noun. The fluid-filled cavity within the body of most multicellular animals, except some invertebrates such as flatworms and cnidarians, that lies between the body wall and the digestive tract and is formed by the splitting of the embryonic mesoderm into two layers.

noun. A fluid-filled cavity within the body of an animal. The digestive system is suspended within the cavity, which is lined by a tissue called the peritoneum.

noun. a cavity in the mesoderm of an embryo that gives rise in humans to the pleural cavity and pericardial cavity and peritoneal cavity

"The same kind of coelom-formation as in sagitta was afterwards found by Kowalevsky in brachiopods and other invertebrates, and in the lowest vertebrate -- the amphioxus."

coelomate  coelomic  

bodily cavity  cavity  cavum