A body cavity can be defined as the space that remains after the organs inside it are removed, but this definition does not do justice to the variety and
In humans all but the cranial cavity develop from the coelom (pronounced SEE-lum). A coelom is a cavity that is entirely enclosed within cells derived from the middle layer of embryonic tissue. A few groups of animals, such as roundworms (Nematoda), have a body cavity that is only partly enclosed by tissue from the middle layer. Such a body cavity is called a pseudocoelom (pronounced SOO-doe-SEE-lum). In a few other groups, such as flatworms (Platyhelminthes), there is no body cavity.
C. Leon Harris
Pechenik, Jan A. "Classification by Developmental Pattern." In Biology of the Invertebrates, 4th ed. Boston: McGraw-Hill Higher Education, 2000.
Saladin, Kenneth S. "Body Cavities and Membranes." In Anatomy and Physiology. Boston: McGraw-Hill, 1998.