Body-cavity

Body-cavity, the space inclosed by the body-walls of an animal, and inclosing the gut and other organs. It acquires special names in different regions. In some cases, such as flat-worms, such a cavity is not present, either through non-development or in consequence of degeneration. In the Cœlenterata (hydra, jelly-fish, sea-anemone) the body-cavity is not yet distinct from the alimentary system. The origin of the body-cavity is a very difficult problem which is not yet by any means satisfactorily solved. The brothers Hertwig have (1881) sought to distinguish two main ways in which the body-cavity arises—(a) from (cœlomic or mesodermic) pouches growing out from the primitive gut, and gradually inclosing it (Enterocæla); or (b) from a splitting in a solid middle layer or mesoderm, which does not arise as pouches from the gut, but appears in the form of indifferent cells budded in between the two primitive germinal layers. The latter set includes Molluscs, Polyzoa, Rotifers, and flat-worms, the cavity being undeveloped in the latter. The former set includes all the other classes. This view cannot be said, however, to be generally accepted.