The coelom (or celom) is the main

body cavity A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...

in most animals and is positioned inside the

body Body may refer to: In science * Physical body, an object in physics that represents a large amount, has mass or takes up space * Body (biology), the physical material of an organism * Body plan, the physical features shared by a group of anima ...

to surround and contain the

digestive tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organs of the digestive system, in humans and ...

and other

organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...

. In some

animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...

s, it is lined with mesothelium. In other animals, such as

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

, it remains undifferentiated. In the past, and for practical purposes, coelom characteristics have been used to classify bilaterian animal phyla into informal groups.

Etymology

The term ''coelom'' derives from the

Ancient Greek Ancient Greek includes the forms of the Greek language used in ancient Greece and the ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Dark Ages (), the Archaic p ...

word (), meaning 'cavity'.

Structure

Development

The coelom is the mesodermally lined cavity between the gut and the outer body wall. During the development of the embryo, coelom formation begins in the gastrulation stage. The developing digestive tube of an embryo forms as a blind pouch called the

archenteron The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal. Formation in sea urchins As primary mesen ...

. In Protostomes, the coelom forms by a process known as

schizocoely Schizocoely (adjective forms: schizocoelous or schizocoelic) is a process by which some animal embryos develop. The schizocoely mechanism occurs when secondary body cavities ( coeloms) are formed by splitting a solid mass of mesodermal embryonic ...

. The

initially forms, and the mesoderm splits into two layers: the first attaches to the body wall or ectoderm, forming the

parietal layer The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...

and the second surrounds the endoderm or alimentary canal forming the

visceral layer The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm.Langman's Medical Embry ...

. The space between the parietal layer and the visceral layer is known as the coelom or body cavity. In

Deuterostomes Deuterostomia (; in Greek) are animals typically characterized by their anus forming before their mouth during embryonic development. The group's sister clade is Protostomia, animals whose digestive tract development is more varied. Some exampl ...

, the coelom forms by

enterocoely Enterocoely (adjective forms: enterocoelic and enterocoelous) describes both the process by which some animal embryos develop and the origin of the cells involved. In enterocoely, a mesoderm (middle layer) is formed in a developing embryo, in whic ...

. The archenteron wall produces buds of mesoderm, and these mesodermal diverticula hollow to become the coelomic cavities. Deuterostomes are therefore known as ''enterocoelomates''. Examples of deuterostome coelomates belong to three major clades: chordates (

vertebrate Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (chordates with backbones), including all mammals, birds, reptiles, amphibians, and fish. Vertebrates represent the overwhelming majority of the phylum Chordata, with c ...

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

s, and

lancelet The lancelets ( or ), also known as amphioxi (singular: amphioxus ), consist of some 30 to 35 species of "fish-like" benthic filter feeding chordates in the order Amphioxiformes. They are the modern representatives of the subphylum Cephalochord ...

s), echinoderms ( starfish, sea urchins, sea cucumbers), and

hemichordates Hemichordata is a phylum which consists of triploblastic, enterocoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and includ ...

(

acorn worm The acorn worms or Enteropneusta are a hemichordate class of invertebrates consisting of one order of the same name. The closest non-hemichordate relatives of the Enteropneusta are the echinoderms. There are 111 known species of acorn worm in th ...

s and

graptolite Graptolites are a group of colonial animals, members of the subclass Graptolithina within the class Pterobranchia. These filter-feeding organisms are known chiefly from fossils found from the Middle Cambrian ( Miaolingian, Wuliuan) through t ...

s).

Origins

The

evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

ary origin of the coelom is uncertain. The oldest known animal to have had a body cavity was the ''

Vernanimalcula ''Vernanimalcula guizhouena'' is an acritarch dating from ; it was between 0.1 and 0.2 mm across (roughly the width of one or two human hairs). ''Vernanimalcula'' means "small spring animal", referring to its appearance in the fossil record ...

''. Current hypothesis include: * The acoelomate theory, which states that coelom evolved from an acoelomate ancestor. * The enterocoel theory, which states that coelom evolved from gastric pouches of

cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that ...

n ancestors. This is supported by research on flatworms and small worms recently discovered in marine fauna ("coelom").

Functions

A coelom can absorb shock or provide a

hydrostatic skeleton A hydrostatic skeleton, or hydroskeleton, is a flexible skeleton supported by fluid pressure. Hydrostatic skeletons are common among simple invertebrate organisms. While more advanced organisms can be considered hydrostatic, they are sometimes refe ...

. It can also support an immune system in the form of

coelomocyte A coelomocyte () is a phagocytic leukocyte that appears in the bodies of animals that have a coelom. In most, it attacks and digests invading organisms such as bacteria and viruses through encapsulation and phagocytosis, though in some animals (e.g. ...

s that may either be attached to the wall of the coelom or may float about in it freely. The coelom allows muscles to grow independently of the body wall — this feature can be seen in the digestive tract of

tardigrade Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...

s (water bears) which is suspended within the body in the mesentery derived from a mesoderm-lined coelom.

Coelomic fluid

The fluid inside the coelom is known as coelomic fluid. This is circulated by mesothelial cilia or by contraction of muscles in the body wall which are themselves of mesin. The coelomic fluid serves several functions: it acts as a hydroskeleton; it allows free movement and growth of internal organs; it serves for transport of gases, nutrients and waste products around the body; it allows storage of sperm and eggs during maturation; and it acts as a reservoir for waste.

Classification in zoology

In the past, some zoologists grouped

bilaterian The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly an ...

animal phyla based on characteristics related to the coelom for practical purposes, knowing, and explicitly stating, that these groups were ''not'' phylogenetically related. Animals were classified in three informal groups according to the type of body cavity they possess, in a non-taxonomic, utilitarian way, as the Acoelomata, Pseudocoelomata, and Coelomata. These groups were never intended to represent related animals, or a sequence of evolutionary traits. However, although this scheme was followed by a number of college textbooks and some general classifications, it is now almost totally abandoned as a formal classification. Indeed, as late as 2010, one author of a

molecular phylogeny Molecular phylogenetics () is the branch of phylogeny that analyzes genetic, hereditary molecular differences, predominantly in DNA sequences, to gain information on an organism's evolutionary relationships. From these analyses, it is possible to ...

study mistakenly called this classification scheme the "traditional, morphology-based phylogeny". Figure 27 02 05

Coelomate animals or Coelomata (also known as eucoelomates – "true coelom") have a body cavity called a coelom with a complete lining called

peritoneum The peritoneum is the serous membrane forming the lining of the abdominal cavity or coelom in amniotes and some invertebrates, such as annelids. It covers most of the intra-abdominal (or coelomic) organs, and is composed of a layer of meso ...

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

s, are coelomates. Pseudocoelomate animals have a pseudocoelom (literally "false cavity"), which is a fluid filled body cavity. Tissue derived from mesoderm 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 Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryogenesis, embryonic development. This nature has since been discovered to be extremely variable among Pro ...

s; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Blastocoelomate. Acoelomate animals, like flatworms, have no body cavity at all. Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.

Coelomates

Coeloms developed in

triploblast Triploblasty is a condition of the gastrula in which there are three primary germ layers: the ectoderm, mesoderm, and endoderm. Germ cells are set aside in the embryo at the blastula stage, which are incorporated into the gonads during organogenes ...

s but were subsequently lost in several lineages. The lack of a coelom is correlated with a reduction in body size. Coelom is sometimes incorrectly used to refer to any developed digestive tract. Some organisms may not possess a coelom or may have a false coelom (

pseudocoelom A body cavity is any space or compartment, or potential space, in an animal body. Cavities accommodate organs and other structures; cavities as potential spaces contain fluid. The two largest human body cavities are the ventral body cavity, and ...

). Animals having coeloms are called coelomates, and those without are called

acoelomate The coelom (or celom) is the main body cavity in most animals and is positioned inside the body to surround and contain the digestive tract and other organs. In some animals, it is lined with mesothelium. In other animals, such as molluscs, it ...

s. There are also subtypes of coelom: * schizocoelom: develops from split in mesoderm found in

annelids The annelids (Annelida , from Latin ', "little ring"), also known as the segmented worms, are a large phylum, with over 22,000 extant species including ragworms, earthworms, and leeches. The species exist in and have adapted to various ecolog ...

, arthropods and

* haemocoelom: true coelom reduced and cavity filled with blood found from

arthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...

to mollusca * enterocoelom: develops from wall of embryonic gut found from

echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

to chordata

Coelomate phyla

According to Brusca and Brusca,R. C. Brusca, G. J. Brusca. ''Invertebrates''. Sunderland, Massachusetts: Sinauer Associates, 2003 (2nd ed.), p. 47, . the following

phyla possess a coelom: *

Nemertea Nemertea is a phylum of animals also known as ribbon worms or proboscis worms, consisting of 1300 known species. Most ribbon worms are very slim, usually only a few millimeters wide, although a few have relatively short but wide bodies. Many h ...

, traditionally viewed as acoelomates *

Priapulida Priapulida (priapulid worms, from Gr. πριάπος, ''priāpos'' 'Priapus' + Lat. ''-ul-'', diminutive), sometimes referred to as penis worms, is a phylum of unsegmented marine worms. The name of the phylum relates to the Greek god of fertility ...

* Annelida *

Onychophora Onychophora (from grc, ονυχής, , "claws"; and , , "to carry"), commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, '' Peripatus ...

Tardigrada Tardigrades (), known colloquially as water bears or moss piglets, are a phylum of eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them Kleiner Wasserbä ...

Arthropoda Arthropods (, (gen. ποδός)) are invertebrate animals with an exoskeleton, a segmented body, and paired jointed appendages. Arthropods form the phylum Arthropoda. They are distinguished by their jointed limbs and cuticle made of chitin, ...

* Mollusca * Phoronida *

Ectoprocta Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called ...

* Brachiopoda *

Echinodermata An echinoderm () is any member of the phylum Echinodermata (). The adults are recognisable by their (usually five-point) radial symmetry, and include starfish, brittle stars, sea urchins, sand dollars, and sea cucumbers, as well as the s ...

Chaetognatha The Chaetognatha or chaetognaths (meaning ''bristle-jaws'') are a phylum of predatory marine worms that are a major component of plankton worldwide. Commonly known as arrow worms, about 20% of the known Chaetognatha species are benthic, and ca ...

* Hemichordata * Chordata

Pseudocoelomates

In some

s, the embryonic

blastocoele The blastocoel (), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammal ...

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

. A pseudocoelomate or blastocoelomate is any

invertebrate Invertebrates are a paraphyletic group of animals that neither possess nor develop a vertebral column (commonly known as a ''backbone'' or ''spine''), derived from the notochord. This is a grouping including all animals apart from the chordate ...

with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of

mutation In biology, a mutation is an alteration in the nucleic acid sequence of the genome of an organism, virus, or extrachromosomal DNA. Viral genomes contain either DNA or RNA. Mutations result from errors during DNA replication, DNA or viral repl ...

s in certain types of

gene In biology, the word gene (from , ; "... Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a b ...

s that affected early development. Thus, pseudocoelomates evolved from coelomates. "Pseudocoelomate" is no longer considered a valid

taxonomic group In biology, a taxon (back-formation from ''taxonomy''; plural taxa) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular nam ...

, since it is not monophyletic. However, it is still used as a descriptive term. Important characteristics: * lack a vascular blood system **

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemica ...

and

osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region o ...

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 A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...

** usually covered by a secreted cuticle * most are microscopic * parasites of almost every form of life (although some are free living) *

eutely Eutelic organisms have a fixed number of somatic cells when they reach maturity, the exact number being relatively constant for any one species. This phenomenon is also referred to as cell constancy. Development proceeds by cell division until matur ...

in some * loss of larval stage in some * possibly

pedomorphism Neoteny (), also called juvenilization,Montagu, A. (1989). Growing Young. Bergin & Garvey: CT. is the delaying or slowing of the physiological, or somatic, development of an organism, typically an animal. Neoteny is found in modern humans compare ...

Pseudocoelomate phyla

According to Brusca and Brusca, bilaterian pseudocoelomate phyla include: *

Rotifera The rotifers (, from the Latin , "wheel", and , "bearing"), commonly called wheel animals or wheel animalcules, make up a phylum (Rotifera ) of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John H ...

Kinorhyncha Kinorhyncha ( grc, κινέω, kīnéō, I move, ' "snout") is a phylum of small marine invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos. They are also called mud dragons. Modern species are or less, ...

Nematoda The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broa ...

Nematomorpha Nematomorpha (sometimes called Gordiacea, and commonly known as horsehair worms, hairsnakes, or Gordian worms) are a phylum of parasitoid animals superficially similar to nematode worms in morphology, hence the name. Most species range in size f ...

Acanthocephala Acanthocephala (Greek , ', thorn + , ', head) is a phylum of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to p ...

Loricifera Loricifera (from Latin, '' lorica'', corselet (armour) + ''ferre'', to bear) is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals that had been determined to be 37 described species, in 9 genera, bu ...

Some authors list the following phyla as pseudocoelomates: Ecdysozoan pseudocoelomates *

(roundworms) *

(nematomorphs or horsehair worms) *

Spiralian pseudocoelomates *

Gastrotricha The gastrotrichs (phylum Gastrotricha), commonly referred to as hairybellies or hairybacks, are a group of microscopic (0.06-3.0 mm), worm-like, acoelomate animals, and are widely distributed and abundant in freshwater and marine environmen ...

Entoprocta Entoprocta (), or Kamptozoa , is a phylum of mostly sessile aquatic animals, ranging from long. Mature individuals are goblet-shaped, on relatively long stalks. They have a "crown" of solid tentacles whose cilia generate water currents that ...

(rotifers) *

(spiny-headed worms)

Acoelomates

Acoelomates lack a fluid-filled body cavity between the body wall and digestive tract. This can cause some serious disadvantages. Fluid compression is negligible, 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. 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 The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegme ...

, traditionally viewed as blastocoelomates *

Gnathostomulida Gnathostomulids, or jaw worms, are a small phylum of nearly microscopic marine animals. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively anoxic environments. They were first recognised and described in 1956. ...

, traditionally viewed as blastocoelomates *

Cycliophora ''Symbion'' is a genus of commensal aquatic animals, less than 0.5 mm wide, found living attached to the mouthparts of cold-water lobsters. They have sac-like bodies, and three distinctly different forms in different parts of their two-sta ...

R.C.Brusca, G.J.Brusca 2003, p. 379. According to others, acoelomates include the

ns (

jellyfish Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrell ...

and allies), and the

ctenophores Ctenophora (; ctenophore ; ) comprise a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), a ...

(comb jellies),

platyhelminthes The flatworms, flat worms, Platyhelminthes, or platyhelminths (from the Greek πλατύ, ''platy'', meaning "flat" and ἕλμινς (root: ἑλμινθ-), ''helminth-'', meaning "worm") are a phylum of relatively simple bilaterian, unsegme ...

( flatworms including

tapeworm Eucestoda, commonly referred to as tapeworms, is the larger of the two subclasses of flatworms in the class Cestoda (the other subclass is Cestodaria). Larvae have six posterior hooks on the scolex (head), in contrast to the ten-hooked Cesto ...

s, etc.),

, and