Biology and Zoology.

http://en.wikipedia.org/wiki/Coelom

Coeloms developed in diploblasts but were subsequently lost in several lineages. The lack of a coelom is correlated with a reduction in body size. Coelom is sometimes incorrectly used to refer to any developed digestive tract. Some organisms may not possess a coelom or may have a false coelom (pseudocoelom). Animals having coeloms are called coelomates, and those without are called acoelomates.

• acoelomate porifera to platyhelminthes

• pseudocoelomate nemathelminthes(roundworms)

• schizocoelomate develops from split in mesoderm found in annelids, arthropods and molluscs

• haemocoelomate true coelom reduced & cavity filled with blood found from arthropoda and mollusca

• enterocoelomate develops from wall of embryonic gut found from echinodermata to chordata