Editing Polyp (zoology) (section)

==Anatomy==
[[Image:Coral polyp.jpg|thumb|Anatomy of a [[coral]] polyp]]{{One source|date=January 2025|section}}
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 [[Germ layer|layer]] is known technically as the ''[[ectoderm]]'', with the inner layer as the ''[[endoderm]]'' (or ''gastroderm''). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed [[mesogloea|mesoglea]], secreted by the cell layers of the body wall.<ref name="EB1911" /> The mesoglea can be thinner than the endoderm or ectoderm or comprise the bulk of the body as in larger [[jellyfish]]. The mesoglea can contain [[endoskeleton|skeletal]] elements derived from cells [[cell migration|migrated]] from ectoderm.{{Citation needed|date=January 2025}}

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.<ref name="EB1911" /> Polyps extend their tentacles, particularly at night, containing coiled stinging nettle-like cells, or [[nematocyst]]s, which pierce, poison, and firmly hold living prey paralysing or killing them. Polyp prey includes [[copepods]] and fish [[larva]]e.<ref>Chang, T.D. and Sullivan, J.M. "[http://www.dartmouth.edu/~biofsp/pdf08/133_SIFP1_ACCEPTED_coral_zoops.pdf Temporal associations of coral and zooplankton activity on a Caribbean reef] {{webarchive|url=https://web.archive.org/web/20110606021125/http://www.dartmouth.edu/~biofsp/pdf08/133_SIFP1_ACCEPTED_coral_zoops.pdf |date=2011-06-06 }}" Dartmouth Studies in Tropical Ecology. 2008. Accessed 2009-06-21.</ref> Longitudinal [[muscular fibril]]s formed from the cells of the ectoderm allow tentacles to contract when conveying the food to the mouth. Similarly, circularly disposed muscular fibrils formed from the endoderm permit tentacles to be protract or thrust out once they are contracted. These muscle fibres belong to the same two systems, allowing the whole body to retract or protrude outwards.<ref name="EB1911" />

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. Generally, there is no other opening to the body except the mouth, but in some cases [[excretory]] pores are known to occur in the foot, and pores may occur at the tips of the tentacles. A polyp is an animal of very simple structure,<ref name="EB1911" /> a [[living fossil]] that has not changed significantly for about half a billion years (per generally accepted dating of [[Cambrian]] [[sedimentary rock]]).{{Citation needed|date=January 2025}}

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 Anthozoa.<ref name=EB1911/>

In the class Hydrozoa, the polyps are indeed often very simple, like the common little [[fresh water]] species of the genus ''[[Hydra (genus)|Hydra]]''. Anthozoan polyps, including the corals and sea anemones, 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 (zoology)|mesenteries]]. Many of the mesenteries project into the enteric cavity but some extend from the body wall to the central stomodaeum.{{Citation needed|date=January 2025}}