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
1. Acoelomates
The animals in wlfch the body cavity is absent are called
acoelomates, e.g., poriferans, platyhelminthes, coelenterates,
ctenophors and flatworms.
2. Pseudocoelomates
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, e.g., aschelminthes.
3. Coelomates
The animals having true coelom are called coelomates. A
true coelom arises within mesoderm and is therefore, lined
by mesodermal tissues,
externally by parietal I.e.,
peritoneum and internally by visceral peritoneum.