The coelom (or celom)
Etymology
The term
coelom derives from the
Ancient Greek word () 'cavity'.
Structure
Development
The coelom is the mesodermally lined cavity between the gut and the outer body wall.
During the embryogenesis, coelom formation begins in the gastrulation stage. The developing digestive tube of an embryo forms as a blind pouch called the archenteron.
In protostomes, the coelom forms by a process known as schizocoely.
In deuterostomes, the coelom forms by enterocoely.
Origins
The
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 ancestors. This is supported by research on 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
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
(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.
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 Phylogenetics 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".[Nielsen, C. (2010). " The 'new phylogeny'. What is new about it?" Palaeodiversity 3, 149–150.]
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 germ layer). 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 , 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 ; however, not all protostomes are pseudocoelomates. An example of a pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Blastocoele.
Acoelomate animals, like , 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
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
, and those without are called
. There are also subtypes of coelom:
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 phylum possess a coelom:
Pseudocoelomates
In some
, 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.
A pseudocoelomate or blastocoelomate is any invertebrate animal with a three-layered body and a Body cavity. The coelom was apparently lost or reduced as a result of in certain types of that affected early development. Thus, pseudocoelomates evolved from coelomates.[Evers, Christine A., Lisa Starr. Biology:Concepts and Applications. 6th ed. United States:Thomson, 2006. .] "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.
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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
Bilaterian pseudocoelomate phyla according to Brusca and Brusca,:
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.
A body cavity is also absent in , (jellyfish and allies) and the Ctenophora (comb jellies), but these animals are neither or Triploblasty.
See also
Further reading
