Coelom, << SEE luhm, >> is the body cavity of vertebrates and higher invertebrates. The coelom is an important characteristic in distinguishing the lower animal phyla from the higher phyla. A number of internal organs, such as the liver and the kidneys, are contained within the coelom.
Animals that have a coelom are called eucoelomates. They have a body plan that can be described as a “tube within a tube.” The digestive tract forms the inner tube, and the body wall forms the outer tube. The coelom is the space between the tubes. This arrangement results from the animal’s development as an embryo.
In its early stages, the embryo consists of a hollow sphere of cells called the blastula. The cavity within the blastula is called the blastocoele. Eventually, the cells located at one point on the blastula begin to invaginate–that is, they start to fold inward into the blastocoele. This tubelike invagination forms a primitive gut, lined by a layer of cells called endoderm.
The cells on the outer wall of the blastula make up a layer called the ectoderm. A third layer of cells, the mesoderm, develops between the ectoderm and endoderm. The mesoderm completely fills what remained of the blastocoele. The coelom, a totally new cavity, develops within the mesoderm and, therefore, is lined entirely by mesodermal cells.
Certain lower invertebrates have a tube-within-a-tube body plan but lack a true coelom. These pseudocoelomates, including roundworms and rotifers, have a body cavity that serves some of the functions of a true coelom. In pseudocoelomate embryos, the mesodermal cells do not completely fill in the blastocoele. The blastocoele is retained and becomes the body cavity in the adult animal. Unlike a true coelom, this pseudocoelom is not lined completely by mesoderm.
Invertebrates called acoelomates, which are simpler than pseudocoelomates, have a solid mesodermal layer with no body cavity. Flatworms and ribbon worms are acoelomates.
