Presence or absence of a cavity between the body wall and the gut wall is very important is classification. The body cavity which is laid by mesoderm is called coelom. Animals possessing coelom are called coelomates (e.g.: annelids, molluscs, echinoderms, chordates etc.,)
In some animals, body cavity is not lined by mesoderm but mesoderm is present as scattered pouches in between the ectoderm and endoderm. Such a body cavity is called pseudocoelom and animal pseudocoelomates.
e.g: aschelminthes The animals in which the body cavity is absent is called acoelomates
e.g.: platyhelminthes
by the above example we can see that absence of coelom indicates that the animal is primitive, hence is classified as lower in the classification ladder, where as coelomates are highly developed and pseudo coelomates are in middle of classification ladder.
