By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The body cavity" of human body cavities normally refers to the ventral body cavity, because it is by far the largest one in area.
The type of body cavity places an organism into one of these three groups:
Mammalian embryos develop two coelomic cavities: the intraembryonic coelom and the extraembryonic coelom (or chorionic cavity). The intraembryonic coelom is lined by somatic and splanchnic lateral plate mesoderm, while the extraembryonic coelom is lined by extraembryonic mesoderm. The intraembryonic coelom is the only cavity that persists in the mammal at term, which is why its name is often contracted to simply coelomic cavity. Subdividing the coelomic cavity into compartments, for example, the pericardial cavity / pericardium, where the heart develops, simplifies discussion of the anatomies of complex animals.
Coelom formation begins in the gastrula stage. The developing digestive tube of an embryo forms as a blind pouch called the archenetron.
In Protostomes, a process known as schizocoelus happens: as the archenteron initially forms, the mesoderm splits to form the coelomic cavities. In Deuterostomes, a process known as enterocoelus happens: the mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities.
The origin of the coelom is uncertain. The oldest known animal to have had a body cavity is Vernanimalcula.
Current evolutionary theories:
In some protostomes, the embryonic blastocoele persists as a body cavity. These protostomes have a fluid filled main body cavity unlined or partially lined with tissue derived from mesoderm. This fluid-filled space surrounding the internal organs serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton.
A pseudocoelomate is any invertebrate animal with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates (Evers, 355).
Lacking a fluid filled body cavity presents some serious disadvantages. Fluids do not compress, while the tissue surrounding the organs of these animals will compress. Therefore, acoelomate organs are not protected from crushing forces applied to the animal’s outer surface.
Organisms showing acoelomate formation include the platyhelminthes (flatworms, tapeworms etc.) The coelom can be used for diffusion of gases and metabolites etc. These creatures do not have this need, as the surface area to volume ratio is large enough to allow absorption of nutrients and gas exchange by diffusion alone, due to dorso-ventral flattening.
Platyhelminthes along with members of the phylum Nemertea (ribbon worms), lack a coelom* - a fluid-filled cavity between the outer body wall and the gut. These animals are described as being acoelomate* .