Presence or absence of a cavity between the body wall and the gut wall is very important in classification. The body cavity, which is lined by mesoderm is called coelom.
Animals possessing coelom are called coelomates, e.g., annelids, molluscs, arthropods, echinoderms, hemichordates and chordates (Figure: a). In some animals, the body cavity is not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and the animals possessing them are called pseudocoelomates, e.g., aschelminthes (Figure: b). The animals in which the body cavity is absent are called acoclomates, e.g., platyhelminthes (Figure: c).
Fig: Diagrammatic sectional view of: (a) Coelomate (b) Pseudocoelomate (c) Acoelomate