The coelom is one of the characteristic features of metazoans.
Coelom refers to the fluid-filled body cavity lined by the mesoderm between the body wall and gut wall. A true coelom is entirely lined by mesoderm.
Based on the presence or absence of coelom, animals are classified into two groups:
Coelomate animals possess coelom between the body wall and digestive tract. For example, annelids, molluscs, arthropods.
Pseudocoelomate animals have a body cavity that is not lined by mesoderm. In these animals, mesoderm is scattered between ectoderm and endoderm. For example, Aschelminthes.
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