This important change in respiration is the chief modification that the animal organism underwent in passing from the water to the solid land. The first consequence was the formation of lungs for breathing air; up to that time the gills alone had served for respiration. But there was at the same time a great change in the circulation and its organs; these are always very closely correlated to the respiratory organs. Moreover, the limbs and other organs were also more or less modified, either in consequence of remote correlation to the preceding or owing to new adaptations. (FIGURE 2.252. Tooth of a gigantic shark (Carcharodon megalodon), from the Pliocene at Malta. Half natural size. (From Zittel.)) In the vertebrate stem it was unquestionably a branch of the fishes--in fact, of the Ganoids--that made the first fortunate experiment during the Devonian period of adapting themselves to terrestrial life and breathing the atmosphere. This led to a modification of the heart and the nose. The true fishes have merely a pair of blind olfactory pits on the surface of the head; but a connection of these with the cavity of the mouth was now formed. A canal made its appearance on each side, and led directly from the nasal depression into the mouth-cavity, thus conveying atmospheric air to the lungs even when the mouth was closed. Further, in all true fishes the heart has only two sections--an atrium that receives the venous blood from the veins, and a ventricle that propels it through a conical artery to the gills; the atrium was now divided into two halves, or right and left auricles, by an incomplete partition. The right auricle alone now received the venous blood from the body, while the left auricle received the venous blood that flowed from the lungs and gills to the heart. Thus the double circulation of the higher vertebrates was evolved from the simple circulation of the true fishes, and, in accordance with the laws of correlation, this advance led to others in the structure of other organs. (FIGURE 2.253. A Devonian Crossopterygius (Holoptychius nobilissimus, from the Scotch old red sandstone. (From Huxley.) FIGURE 2.254. A Jurassic Crossopterygius (Undina penicillata), from the upper Jurassic at Eichstatt. (From Zittel.) j jugular plates, b three ribbed scales. FIGURE 2.255. A living Crossopterygius, from the Upper Nile (Polypterus bichir). FIGURE 2.256. Fossil Dipneust (Dipterus Valenciennesi), from the old red sandstone (Devon). (From Pander.) FIGURE 2.257. The Australian Dipneust (Ceratodus Forsteri). B view from the right, A lower side of the skull, C lower jaw. (From Gunther.) Qu quadrate bone, Psph parasphenoid, PtP pterygopalatinum, Vo vomer, d teeth, na nostrils, Br branchial cavity, C first rib. D lower-jaw teeth of the fossil Ceratodus Kaupi (from the Triassic).)