600-555 million years ago

worm blood vessels

Modern coelomates can possess a variety of different types of circulatory systems. Ancestral coelomates probably possessed an open circulatory system in which blood vessels were not lined by cells and the coelom could help distribute materials.

The circulatory systems of most invertebrates are not closed systems, as seen in the vertebrates. Blood vessels are typically not lined by cells and, in groups where there is a cellular lining (such as certain mollusks and echinoderms), the lining is incomplete. Coelomate animals (such as hemichordates, echinoderms, and some annelids) often use their coeloms for distribution of materials, forming a coelomic circulatory system, supplementing the functions of the cardiovascular system. In coelomates other than chordates, the respiratory pigments are found primarily in the coleom (Beklemishev 2, 356). Open circulatory systems have the advantage of maintaining low blood pressure and thus the risk of blood loss after injury is less.
Lophophorates, whose ancestors may have diverged near the split of coelomates into protostomes and deuterostomes, possess a closed circulatory system with 2 main vessels, (1 dorsal, 1 ventral) which are connected through a plexus of smaller vessels. There is no heart and muscle contractions in blood vessels propel the blood. The blood flows directionally through the major blood vessels-the dorsal vessel is afferent (moving blood away from the heart) and the ventral vessel is efferent (Hickman). Protostomes do not possess a single type of circulatory system: circulatory systems may be open (as in most arthropods and molluscs), closed (as in annelids), or nonexistent (Priapuloidea, Sipunculoidea, and Bryozoa among the protostomes and Chaetognatha among the deuterostomes) (Beklemishev 2, p. 356).