By many ornithologists the pigeons and ostriches are considered to form distinct orders, called respectively Columbæ and Cursores.
THE REPTILIA,
Or Reptiles, have neither hair, wool, nor feathers, and their bodies are either naked, or covered with scales. Some lay eggs, and some bring forth their young alive. Some have gills, and others lungs, but the latter have only a portion of the blood passing through them; and thus the blood of reptiles is cold, as it is respiration which gives the blood heat. The senses of reptiles are dull, and their movements are either slow or laborious. The following are the four orders into which this class is divided:—
1. Chelonian Reptiles. These animals have four legs. The body is enclosed in an upper buckler, called the carapace, and an under one, called the plastron. They have lungs which are much expanded; but they have no teeth, though they have hard horny jaws. The females lay eggs covered with a hard shell. The principal animals belonging to this division are the tortoises, which live on land or in fresh waters, and the turtles, which inhabit the sea.
2. The Saurian Reptiles. These animals have also expanded lungs, and generally four legs, but some have only two. Their bodies are covered with scales, and their mouths filled with teeth. This order includes all the crocodiles and lizards. The crocodiles have broad flat tongues, attached throughout to the jaws, and the lizards have long narrow tongues, which many of them can extend to a great distance from the mouth.
3. The Ophidian Reptiles are the snakes and serpents. The body is covered with scales, but it is destitute of feet. The lungs are generally well developed, only on one side. Serpents are frequently furnished with poison-bags at the base of some of their teeth.
4. The Batrachian Reptiles include the frogs and toads. The body is naked. The greater part of these reptiles undergo a transition from a fish-like tadpole furnished with gills to a four-legged animal with lungs. Others never lose their gills, though they acquire lungs, and of this kind are the siren and the proteus.
THE PISCES,
Or Fishes, are defined by Cuvier to be vertebrated animals with red blood, breathing through the medium of water by means of their branchiæ or gills. To this definition may be added, that fishes have no neck, and that the body generally tapers from the head to the tail; that most of the species are furnished with air-bladders which enable them to swim; and that their bodies are generally covered with scales. The heart has only one auricle, and the blood is cold. The gills require to be kept moist to enable the fish to breathe, and as soon as they become dry, the fish dies. Thus fishes with large gill openings die almost as soon as they are taken out of the water; while those with very small openings, like the eel, live a long time. Fishes have no feet, but are furnished with fins. The scientific knowledge of Fishes is called Ichthyology. Fishes are first divided into two great series, viz. the Bony Fishes, and the Cartilaginous Fishes, and these are again subdivided into nine orders, as follows:—