The heart has one auricle and one ventricle, except in a single group which we shall afterwards mention. The heart is situated immediately behind the gills, to which the blood is pumped directly by the ventricle. From the gills, the blood is collected and distributed throughout the body, is re-collected and returned to the auricle. The circulatory system is provided with a set of blood-glands, essentially similar to those in man himself. There is a spleen, a thymus and a thyroid gland, and a pair of suprarenal bodies. The several functions of these glands form an extremely difficult chapter of physiology, but, broadly speaking, they are concerned in the formation of the white blood corpuscles, the removal of worn-out red corpuscles, and in certain obscure but important chemical changes in the composition of the blood. The blood itself consists of a fluid plasma in which float white and red blood corpuscles, the latter being flat and oval, and containing the same oxygen-carrying substance, hæmoglobin, as is found in mammals. The alimentary canal is simple. The mouth cavity is succeeded by the pharynx, the walls of which are perforated by the gill clefts. Next follow the gullet, the stomach, and the intestine, the division into the three portions being apparent often only after close examination. There are generally gastric glands, of simple form, a large liver, and almost always a pancreas. The kidneys and the reproductive organs open to the exterior by a common duct. A further characteristic feature of the fishes is their external covering of scales. True teeth, comparable to those of the higher vertebrates, appear first in this group. Some of the main features that we have mentioned are illustrated in Fig. 70. Careful study of the fishes makes it evident that they have very much in common with the higher groups of vertebrates. It is not too much to say, with Haeckel, that there is far more difference between Amphioxus and the fishes than between the fishes and man. There are four main divisions of the fish group. The first, that of the Elasmobranchs, comprises the sharks and dog-fishes, the skates and the rays. The second group, the Ganoids, includes the sturgeon and a few less well-known forms. The third, the so-called bony or food fishes, includes the vast majority of ordinary species, such as the salmon and trout, the cod, herring, eel, and all our ordinary freshwater species. The fourth, the 'lung fishes,' consists of three very remarkable species, which we shall later describe in detail. The mutual relationships of these groups is well understood, and it is possible to make fairly definite statements regarding their evolution.