What is coelom ? Describe its types. 

Explanation

The body cavity, (between the body wall and gut wall) which is lined by mesoderm is called coelom.
The presence or absence of coelom is very important in classification
On the basis of coelom animals can be classified in three different groups.

Acoelomate : The animals in which the body cavity is absent are called acoelomates.
e.g. Poriferan, platyhelminths, coelenterates ctenophores and flatworms.
Pseudocoelomate :
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, Aschelminthes.
Coelomate :
The animals having truc coclom are called coelomates.
Annelids, molluscs, arthropods, echinoderms, hemichordates and chordates.