How useful is the study of Body cavity and coelom, in the classification of animals?

Explanation

Presence of absence of a cavity between the body wall and the gut wall , is very important in classification. The body cavity which is lined by coelomic epithelium is called Eucoelom. Animals possessing eucoelom are called Eucoelomates (e.g: Annelids, Molluscs, Echinoderms, Chordates etc).
In some animals , body cavity is not lined by mesoderm lining of coelomic epithelium , but mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and these animals are called Acoelomates.