Coelom is a fluid-filled cavity between the body wall and gut wall. It is lined by mesoderm. On the basis of the presence or absence of coelom, animals are classified into three groups, namely coelomates and pseudocoelomates and acoelomates. Coelomate animals possess coelom between the body wall and digestive tract. For example, annelids, molluscs, arthropods. Pseudocoelomate animals have the body cavity that is not lined by mesoderm. In these animals, mesoderm is scattered between ectoderm and endoderm. For example, Aschelminthes. In acoelomate animals, the body cavity is absent. For example, Platyhelminthes.