The cavity between the body wall and the gut wall is called coelom. If the animals do not have coelom, they are called acoelomates. e.g., flatworms. In some animals, the body cavity is not fully lined by the mesodermal epithelium. The mesoderm is formed as scattered pouches between the ectoderm and endoderm. Such a body cavity is called a pseudocoel. The animals which have pseudocoel e.g. round worms.
If the coelom develops within the mesoderm and is lined by mesodermal epithelium it is called eucoelom. The animals which have true coelom are called eucoelomates. If the body cavity is formed by splitting of mesoderm, the animals are called schizocoelomates e.g., Annelids, arthropods and molluscs. If the body cavity is formed from the mesodermal ‘ pouches of archenteron, the animals are called enterocoelomate animals, e.g., echinoderms, hemichordates and chordates.
