By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism, other than those of vessels (such as blood vessels and lymph vessels). 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 human body cavity", normally refers to the ventral body cavity, because it is by far the largest one in volume.
The type of body cavity places an organism into one of three groups:
- Coelomates or Coelomata (also known as eucoelomates — "true coelom") have a fluid filled body cavity called a coelom (pron.: //) with a complete lining called peritoneum derived from mesoderm (one of the three primary tissue layers). The complete mesoderm lining allows organs to be attached to each other so that they can be suspended in a particular order while still being able to move freely within the cavity. Most bilateral animals, including all the vertebrates, are coelomates.
- Pseudocoelomate animals have a pseudocoelom (literally “false cavity”), which is a fully functional body cavity. Tissue derived from mesoderm only partly lines the fluid filled body cavity of these animals. Thus, although organs are held in place loosely, they are not as well organized as in a coelomate. All pseudocoelomates are protostomes; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate.
- Acoelomate animals, like flatworms, have no body cavity at all. Organs have direct contact with the epithelium. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.
A coelom is a cavity lined by an epithelium derived from mesoderm. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.
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 archenteron.
In Protostomes the coleom forms by a process known as schizocoely. The archenteron initially forms, and the mesoderm splits into two layers: the first attaches to the body wall or ectoderm, forming the parietal layer and the second surrounds the endoderm forming the visceral layer or alimentary canal. The space between the parietal layer and the visceral layer is known as the coelom or body cavity. In Deuterostomes, the coelom forms by enterocoely: 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:
- The acoelomate theory
- Coelom evolved from an acoelomate ancestor.
- The enterocoel theory
- Coelom evolved from gastric pouches of cnidarian ancestors.
- Supported by research on flatworms and small worms recently discovered in marine fauna ("coelom")
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.
- Pseudocoelomate is no longer considered a valid taxonomic group, since it is not monophyletic. However, it is still used as a descriptive term.
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.
- lack a vascular blood system
- lack a skeleton
- hydrostatic pressure gives the body a supportive framework that acts as a skeleton.
- no segmentation
- body wall
- epidermis and muscle
- often syncytial
- usually covered by a secreted cuticle
- most are microscopic
- parasites of almost every form of life (although some are free living)
- eutely in some
- loss of larval stage in some
- possibly pedomorphism
Examples of pseudocoelomates 
- Nematoda (roundworms)
- Rotifera (rotifers)
- Nematomorpha, nematomorphs, or horsehair worms
- Acanthocephala (spiny-headed worms)
- Aschelminth animals
No coelom (Acoelomate) 
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.
Acoelomates include the platyhelminthes (flatworms, tapeworms etc.), the cnidarians (jellyfish and allies), and the ctenophores (comb jellies). 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.
- "Animals III — Pseudocoelomates and Protostome Coelomates".
- Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.