Body cavity
Overview
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

. However, the term usually refers to the space located between an animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The human body cavity
Human body cavities
The human body consists of the following body cavities:*dorsal body cavity**cranial cavity, enclosed by the skull and contains the brain, eyes, and ears.**spinal canal, enclosed by the spine and contains the spinal cord.*ventral body cavity...

" normally refers to the ventral body cavity
Ventral body cavity
The ventral body cavity is a human body cavity that is in the anterior aspect of the human body. It is made up of the thoracic cavity, and the abdominopelvic cavity. The abdominopelvic cavity is further separated into the abdominal cavity and pelvic cavity, but there is no physical barrier...

, because it is by far the largest one in volume.
The type of body cavity places an organism into one of three groups:
  • Coelomates (also known as eucoelomates — "true coelom") have a fluid filled body cavity called a coelom
    Coelom
    The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

     (icon) with a complete lining called peritoneum
    Peritoneum
    The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

     derived from mesoderm (one of the three primary tissue layers
    Germ layer
    A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...

    ).
Encyclopedia
By the broadest definition, a body cavity is any fluid-filled space in a multicellular organism
Multicellular organism
Multicellular organisms are organisms that consist of more than one cell, in contrast to single-celled organisms. Most life that can be seen with the the naked eye is multicellular, as are all animals and land plants.-Evolutionary history:Multicellularity has evolved independently dozens of times...

. However, the term usually refers to the space located between an animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The human body cavity
Human body cavities
The human body consists of the following body cavities:*dorsal body cavity**cranial cavity, enclosed by the skull and contains the brain, eyes, and ears.**spinal canal, enclosed by the spine and contains the spinal cord.*ventral body cavity...

" normally refers to the ventral body cavity
Ventral body cavity
The ventral body cavity is a human body cavity that is in the anterior aspect of the human body. It is made up of the thoracic cavity, and the abdominopelvic cavity. The abdominopelvic cavity is further separated into the abdominal cavity and pelvic cavity, but there is no physical barrier...

, because it is by far the largest one in volume.

Grouping

The type of body cavity places an organism into one of three groups:
  • Coelomates (also known as eucoelomates — "true coelom") have a fluid filled body cavity called a coelom
    Coelom
    The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...

     (icon) with a complete lining called peritoneum
    Peritoneum
    The peritoneum is the serous membrane that forms the lining of the abdominal cavity or the coelom — it covers most of the intra-abdominal organs — in amniotes and some invertebrates...

     derived from mesoderm (one of the three primary tissue layers
    Germ layer
    A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or 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 vertebrate
    Vertebrate
    Vertebrates are animals that are members of the subphylum Vertebrata . Vertebrates are the largest group of chordates, with currently about 58,000 species described. Vertebrates include the jawless fishes, bony fishes, sharks and rays, amphibians, reptiles, mammals, and birds...

    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
    Protostome
    Protostomia 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; however, not all protostomes are pseudocoelomates. An example of a Pseudocoelomate is the roundworm. Pseudocoelomate animals are also referred to as Hemocoel and Blastocoelomate
    Blastocoele
    A blastocoel or blastocele is the fluid-filled central region of a blastocyst...

    .

  • Acoelomate animals, like flatworm
    Flatworm
    The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

    s, have no body cavity at all. Organs have direct contact with the epithelium
    Epithelium
    Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

    . Semi-solid mesodermal tissues between the gut and body wall hold their organs in place.

Coelom

A coelom is a cavity lined by an epithelium
Epithelium
Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. Epithelial tissues line the cavities and surfaces of structures throughout the body, and also form many glands. Functions of epithelial cells include secretion, selective...

 derived from mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

. 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
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...

s and mollusks 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
Intraembryonic coelom
In the development of the human embryo the intraembryonic coelom is a portion of the conceptus forming in the mesoderm. During the second week of development, the lateral mesoderm splits into a dorsal somatic mesoderm and a ventral splanchnic mesoderm...

 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 pericardial cavity
Pericardial cavity
The pericardial cavity is a potential space between the parietal pericardium and visceral layer. It contains a supply of serous fluid. The serous fluid that is found in this space is known as the pericardial fluid....

 / pericardium
Pericardium
The pericardium is a double-walled sac that contains the heart and the roots of the great vessels.-Layers:...

, where the heart develops, simplifies discussion of the anatomies
Anatomy
Anatomy is a branch of biology and medicine that is the consideration of the structure of living things. It is a general term that includes human anatomy, animal anatomy , and plant anatomy...

 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 archenteron
Archenteron
The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....

.

In Protostomes the coleom forms by a process known as schizocoely. The archenteron
Archenteron
The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....

 initially forms, and the mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

 splits into two layers: the first attaches to the body wall or ectoderm
Ectoderm
The "ectoderm" is one of the three primary germ cell layers in the very early embryo. The other two layers are the mesoderm and endoderm , with the ectoderm as the most exterior layer...

, forming the parietal layer and the second surrounds the endoderm
Endoderm
Endoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and mesoderm , with the endoderm as the intermost layer...

 forming the visceral layer or alimentary canal. 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: mesoderm
Mesoderm
In all bilaterian animals, the mesoderm is one of the three primary germ cell layers in the very early embryo. The other two layers are the ectoderm and endoderm , with the mesoderm as the middle layer between them.The mesoderm forms mesenchyme , mesothelium, non-epithelial blood corpuscles and...

 buds from the walls of the archenteron
Archenteron
The primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal....

 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
Vernanimalcula
Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to . The fossils are between 0.1 and 0.2 mm across...

.

Current evolutionary
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

 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 ("coelom")

Pseudocoelom

In some protostome
Protostome
Protostomia 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, the embryonic blastocoele
Blastocoele
A blastocoel or blastocele is the fluid-filled central region of a blastocyst...

 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 (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

s serves several functions like distribution of nutrients and removal of waste or supporting the body as a hydrostatic skeleton
Hydrostatic skeleton
A 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 circular and longitudinal muscles are used to change an...

.

Pseudocoelomates

Pseudocoelomate is no longer considered a valid taxonomic group
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

, since it is not monophyletic. However, it is still used as a descriptive term.


A pseudocoelomate is any invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...

 animal
Animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

 with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutation
Mutation
In molecular biology and genetics, mutations are changes in a genomic sequence: the DNA sequence of a cell's genome or the DNA or RNA sequence of a virus. They can be defined as sudden and spontaneous changes in the cell. Mutations are caused by radiation, viruses, transposons and mutagenic...

s in certain types of gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...

s that affected early development. Thus, pseudocoelomates evolved from coelomates.

Important characteristics:
  • lack a vascular blood system
    • diffusion
      Diffusion
      Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...

       and osmosis
      Osmosis
      Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...

       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 (roundworms)
  • Rotifera (rotifer
    Rotifer
    The rotifers make up a phylum of microscopic and near-microscopic pseudocoelomate animals. They were first described by Rev. John Harris in 1696, and other forms were described by Anton van Leeuwenhoek in 1703...

    s)
  • Kinorhyncha
    Kinorhyncha
    Kinorhyncha is a phylum of small marine pseudocoelomate invertebrates that are widespread in mud or sand at all depths as part of the meiobenthos...

  • Nematomorpha
    Nematomorpha
    Nematomorpha is a phylum of parasitic animals that are superficially morphologically similar to nematode worms, hence the name. They range in size in most species from long and can reach in extreme cases up to 2 metres, and in diameter...

    , nematomorphs, or horsehair worms
  • Gastrotrich
    Gastrotrich
    The gastrotrichs are a phylum of microscopic animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos...

    a
  • Loricifera
    Loricifera
    Loricifera is a phylum of very small to microscopic marine sediment-dwelling animals with twenty-two described species, in eight genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their size ranges from 100 µm to ca....

  • Priapulida
    Priapulida
    Priapulida is a phylum of marine worms. They are named for their extensible spiny proboscis, which, in some species, may have a shape like that of a human penis...

  • Acanthocephala
    Acanthocephala
    Acanthocephala is a phylum of parasitic worms known as acanthocephales, thorny-headed worms, or spiny-headed worms, characterized by the presence of an evertable proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host...

     (spiny-headed worms)
  • Aschelminth
    Aschelminth
    The Aschelminthes , closely associated with the Platyhelminthes, are an obsolete phylum of pseudocoelomate and other similar animals that are no longer considered closely related and have been promoted to phyla in their own right...

     animals
  • Entoprocta
    Entoprocta
    Entoprocta, whose name means "anus inside", 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 draw food particles towards the mouth, and...


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.

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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