The _Ophidia_ have many enemies, as the well-known Mongoose among mammalia, also Swine, and various ruminating quadrupeds, as Deer and Goats. In the bird class, the famous Serpent-eater, or Secretary-bird of South Africa, is one of their chief destroyers; and there are various other Snake-devouring birds of prey, besides the great African Ground Hornbill,--even the Pea-fowl and sundry Storks and other waders. Comparatively large birds of the King-fisher family prey chiefly upon Snakes and Lizards in Australia; and of reptiles, besides those Snakes which prey upon other Snakes, the Monitor Lizards frequently seize and devour them. The series of Ophidians is arranged by our most eminent herpetologist, Dr. A. Günther, into five subordinate groups, which he characterises as follows:-- I. _Burrowing Snakes_, living under ground, only occasionally appearing above the surface. They are distinguished by a rigid cylindrical body, short tail, narrow mouth, small head not distinct from the neck, little teeth in small number, and by the absence or feeble development of the ventral shields. They feed chiefly on small invertebrate animals. Not any of them are Venemous. II. _Ground Snakes_, or species which live above ground, and only occasionally climb bushes or enter the water; their body is more or less cylindrical, very flexible in every part, and of moderate proportions. Their ventral shields are broad. They feed chiefly on terrestrial vertebrate animals. By far the greater number of Snakes belong to this category, and it is represented by many variations in all of the three sub-orders to be noticed presently. III. _Tree Snakes_, or species passing the greater part of their life on bushes and trees, which they traverse with the utmost facility. They are distinguished either by an exceedingly slender body, with broad, sometimes carinated, ventral shields, or by a prehensile tail. Many of the species are characterised by their vivid coloration, of which green forms the principal part. We shall see, in the sequel, that the first and third sub-orders offer numerous instances of Tree Snakes; the Tree Snakes of the second sub-order being confined to Tropical Africa. They feed on animals which have a mode of life similar to their own; only a few species on eggs. IV. _Fresh-water Snakes_, distinguished by the position of the nostrils, which are placed on the top of the snout, and by a tapering tail. They inhabit fresh-waters, and are, therefore, excellent swimmers and divers; only a few species (which also in external characters approach the following group, that of the true Sea Snakes) venture out to sea. They feed on fishes, frogs, crustaceans, and other water animals, and are viviparous. Not any of them are Venemous.