The difference between an open and a closed circulatory system is...

In the closed circulatory system, blood is confined to a heart and blood vessels, whereas in an open system, hemolymph (blood) moves from a heart through vessels into interconnected spaces called a "hemocoel."

Explanation:

An open circulation the blood is not blood vessels, it instead is pumped into a cavity called Hemocoel.

A closed circulatory system, the blood is pumped through vessels SEPARATE from the interstitial fluid of the body.

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