Geplaatst op 29 maart 2010 - 19:47
Alvast bedankt voor jullie hulp
Geplaatst op 02 april 2010 - 20:56
Primary and secondary body cavities, acoelomates, pseudocoelomates, and coelomates
Some animals lack any cavity; their cells are in close contact with each other, separated only by the extracellular matrix. Such organisms are known as acoelomates and have what can be called a "compact organization." However, many organisms have some type of cavity: Small interstitial spaces between cells, tube-like systems, large spaces, repeating units, and so forth (Schmidt-Rhaesa 2007).
Generally, two structural types of body cavities are recognized. One type of body cavity may be termed a primary body cavity and the other termed a secondary body cavity. More common terminology is to call one type of body cavity a pseudocoelom, and animals with this body plan pseudocoelomates, and the other type of body cavity a coelom, and animals with this body plan coelomates.
Since a cavity itself lacks features, body cavities can only be characterized on the basis of the surrounding structures or those structures internal to the cavity (Jenner 2004). A coelom is a fluid-filled body cavity that separates the digestive tract and the outer body wall and is completely lined with mesoderm (Simmons 2004). The surfaces of the coelom are covered with a peritoneum, which is a slick epithelial layer (Yeh 2002). Schmidt-Rhaesa (2007), among others, equates coelom with secondary body cavity; "the secondary body cavity is commonly named the coelom." The pseudocoelom is a fluid-filled body cavity that separates the digestive tract and the outer body wall and is not completely lined with mesoderm (Simmons 2004). This pseudocolom, which develops between the mesoderm and the endoderm, is a persistent blastocoel, or fluid-filled cavity, of the blastula stage of the embryo (Yeh 2002). Schmidt-Rhaesa (2007), among others, equates the term primary body cavity with the pseudocoelom: "The primary body cavity is sometimes called a pseudocoel." Schmidt-Rhaesa (2007), in the book The Evolution of Organs, actually differentiates the two types of cavities as the primary body cavity has an extracellular matrix that borders the entire cavity, whereas in the secondary body cavity, there is a cellular layer (epithelium) that itself rests on the extracellular matrix.
However, although coelom is unambiguously defined (Schmidt-Rhaesa 2007), the terminology of primary and secondary cavities, and aceoelomate and pseudocoelomate, although long appearing in the literature, are not rigorously defined and in some cases there has been a misleading use of the terms (Jenner 2004). For example, Jenner (2004) references the use of acoelomate also for some animals with a primary body cavity. And Yeh (2002) refers to the primary body cavity as including the digestive system (gut tube or visceral tube) and the secondary body cavity as including both organisms with a pseudocoelom or with a true coelom (for example, "animal species with a secondary body cavity, either a pseudocoelom or a true coelom"). That is, according to Yeh, acoelomates, such as sponges and flatworms, have a single body cavity, and pseudocoelomates, such as roundworms and rotifers, have a secondary body cavity. Simmons (2004) similarly notes that "primitive animals … developed only one major body cavity, the digestive tract" and "all triploblastic animals pass the Playthelminthes have some form of secondary body cavity."
Note that the term human body cavities normally refers to the ventral body cavity, because it is by far the largest one in area.
Blijkbaar zijn de termen niet zo duidelijk gedefinieerd en zorgen ze wel eens voor verwarring...
Of anders eventueel nog: http://www.tutorvist...ary-body-cavity
Geplaatst op 09 april 2010 - 13:19
Geplaatst op 11 april 2010 - 16:52
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