Cavity Wall: A body cavity is any space in a multicellular organism that is fluid-filled. Surrounding this cavity is the wall of the cavity. The term body cavity also refers to the space that is located in between an animal’s epidermis or outermost covering and the outermost lining of the gut cavity and this is where organs internally develop. The human body cavity usually refers to the ventral body cavity, due to the fact that it is the largest one of the cavities in volume.

The type of body cavity puts an organism into one of three groupings: coelomates also referred to as eucoelomates or “true coelom”, have a body cavity that is fluid filled known as a coelom with a total cavity lining or wall called peritoneum which is derived from mesoderm – one of the three primary layers of tissue. This complete mesoderm lining lets organs attach to each other so that they may be “suspended” in a specific order while still able to freely move within the cavity. Most animals including all vertebrates are in this class.

Animals classified as Pseudocoelomate have a “false cavity” or pseudocoel that is a body cavity that is fully functional. Tissue which is resulting from mesoderm only partially lines the cavity walls of these animals. So, although this class of animal has organs that are kept in place they are rather loose and not organized as well as coelomate. Example of a Pseudocoelomate would be the roundworm. Pseudocoelomate animals are referred to as Blastocoelomate and Hemocoel.

Acoelomate animals, for example the flatworms, have no body cavities whatsoever. Organs are directly contacted with the epithelium. Mesodermal tissue that is semi-solid between the gut and body cavity wall hold these organs in place.

In the human body the largest body cavities are the abdominal cavity and the thoracic cavity with the abdominal cavity wall consisting of muscles, with another muscular wall that separates the abdominal and thoracic cavities from each other.