Primary Body Cavity

the space containing the internal organs, situated between the body wall and the intestine in certain multicellular animals. The primary body cavity is well developed only in the phyla Nemathelminthes; here it has no cell wall. In mollusks, the primary body cavity is a system of lacunae and sinuses; in higher multicellular animals it has been superseded by the secondary body cavity, or coelom. In the phyla Arthropoda the vestiges of the primary body cavity, merging with the reduced coelom during embryonic development, form the mixed body cavity, or mixocoel. In embryos of animals in the blastula stage, the primary body cavity is called the blastocoel.