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Given a scientific passage and an answer, generate a question for the given answer.

Passage: Later, a true coelom evolved. This is a fluid-filled body cavity that is completely enclosed by mesoderm. The coelom lies between the digestive cavity and body wall. You can see it in the invertebrate in Figure below . Modern invertebrates with a coelom include mollusks (Phylum Mollusca) and annelids (Phylum Annelida). Answer: coelom
Output:
What is a fluid-filled body cavity that is completely enclosed by mesoderm called?