SAUROPSIDA. This name was introduced by T. H. Huxley in his Introduction to the Classification of Animals (1869), to designate a province of the Vertebrata formed by the union of the Aves with the Reptilia. In his Elements of Comparative Anatomy (1864) he had used the term " Sauroids" for the same province. The five divisions of the Vertebrata Pisces, Amphibia, Reptilia, Aves, and Mammalia are all distinctly definable, but their relations to one another differ considerably in degree. Whilst it was Huxley's great merit to emphasize by the term Sauropsida the close and direct relationship between the classes of reptiles and birds, it was an unfortunate innovation to brigade the Amphibia and fishes as Ichthyopsida, thereby separating the Amphibia much more from the reptiles than is justifiable, more than perhaps he himself Intended. The great gulf within the recent Vertebrata lies between fishes, absolutely aquatic creatures with internal gills and " fins " on the one side, and on the other side all the other, tetrapodous creatures with lungs and fingers and toes, for which H. Credner has found the excellent term of Tetrapoda. Another drawback of Huxley's divisions resulted in the tendency of alienating the Mammalia, the third division, from the reptiles whilst trying to connect their ancestry with the Amphibia, a view which even now has some vigorous advocates.
The characters which distinguish the Sauropsida, that is, which are common to birds and reptiles, and not found combined in the other classes, have been thus summarized by Huxley : no branchiae at any period of existence; a well-developed amnion and allantois present in the embryo; a mandible composed of many bones and articulated to the skull by a quadrate bone; nucleated bloodcorpuscles; no separate parasphenoid bone in the skull; and a single occipital condyle. In addition to these principal characters others exist which are found in all birds and reptiles, but are not exclusively confined to them. The oviduct is always a Mullerian duct separate from the ovary and opening from the body cavity. The adult kidney is a metanephros with separate ureter; the mesonephros and mesonephric duct become in the adult male the efferent duct of the testis. The intestine and the reproductive and urinary ducts open into a common cloaca. There is usually an exoskeleton in the form of scales; in the birds the scales take the form of feathers. There are two aortic arches in reptiles, in birds only one the right. The heart is usually trilocular, becoming quadrilocular in crocodiles and birds. In all the eggs are meroblastic and large, possessing a large quantity of yolk; in all the egg is provided in the oviduct with a layer of albumen and outside this with a horny or calcareous shell. In a few cases the egg is hatched in the oviduct, but in these cases there is no intimate connexion between the embryo and the walls of the duct. Fertilization takes place internally, occurring at the upper end of the oviduct previously to the deposition of the albuminous layer and egg shell.
Comparative anatomy clearly shows that birds are closely allied to reptiles; enthusiasts even spoke of them as " glorified reptiles," and this view seemed to receive its proof by the discoveries of Archaeopteryx (q.v.), and the numerous bipedal Dinosaurs. But Arcnaeopteryx was after all a bird, although still somewhat primitive, and the question, what group of reptiles has given rise to the birds? is still unanswered. By irony of fate, mere lack of the fossil material, it has come to pass that the bridges between Amphibia and reptiles and from them to Mammals are in a fairer way of reconstruction than is that between reptiles and birds, the very two classes of which we know that they " belong together." (H. F. G.)
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)