Which describes a difference between bivalves and cephalopods?

Bivalves, such as clams and mussels, have an open circulatory system: their circulatory fluid is not always contained in blood vessels. Cephalopods, such as octopuses and squids, have a closed circulatory system, in which blood remains within vessels. This makes “Bivalves have an open circulatory system, and cephalopods have a closed circulatory system” the correct difference.

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