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.

Differences between direct and indirect development
are as follows
(i) Direct development In case of direct development
the life cycle of an adult individual lacks any larval
stage in its life cycle. The adults give rise to young
ones which directly develop into the adult. e.g„
mammals.
(ii) Indirect development It is present mostly in case
lower animals, the adult individual gives rise to eggs
mostly which develop into the adult after bang
passing through several larval
stages,
echinoderms, Ascaris, cockroach have prolonged
larval stages in their life cycle.