The second great change which required to be made was of course in the method of breathing. An ordinary fish, when taken out of the water, dies of suffocation, because its gills become inefficient for respiration as soon as they become dry. An entirely new type of organ had therefore to be evolved, and this occurred on the same lines as in the Dipnoi, by the development of a pair of sacs from the upper part of the digestive canal, in which the blood is made to circulate, and which are kept filled with air taken direct from the atmosphere. It is of course very well known that an ordinary amphibian is not a lung breather throughout its whole life. The metamorphosis of a gill-breathing tadpole into a lung-breathing frog, illustrated in Fig. 85, is a phenomenon with which everyone is familiar. And this condition, in which a change in the mode of life is made by each individual in the course of its development, is the typical one. But the modern amphibians include types ranging from completely water to perfect land forms. Some, like the Austrian Olm (Fig. 86), are gill breathers throughout their whole life. One which is normally of this type, the Axolotyl, illustrated in Fig. 87, can be made to acquire lungs and assume a land mode of life. Others, which normally make the metamorphosis, can be prevented from doing so by being confined to the water, and complete their life-histories in the condition of gill breathers. In still other forms (_e.g._ the Cœcilians, Fig. 89) the change is made before the young creature leaves the egg, and the independent life is commenced in the condition of a land animal. Correlated with the development of the lungs is a change in the structure of the nostrils, from the condition of blind sacs, as they occur in the fishes, to that of air passages, communicating with the upper part of the alimentary canal, and thence with the lungs. [Illustration: FIG. 86.--The Olm--_Proteus anguincus_.] [Illustration: FIG. 87.--Axolotyl. The ordinary gill-breathing form, and the artificially developed land form.] [Illustration: FIG. 88.--Amphibians--The Fire Salamander.] The living forms of the Amphibia differ considerably from the types which constituted the group in those long-distant ages when it was in the heyday of its prosperity. The latter forms were characterised especially by a system of armour-plating over the head, which frequently extended under the breast and even covered the greater part of the lower surface, and which appears to have formed a protection against the multitude of sharks which populated the waters in which the amphibians partly lived. The armour-plated type has long ago become extinct, but it is in it, rather than in the modern forms of Amphibia, that we must look for the direct ancestor of man. A fossil of this earlier group is shown on Fig. 90. [Illustration: FIG. 89.--The Ceylon Cœcilian, _Ichthyophis glutinosa_, with eggs.]