Question: what is the key difference between a coelom and pseudocoelom?
Answer: Specifically, a coelom is a body cavity completely within the mesoderm. A pseudocoelom is a cavity between the mesoderm and the endoderm. As David mentioned, both are used as hydrostatic skeletons. Most animals with bilateral symmetry have coeloms (the human coelom is the pericardial sac around the heart).