The Elasmobranchs are at once the most primitive and, so far as is known, the oldest of the four. From these evolved the lower Ganoids, which then divided into two main branches, the first of which led up to the higher Ganoids and through them, at a comparatively late date, to the bony fishes. The second led to the lung fishes and, either through them or along a somewhat parallel line, to the amphibians and the land vertebrates generally. It is with the second line, therefore, that we shall be mainly concerned. The Elasmobranchs are characterised by the fact that the gill slits open individually to the exterior, there being no gill cover, such as is found in the other groups. Their scales are simple, tooth-like projections, and in fact there is no essential difference between them and the teeth. The skull is more primitive than in the other groups, but a discussion of its details would necessarily be very involved. The living members of the group show a fairly high stage of development of the vertebræ--considerably higher, in fact, than that found in the lung fishes--but some extinct members showed a very primitive condition with regard to this point. In the fossil skeleton shown in Fig. 71, for instance, it is apparent that the notochord was present as a simple continuous rod. The skeleton in question is from the Permian and belongs to what is regarded as the most primitive type of fish known. Two specimens of Elasmobranchs are shown in Figs. 72 and 73, and the teeth of a shark in Fig. 74. [Illustration: FIG. 71.--Fossil skeleton of _Pleuracanthus Decheri_, a primitive Elasmobranch.] [Illustration: Photo: Thiele. FIG. 72.--Spotted dog-fish.] [Illustration: Photo: Thiele. FIG. 73.--Blue Shark.] [Illustration: FIG. 74.--Teeth of Shark.] In the Ganoids and bony fishes there is a gill cover, and in all but a few Ganoids there is some formation of true bone, whereas in the Elasmobranchs the skeleton is wholly cartilaginous. One of the most striking anatomical features of these groups, and one which distinguishes them from the Elasmobranchs, is the presence of a swim bladder, a large sac-like outgrowth from the upper part of the gut. The function of the swim bladder is that of regulating the specific gravity of the fish, which becomes greater or less according as air is expelled or taken in. The Ganoids and bony fishes are illustrated in Figs. 75 to 79. [Illustration: Photo: Underwood. FIG. 75.--Ganoid fishes--The Sturgeon.]