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Which was one of the first animals to develop a true body cavity?

1 Answer

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The answer is mollusks and annelids.

Coelomates are animals which body cavity (coelom) is filled with fluid. Fluid surrounds organs, protects them and allows their free movement and growth. All vertebrates and some invertebrates, such as mollusks, annelids, and arthropods, are coelomates.

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