The nature of body cavity and coelom is very important in classification.
(a) The body cavity which is lined by mesoderm is called coelom and the animal possessing coelom are called coelomates.
(b) The animals in which the coelom is absent are called acoelomate animals.
(c) In some animals, the body cavity in not lined by mesoderm, instead, the mesoderm is present as scattered pouches in between the ectoderm and endoderm.
(d) Such a body cavity in known as pseudocoel and the animals are called pseudocoelomates.