Body cavity
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[[Image:Illu body cavities.jpg|thumb|400px|Picture of [[Human body cavities]] - dorsal body cavity to the left and ventral body cavity to the right.]]
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 (skin)|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:
* '''Coelomates''' (also known as eucoelomates--"true coelom") have a fluid filled body cavity called a '''coelom''' ({{pronEng|ˈsiːləm}}) with a complete lining called [[peritoneum]] derived from mesoderm (one of the three [[germ layer|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 [[vertebrate]]s, are coelomates.
* '''Pseudocoelomate animals''' have a '''pseudocoel,''' (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 [[protostome]]s; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as [[Hemocoel]] and [[Blastocoele|Blastocoelomate]].
* '''Acoelomate animals''', like [[flatworm]]s, 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.
== Coelom ==
{{main|Coelom}}
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.
[[Arthropod]]s and [[mollusk]]s have a reduced (but still true) coelom. Their principal body cavity is the ''hemocoel'' of an [[open circulatory system]].
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 [[pericardium|pericardial cavity]], where the heart develops, simplifies discussion of the [[anatomy|anatomies]] of complex animals.
=== Formation ===
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.
=== Origins ===
The origin of the coelom is uncertain. The oldest known animal to have had a body cavity is '''[[Vernanimalcula]]'''.
Current [[evolution|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 ([http://www.answers.com/body%20cavity "coelom"])
== Pseudocoel ==
In some [[protostome]]s, 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 [[organ (anatomy)|organ]]s serves several functions like distribution of nutrients and removal of waste or supporting the body as a [[hydrostatic skeleton]].
=== Pseudocoelomates ===
: '' '''Pseudocoelomate''' is no longer considered a valid [[taxonomy|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 [[mutation]]s in certain types of [[gene]]s that affected early development. Thus, pseudocoelomates evolved from coelomates ([[body cavity#References|Evers]], 355).
Important characteristics:
* lack a vascular blood system
** [[diffusion]] and [[osmosis]] circulate nutrients and waste products throughout the body.
* 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)
=== Examples of pseudocoelomates ===
*Nematoda ([[roundworm]]s)
* Rotifera ([[rotifer]]s)
* [[Kinorhyncha]]
* [[Nematomorpha]], nematomorphs, or horsehair worms
* [[Gastrotrich]]a
* [[Loricifera]]
* [[Priapulida]]
* [[Acanthocephala]] (spiny-headed worms)
* [[Aschelminth]] animals
* [[Entoprocta]]
== 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.
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* .
== References ==<!-- This section is linked from [[Body cavity]] -->
* ''Biology''. Sixth Edition by Solomon, Berg and Martin. ISBN 0-534-39175-3.
* Evers, Christine A., Lisa Starr. ''Biology:Concepts and Applications.'' 6th ed. United States:Thomson, 2006. ISBN 0-534-46224-3.
* "coelom." The Columbia Electronic Encyclopedia, Sixth Edition. Columbia University Press., 2003. Answers.com 02 . 2006. http://www.answers.com/topic/body-cavity
* {{BiowebUW|zoolab/Table_of_Contents/Lab-4a/Pseudocoelomate_Introduction/pseudocoelomate_introduction.htm|Zoolab}}
* [http://orion1.paisley.ac.uk/courses/Tatner/biomedia/units/mino5.htm Glasgow University Science Museum]
== External links ==
*[http://www.gwu.edu/~darwin/BiSc151/Coelom/Coelom.html Further discussion]
[[Category:Animal anatomy]]
[[Category:Developmental biology]]
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