Body Cavity. In the article on blastosphere it was shown that a central cavity is formed in an egg in an early stage of its development; this cavity is known as the "blastocoele" and it usually communicates with the exterior by a "blastopore." In some of the lower coelenterata (see e.g. Actinia and Hydra) the blastocoele is the only body cavity and the blastopore remains as the mouth and anus. But in that division of the animal kingdom known as the Coelomata this simple body cavity is usually obliterated, though remnants of it may persist in the adult as in the head cavities of some worms (see Archiannelida) and in the Rotifera (q.v.); such are known as "archicoeles." But in most cases the conspicuous body cavity of the adult has no connection with this primitive "blastocoele," but has been formed by the excavation of a series of spaces; such are known as "pseudocodes" or false coelomes, and examples are met with among the mollusca, arthropoda, and the remarkable Peripatus; in the prawn, however, it has been proved that a large true archicoele is also present. A third type of body cavity is the "enterocoele" of Starfish, Balanoglossus, etc., which is formed from an outgrowth of the primitive alimentary canal (archenterm) of the embryo. In the vertebrates and many worms the body cavity is of a similar origin, but as the development is shortened it is known as a "cryptenterocoele."