to devour them before they are hatched, and others to attack them afterwards. Some of the rivers of France have lately been artificially stocked with Fish by causing the fecundated spawn to be preserved till they are of a bulk and strength to protect themselves. This was effected by confining them in tanks floating in the water and perforated with holes, and afterwards placing them in a part of the river, inclosed both above and below by means of nets, so as to keep away their enemies—the larger Fishes.
Fishes were the first of the Vertebrated animals created, and as a proof of the incorrectness of the doctrine of development, these first-created Fish were of a class corresponding to the most highly organised Fishes of the present day.
Fishes are divided into two families: 1, Osseous Fishes (with a bony skeleton); 2, Cartilaginous Fishes (with a cartilaginious skeleton).
The Osseous Fishes are divided into six Orders:—
1. Acanthopterygii (Thorny-fin).
