The circulatory system in chordates.

Chordates are animals in which the skeleton is presented in the form of a notochord or spine. The circulatory organ of these animals is the heart or a blood vessel that replaces it, and venous blood flows through it. Chordates have a closed circulatory system, that is, they have one circle of blood circulation.

For example, in the lancelet through the abdominal vessel, blood flows into the gill vessels, and after oxygenation it goes into the dorsal vessel, and from there the blood flows to the entire body. Then the blood is collected in the vessels of the front and back of the body and returns to the abdominal vessel.