Evolution of the circulatory system in vertebrate and invertebrate animals.

For the first time, the circulatory system appeared in annelids: closed, no heart. They were the ancestors of vertebrates. It is the same for lancelets. In fish, a 2-chambered heart appears. With the emergence of chordates on land, a second circle of blood circulation (pulmonary or small) appeared and the evolution proceeded in the direction of increasing the chambers in the heart for further separation of venous and arterial blood. In amphibians, the heart is 3-chambered, in reptiles too, but there is an incomplete septum in the ventricle. And in birds and mammals – 4-chamber.