Enterocoelous pattern – Coelom
In enterocoelous development of coelom the mesoderm arises in the embryo as paired lateral pouches growing out from the archenteron. The pouches grow 'and later get disconnected from the archenteron. The cavity of the pouch becomes the coelom. The inner wall of the pouch surrounds the developing alimentary canal and the outer layer lines the developing body wall. Those two layers become the future mesoderm. The animals possessing such cavities are known as enterocoelomates. Echinoderms hemichordates and chordates show this pattern of coelom development. The two different patterns of origin of coelom are another expression of the protostome - deuterostome dichotomy in evolution. Protostomes typically show schizocoelous origin of coelom while in deuterostome the coelom arises usually by enterocoely.
Figure: Enterocoelous origin
The coelom provides the coelomic animals with a tube-within a tube body plan that allows greater flexibility when compared to animals that do not have internal body cavity. The fluid filled coelom additionally serves as a hydrostatic skeleton, in some forms such as worms, aiding in burrowing and inmovement. The coelom is of great significance in animal evolution. It is a stepping stone for evolution of more complex and larger forms.