-Kumudinee, Subject Matter Expert at Edumarz.
Solution: Presence or absence of a cavity between the body wall and the gut wall is considered very important in classification of animals.
The body cavity can be lined by mesoderm and called a coelom.
(i) Animals possessing coeloma are known coelomata for example, annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.
(ii) In case of some animals, the body cavity is not made up of mesoderm, instead the mesoderm is present as scattered pouches in between the ectoderm and endoderm and the body cavity is called pseudocoelom.
The animals possessing pseudocoelom are called pseudocoelomates, e.g., Aschelminthes.
(iii) The animals without any body cavity are known as Acoelomates, e.g.Platyhelminthes.