This lymphatic system of the vertebrated animals is, however, expressed in technical language, only the differentiated interstitial sinuses of the lower animals, which has, in the latter, a share in the Fig. 1. Diagram of the Circulation in a Fish. (The portion of the system containing pure blood is black; the part obtaining impure blood is white.) a, auricle, receiving venous blood from the body; v, ventricle; m, bulbus arteriosus; n, branchial artery, carrying venous blood to the gills (b, b) c, aorta, carrying arterialized blood to all parts of the body. venous circulation. Indeed, in the lower vertebrates the lymphatic tubes frequently assume the form of large sinuses, and connect with the veins. They are even found in the birds. In the frog four of these sinuses have muscular walls, and rhythmically contract. These are known as lymphatic hearts.
In various parts of the body the lymphatics form glands, such as the thymus, thyroid gland, and the spleen.
Fishes have a heart resembling that of the mollusks. It is a double force-pump, consisting of a receiving-chamber (auricle), and a propelling chamber (ventricle), with all the valves necessary to prevent a backward flow of the blood. But this heart is respiratory—it sends the blood directly to the breathing organs; consequently, it passes only impure blood. When the blood has traversed the gills and is purified, it passes around the circuit of the body through the systemic and portal capillaries, and back to the heart without any further propulsion.
The low, worm-like fish, lancelet, or amphioxus, has no special heart, but a number of contractile bulbs in the veins. The eel has such an auxiliary heart in its tail, while the hag has the circulation aided by the contractility of the portal vein.
Lepidosiren, one of the mud-fishes, approaches the amphibians in the possession of two auricles; for, in addition to gills, it has true lungs. The vein conveying the purified blood from the lungs joins the left auricle.