Presence or absence ofa cavity between the body wall
and the gut wall is very important in classificatiom The
body cavity which is lined by mesoderm is called
coelom.
(i) Animals possessing coelom are called coelomate, e.g.,
annelids, molluscs, arthropods, echinoderms,
hemichordates and chordates.
(ii) In some animals, the body cavity is not 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, e.g.,
aschelminthes.
(iii) The animals in which the body cavity is absent are
known as acoelomates, e.g., platyhelminthes. This
helps, in assigning the exact taxonomic position of the
animal on the basis of its presence or absence in the
animals’s body.