Coelom

The coelom ( /ˈsiːləm/ SEE-ləm) (also celom) is a fluid-filled cavity formed within the mesoderm of some animals. Coeloms developed in diploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size. Coelom is sometimes (incorrectly) used to refer to any developed digestive tract.