The lung fishes or Dipnoi present a curious mixture of primitive and of highly advanced characters. In their persistent notochord and their inconsiderable formation of bone, they are much more primitive than the food fishes. On the other hand, an extremely important departure is seen in the adaption of the swim bladder as a respiratory organ. In one of the three existing species this organ is single, in the others it is double. The wall of the swim bladder is thick, and contains considerable muscle tissue. Its inner surface is covered with a complex system of pits and blind sacs, the walls of which contain numerous capillary vessels. [Illustration: FIG. 76.--Ganoid fishes--_Polypterus bihir_.] [Illustration: FIG. 77.--Ganoid fishes--American bony pike, Lepidosteus.] [Illustration: Photo: Thiele. FIG. 78.--Bony- or food-fishes--The Salmon.] [Illustration: FIG. 79.--Bony- or food-fishes--The Common Eel. Photo: Underwood.] [Illustration: FIG. 80.--Lung fishes--Australian lung fish, Ceratodus.] [Illustration: FIG. 81.--Lung fishes--Protopterus (Africa).] [Illustration: FIG. 82.--Lung fishes--Lepidosiren (South America).] [Illustration: FIG. 83.--Swim bladder (lung) of Protopterus.] There are three living species of lung fishes, one of which is found in Australia, another in Tropical Africa, and a third in the tributaries of the Amazon. All live under conditions which make ordinary respiration by gills difficult. The Australian species inhabits rivers which become reduced, in the dry season, to stagnant pools of foul water, in which ordinary fish frequently fail to survive. Under such circumstances the creature comes periodically to the surface to breathe. The other two species live in rivers which actually dry up in summer, and the fishes bury themselves in the mud, and remain in a torpid condition, breathing air by their lungs, until the rainy season comes round, perhaps four or even six months later. Correlated with the special method of respiration is a special type of blood system, whereby part of the blood is pumped direct to the lungs, and returns direct to the heart. There are two auricles, to receive the blood from the lungs and the general circulation respectively, but only one ventricle, in which the two streams become mixed. Figs. 80 to 82 illustrate the three existing Dipnoi, and the structure of the lung is shown in Fig. 83. It is obvious, from the distribution of the lung fishes, and also from geological evidence, that the group was once very plentifully represented, and has only been preserved from total extinction by peculiar circumstances. Regarding the position of the group, some zoologists regard them as the direct ancestors of the Amphibians. Others believe that the group had a common origin with the bony fishes and the Amphibia in some early form of Ganoid. In any case, the Dipnoi possess an extraordinary interest as showing the beginnings of adaption to a life out of the water. CHAPTER V