List the features of the similarities in the structure of the circulatory system of polychaetal and small-bristled.

Small bristle and polychaete worms are classes of the annelid worm type. For the first time in the evolutionary process, these worms developed a circulatory system. Despite the fact that polychaetal worms are a more ancient class, the structure of the circulatory system is similar: it is closed – blood flows only through the vessels, there is no heart yet. In addition to the dorsal and abdominal vessels, in each segment of their body there is an annular vessel from which capillaries extend. Several larger annular vessels, by contracting, play the role of the heart.