What is Coelom - Body Cavity explain?

Among the bilaterally symmetrical organisms, animals have three different types of body plans with respect to body cavities. The term coelom is used to describe a specific type of body cavity, one that forms from within the middle layer of tissue in a body, the mesoderm. One type of animal body is solid, lacking an internal cavity, or coelom, between the digestive tract and the layers of the body wall. Organisms with a solid body plan are called Acoelomates. A second group of organisms develops an internal body cavity, but it forms between the endoderm and mesoderm instead of within the mesoderm itself. This group is called the Pseudocoelomates. The third group, the Coelomates, develops the coelom by parting the mesoderm.

While the presence or absence of a coelom may seem like a minor issue, it is in terms of evolution that the development of a body cavity takes on significance. Without a body cavity, the solid body limits circulation of gases and nutrients. Being in direct contact with the muscles of the body, the digestive and circulatory systems are subjected to pressure and distortion whenever the body bends. These factors limit flow in these systems, and therefore their efficiency. The presence of a coelom, then, provides interior space for the increased specialization and effective functioning of tissues and organs involved in circulation and digestion. Interior body space also provides for reproductive organs and systems that utilize internal fertilization and embryo development.