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Reptiles. A class of vertebrate animals, which comprises the tortoises, crocodiles, snakes, lizards, etc. Reptiles are more closely related to birds than any other group of animals; and in some recent classifications birds and reptiles together are made to constitute the great division Sauropsida of the sub-kingdom vertebrata. Reptiles agree with birds and differ from all other vertebrates in the following characters: The skull articulates with the spinal column by a single condyle; the lower jaw articulates with the skull by the intervention of a peculiar bone, termed the os quadratum, or quadrate bone, and each half of the lower jaw consists of several pieces; at no period of their existence are branchial or water-breathing respiratory organs developed. The heart in reptiles consists of three chambers - two auricles and a ventricle, the latter bcing divided into two portions only by a partition which is generally incomplete, and allows the arterial and venous blood to mix, so that the blood is never so perfectly aerated as in the higher animals. The blood is consequently much colder than that of birds and all the more highly organized animals, where the oxygen obtains a freer access to its particles. In the crocodiles the heart has a complete septum, but there is an intermixture of the venous and arterial blood outside the heart. In consequence of this organization of the circulatory system, the whole character of reptiles differs from that of the higher animals. The cavity of the thorax, or chest, in reptiles is not shut off from the abdomen by a complete muscular partition or diaphragm, though traces of it are found in crocodiles. The lungs are usually less cellular than in birds and mammals, but are often of large size, extending into the abdominal cavity. In snakes there is usually only one active lung, the other being rudimentary or completely atrophied. The rectum opens in a common cavity, or cloaca, which receives both excrementitious matters nnd tbe. products of the generative organs. Reptiles are often provided with an exo-skeleton, or hardened skin, consisting of horny plates or scales. The strong and conspicuous outer shell of the body of tortoises and turtles is formed by this exo-skeleton uniting with the true endo-skeleton. Ribs are always present, but differ much in form. Teeth are generally present, but are not sunk in distinct sockets, except in crocodiles. They are perpetually renewed as fast as they wear out. The tortoises and turtles, however, are toothless, but have jaws sheathed with horn like the beak of a bird. The jaws have sharp cutting edges for cutting the food into pieces small enough for swallowing. The young of reptiles are produced from eggs, mostly hatched after being laid, but in some cases the eggs are hatched within the body.