The
coelom (pronounced "sea-lum" ) is a fluid filled cavity formed within the
mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size. Coeloms are only ever present in triploblastic animals, though coelom is sometimes (incorrectly) used to refer to any developed digestive tract.
Functionally, a coelom can absorb shock or provide a
hydrostatic skeletonA hydrostatic skeleton or hydroskeleton is a structure found in many cold-blooded organisms and soft-bodied animals consisting of a fluid-filled cavity, the coelom, surrounded by muscles. The pressure of the fluid and action of the surrounding muscles are used to change an organism's shape and...
. It also allows organs to grow independently of the body wall. This can be seen in the digestive tract of earthworms and other
annelidThe annelids, collectively called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches. They are found in marine environments from tidal zones to hydrothermal vents, in freshwater, and in moist terrestrial environments...
s, which is suspended within the body in a
mesenteryIn anatomy, the mesentary is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentary :The...
derived from a mesoderm-lined coelom. In mammals, the coelom forms the peritoneal, pleural, and
pericardialThere are two Pericardial sinuses: transverse and oblique.* The cul-de-sac enclosed between the limbs of the inverted U of the venous mesocardium lies behind the left atrium and is known as the oblique sinus...
cavities.
In the past, zoologists grouped animals based on characters related to the coelom. The presence or absence of a coelom and the way in which it was formed was believed to be important in understanding the phylogenetic relationships of animal
phylaIn biology, a phylum ["Phylum" is adopted from the Greek phylai, the clan-based voting groups in Greek city-states.] is a taxonomic rank below Kingdom and above Class...
. However, recent molecular phylogenies have suggested this characteristics is not as informative as previously believed. Indeed, the coelom may have arisen twice, once in
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and once among the
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s.