Page "Polychaete" Paragraph 10
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Blood flows forward in the dorsal vessel, above the gut, and returns back down the body in the ventral vessel, beneath the gut.
The blood vessels themselves are contractile, helping to push the blood along, so most species have no need of a heart.
In a few cases, however, muscular pumps analogous to a heart are found in various parts of the system.
Conversely, some species have little or no circulatory system at all, transporting oxygen in the coelomic fluid that fills their body cavity.
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