The heart is the central organ of the circulatory system, and its contractions control the circulation of blood through the vessels. A peculiarity of the circulatory system of fish is the presence of a circulatory circle and a two-chambered heart located just behind the head. The heart consists of a ventricle and an atrium. In front of the atrium is the sinus venosus. The lower fish have an arterial cone (Keysteppers, bivalves, sturgeons, Polyptera) and the higher fish have an aortic bulb. Double-breathing fishes have a pulmonary circulation in connection with which they have an incomplete septum in the atrium, dividing it almost completely into two halves.