Any cavity or no cavity in-between the body wall and gut wall is quite important in classification. The body cavity, which are lined by the mesoderm is called coelom. those Animals having coelom are referred as coelomates, like ,annelids, molluscs, arthropods, echinoderms, hemichordates and chordates In some animals, the body cavity is not lined by the mesoderm, but , the mesoderm there is scattered in pouches in between the ectoderm and
the endoderm. Such a body cavity is named pseudocoelom and therefore the animals possessing them are called pseudocoelomates, e.g. aschelminthes The animals within which the body cavity is absent are called acoelomates, e.g., platyhelminthes
hemocoelomates the animals containing hemocoel. Hemocoel is the blood filled body cavity of many invertebrates like arthropods and molluscs. In such animals, hemolymph circulates through the hemocoel.