The coelom is a body cavity which airse in embryonic mesoderm. The animals which possess true coelom are called eucoelomates. Toue coelom is of two types schizocoelom and enterocoelom. The schizocoelom develop as a split in mesoderm sheet e.g., annelids arthropods, whereas the entrocoelom arise from enterocoelomic pouches e.g., echinoderms, Chordata.
Acoelomates are the group of animals in which body cavity or coelom is absent, e.g., porifers, coelentrates.
Pseudocoelom is the false cavity that develops from blastocoel and is not lined by peritoneum e.g., roundworms.