Polyp
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2008-06-07T17:09:05Z
EmmetCaulfield
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rv [[WP:UNDO|Undid]] revision 215582143 by [[Special:Contributions/69.253.191.175|69.253.191.175]] ([[User talk:69.253.191.175|talk]])
{{two other uses|the biological structure|the medical condition|Polyp (medicine)|the cartoonist|P J Polyp}}
[[Image:Coral polyp.jpg|right|thumb|250px|Anatomy of a coral polyp.]]
In [[zoology]], a '''polyp''' is one of two forms of individuals found in many species of [[cnidaria]]ns. The two are the polyp or hydroid and the [[medusa (biology)|medusa]]. Polyps are approximately [[cylindrical]], elongated on the axis of the body. The [[aboral]] end is attached either to the [[substrate (biology)|substrate]] by means of a disc-like [[holdfast]] if the polyp is [[solitary]], or is connected to other polyps, either directly or indirectly, if the polyp is part of a [[colony (biology)|colony]]. The [[oral]] end bears the [[mouth]], and is surrounded by a circlet of [[tentacle]]s.
==Classes==
In the [[class (biology)|class]] ''[[Coral|Anthozoa]]'', comprising the [[sea anemones]] and [[coral]]s, the individual is always a polyp; in the class ''[[Hydrozoa]]'', however, the individual may be either a polyp or a medusa, with most species undergoing a [[biological life cycle|life cycle]] with both a polyp stage and a medusa stage. In class [[Scyphozoa]], the medusa stage is dominant, and the polyp stage may or may not be present, depending on the [[family (biology)|family]]. In those scyphozoans that have the [[larval]] [[planula]] [[metamorphose]] into a polyp, the polyp, also called a "scyphistoma," grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known as [[strobilation]]. Once strobilation is complete, the polyp may die, or [[regenerate]] itself to repeat the process again later.
==Anatomy==
The body of the polyp may be roughly compared in a structure to a [[sac]], the wall of which is composed of two layers of [[cell (biology)|cell]]s. The outer layer is known technically as the [[germ layer|ectoderm]], the inner layer as the [[germ layer|endoderm]] (or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed [[mesogloea]], secreted by the cell layers of the body wall. The mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains [[endoskeleton|skeletal]] elements formed by cells which have [[cell migration|migrated]] into it from the ectoderm.
The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle of [[tentacle]]s which resemble glove fingers. The tentacles are [[organ (anatomy)|organ]]s which serve both for the tactile sense and for the capture of food. By means of the stinging nettle-cells or [[nematocyst]]s with which the tentacles are thickly covered, living organisms of various kinds are firmly held and at the same time paralysed or killed, and by means of longitudinal [[muscular fibril]]s formed from the cells of the ectoderm the tentacles are contracted and convey the food to the mouth. By means of circularly disposed muscular fibrils formed from the endoderm the tentacles can be protracted or thrust out after contraction. By muscle fibres belonging to the same two systems, the whole body may be retracted or protruded.
We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the [[peristome]], in the centre of which again is the mouth. As a rule there is no other opening to the body except the mouth, but in some cases [[excretory]] [[pore]]s are known to occur in the foot, and pores may occur at the tips of the tentacles. Thus it is seen that a polyp is an animal of very simple structure, a [[living fossil]] that has not changed significantly for about half a billion years (per generally accepted dating of [[Cambrian]] [[sedimentary rock]]).
The external form of the polyp varies greatly in different cases. The column may be long and slender, or may be so short in the vertical direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be long and filamentous, or short and reduced to mere knobs or warts. They may be simple and unbranched, or they may be feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped. As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic of one of the two classes, [[Hydrozoa]] and [[Coral|Anthozoa]].
In the class [[Hydrozoa]], the polyps are indeed often very simple, like the common little [[freshwater]] [[species]] of the [[genus]] ''[[Hydra (genus)|Hydra]]''. [[Coral|Anthozoa]]n polyps, including the [[coral]]s and [[sea anemone]]s, are much more complex due to the development of a tubular [[stomodaeum]] leading inward from the mouth and a series of radial partitions called [[Mesentery|mesenteries]]. Many of the mesenteries project into the enteric cavity but some extend from the body wall to the central stomodaeum.
==Reproduction==
It is an almost universal attribute of polyps to possess the power of reproducing themselves [[asexual reproduction|asexually]] by the method of [[budding]]. This mode of [[reproduction]] may be combined with [[sexual reproduction]], or may be the sole method by which the polyp produces [[offspring]], in which case the polyp is entirely without [[sexual organ]]s. In many cases the buds formed do not separate from the parent but remain in continuity with it, thus forming colonies or stocks, which may reach a great size and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies. The [[reef]]-building corals are polyp-colonies, strengthened by the formation of a firm skeleton.
==Etymology==
The name polyp was given to these organisms from their supposed resemblance to an [[octopus]] (Fr. ''poulpe''), with its circle of writhing arms round the mouth. This comparison, though far-fetched, is certainly more reasonable than the common name "coral-insects" applied to the polyps which form coral.
==References==
* {{1911|article=Polyp|url=http://encyclopedia.jrank.org/PIG_POL/POLYP.html}}
==See also==
* [http://etic.univ-reunion.fr/hydroids/ Hydroids from Reunion Island and Indian Ocean]
*[[Zoanthid]]
*[[Portuguese Man o' War]]
[[Category:Cnidarians]]
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