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 to surround and contain the
digestive tract and other
organs. In some
animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
s, it is lined with
mesothelium. In other animals, such as
molluscs, 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 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.
In
Protostomes
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
, 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 t ...
.
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 or
alimentary canal forming the
visceral layer. 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.
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
A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...
(
vertebrates,
tunicates, and
lancelets),
echinoderms (
starfish,
sea urchins,
sea cucumbers), and
hemichordates (
acorn worms and
graptolites).
Origins
The
evolutionary origin of the coelom is uncertain. The oldest known animal to have had a body cavity was the ''
Vernanimalcula''. 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
cnidarian 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. It can also support an immune system in the form of
coelomocytes 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
tardigrades (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 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 study mistakenly called this classification scheme the "traditional, morphology-based phylogeny".
Coelomate animals or Coelomata (also known as eucoelomates – "true coelom") have a body cavity called a coelom 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 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 embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
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
triploblasts 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). Animals having coeloms are called
coelomate
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 r ...
s, and those without are called
acoelomates. There are also subtypes of coelom:
* schizocoelom: develops from split in
mesoderm found in
annelids,
arthropods and
molluscs
* haemocoelom: true coelom reduced and cavity filled with blood found from
arthropoda 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
bilaterian phyla Phyla, the plural of ''phylum'', may refer to:
* Phylum, a biological taxon between Kingdom and Class
* by analogy, in linguistics, a large division of possibly related languages, or a major language family which is not subordinate to another
Phy ...
possess a coelom:
*
Nemertea, traditionally viewed as acoelomates
*
Priapulida
*
Annelida
*
Onychophora
*
Tardigrada
*
Arthropoda
*
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 a l ...
*
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
protostome
Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
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 mammals ...
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
Organ may refer to:
Biology
* Organ (biology), a part of an organism
Musical instruments
* Organ (music), a family of keyboard musical instruments characterized by sustained tone
** Electronic organ, an electronic keyboard instrument
** Hammond ...
s serves several functions like distribution of nutrients and removal of waste or supporting the body as a
hydrostatic skeleton.
A pseudocoelomate or blastocoelomate is any
invertebrate animal
Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage ...
with a three-layered body and a
pseudocoel. The coelom was apparently lost or reduced as a result of
mutations 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, since it is not
monophyletic. However, it is still used as a descriptive term.
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)
*
eutely in some
* loss of
larval stage in some
* possibly
pedomorphism
Pseudocoelomate phyla
According to Brusca and Brusca,
bilaterian pseudocoelomate phyla include:
*
Rotifera
*
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
*
Nematomorpha
*
Acanthocephala
*
Loricifera
Some authors list the following phyla as pseudocoelomates:
Ecdysozoan pseudocoelomates
*
Nematoda (roundworms)
*
Nematomorpha (nematomorphs or horsehair worms)
*
Loricifera
*
Priapulida
*
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, ...
Spiralian pseudocoelomates
*
Gastrotricha
*
Entoprocta
*
Rotifera (rotifers)
*
Acanthocephala (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
*
Gastrotricha, traditionally viewed as blastocoelomates
*
Entoprocta, traditionally viewed as blastocoelomates
*
Gnathostomulida, 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
cnidarians (
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 umbre ...
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"), ...
(comb jellies),
platyhelminthes (
flatworms including
tapeworms, etc.),
Nemertea, and
Gastrotricha.
See also
*
Abdominal cavity
References
Further reading
*
*
*
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Animal anatomy
Animal developmental biology
Mesoderm