Difference between a coelom and a pseudocoelem

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The coelom is a cavity within the body of all animals higher than the coelenterates and certain primitive worms, formed by the splitting of the embryonic mesoderm into two layers. In mammals it forms the peritoneal, pleural, and pericardial cavities. Also called body cavity.

The pseudocoelom is an internal body cavity of some primitive invertebrates, similar to a coelom but lacking a mesodermal lining.

So the Coelom and Pseudocoelom are both body cavities except the pseudocoelom is lacking a mesodermal lining.