Dictionary:
coe·lom ce·lom or coe·lome (sē'ləm)  |
[German Koelom, from Greek koilōma, cavity, from koilos, hollow.]
coelomic coe·lom'ic (sĭ-lŏm'ĭk, -lō'mĭk) adj.| 5min Related Video: coelom |
| Sci-Tech Encyclopedia: Coelom |
The mesodermally lined body cavity of most animals above the flatworms and nonsegmented roundworms. Its manner of origin provides one basis for classifying the major higher groups.
Annelids, arthropods, and mollusks have a coelom which develops from solid mesodermal bands. Within the trochophore larva of annelids, a single pole cell proliferates two strips of mesoblast lying on either side of the ventral midline. These bands subdivide transversely into bilateral solid blocks, the somites. Each somite then splits internally to form a hollow vesicle, the cavity of which is the coelom. The mollusks also form bands of mesoderm from a single pole cell, but these bands do not segment. They split internally to form single right and left coelomic sacs, but the cavities are soon reduced and the surrounding mesoblast disperses as separate cells, many of which become muscle. The only remnants of the coelom in the adult are the pericardial cavity and the cavities of the gonads and their ducts. In arthropods paired bands of mesoblast may proliferate from a posterior growth center or may separate inward from a blastoderm, a superficial layer of cells, on the ventral surface of the egg. These bands divide into linear series of somites which then hollow out. Their cavities represent the coelom.
Echinoderms and chordates constitute a second major group, characterized by the origin of the coelom from outpocketings of the primitive gut wall. In echinoderms one pair of bilateral pouches evaginates and separates from the archenteron or primitive digestive cavity. Each pouch constricts into three portions, not homologous to the metameres of other animals.
The protochordates of the groups Hemichordata and Cephalochordata have three coelomic pouches formed by separate evaginations of the archenteral roof. In hemichordates the head cavity remains single as the cavity of the proboscis and has a pore to the exterior on each side. The second pouches form cavities within the collar and also acquire external pores. The third pair is contained within the trunk and forms the major perivisceral cavity.
In cephalochordates the head cavity divides into lateral halves. The left side communicates, by a pore, to an ectodermal pit called the wheel organ. The second pair of pouches forms the pair of mesoblastic somites, and the third pouches subdivide transversely to give rise to the remainder of the linear series of somites. The upper or myotomic portion of each somite remains metameric and forms the segmental muscles. As it enlarges, the coelomic space is displaced ventrally and expands above and below the gut to form the perivisceral cavities and mesenteries, as described for annelids.
In vertebrates the mesoderm arises as a solid sheet from surface cells that have been involuted through the blastopore. Lateral to the notochord, beginning at about the level of the ear, the mesoderm subdivides into three parts: (1) the somites; (2) the nephrotomic cord, temporarily segmented in lower vertebrates, which will form excretory organs and ducts; and (3) the unsegmented lateral plate. The coelom arises as a split within the lateral plate. See also Animal kingdom; Gastrulation.
| Columbia Encyclopedia: coelom |
| Veterinary Dictionary: celom |
Body cavity, especially the original cavity in the mammalian embryo between the somatopleure and splanchnopleure, which is both intra- and extraembryonic; the principal cavities of the trunk, the pericardial, pleural and peritoneal sacs, arise from the intraembryonic portion.
| Wikipedia: Body cavity |
By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal’s outer covering (epidermis) and the outer lining of the gut cavity, where internal organs develop. "The body cavity" of human body cavities normally refers to the ventral body cavity, because it is by far the largest one in volume.
Contents |
The type of body cavity places an organism into one of three groups:[1]
A coelom is a cavity lined by an epithelium derived from mesoderm. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.
Arthropods 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 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 / pericardium, where the heart develops, simplifies discussion of the anatomies of complex animals.
Coelom formation begins in the gastrula stage. The developing digestive tube of an embryo forms as a blind pouch called the archenetron.
In Protostomes, a process known as schizocoelus happens: as the archenteron initially forms, the mesoderm splits to form the coelomic cavities. In Deuterostomes, a process known as enterocoelus happens: the mesoderm buds from the walls of the archenteron and hollows to become the coelomic cavities.
The origin of the coelom is uncertain. The oldest known animal to have had a body cavity is Vernanimalcula.
Current evolutionary theories:
In some protostomes, 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 is any invertebrate animal with a three-layered body and a pseudocoel. The coelom was apparently lost or reduced as a result of mutations in certain types of genes that affected early development. Thus, pseudocoelomates evolved from coelomates.[2]
Important characteristics:
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.
This entry is from Wikipedia, the leading user-contributed encyclopedia. It may not have been reviewed by professional editors (see full disclaimer)
 | acoelomate (zoology) |
| coelomostome (invertebrate zoology) |
| myocoel (embryology) |
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved. Read more |
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| Sci-Tech Encyclopedia. McGraw-Hill Encyclopedia of Science and Technology. Copyright © 2005 by The McGraw-Hill Companies, Inc. All rights reserved. Read more |
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| Columbia Encyclopedia. The Columbia Electronic Encyclopedia, Sixth Edition Copyright © 2003, Columbia University Press. Licensed from Columbia University Press. All rights reserved. www.cc.columbia.edu/cu/cup/. Read more |
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| Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved. Read more |
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| Wikipedia. This article is licensed under the Creative Commons Attribution/Share-Alike License. It uses material from the Wikipedia article "Body cavity". Read more |
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