Polyp (zoology)

[citation needed] 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 cells.

[1] Polyps extend their tentacles, particularly at night, containing coiled stinging nettle-like cells, or nematocysts, which pierce, poison, and firmly hold living prey paralysing or killing them.

[2] Longitudinal muscular fibrils 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.

[1] 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.

A polyp is an animal of very simple structure,[1] a living fossil that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian sedimentary rock).

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 mesenteries.

[3] When eggs and sperm (gametes) are formed, they can produce zygotes derived from "selfing" (within the founding clone) or out-crossing, that then develop into swimming planula larvae.

[4] The name polyp was given by René Antoine Ferchault de Réaumur[6] to these organisms from their superficial resemblance to an octopus (French: poulpe, ultimately from Ancient Greek adverb πολύ (poly, "much") + noun πούς (pous, "foot")), with its circle of writhing arms round the mouth.

[1] 75% of the world's corals are threatened[7] due to overfishing, destructive fishing, coastal development, pollution, thermal stress, ocean acidification, crown-of-thorns starfish, and introduced invasive species.

[10][11] There have been many experiments supporting the hypothesis that heat stress in Acropora tenuis juvenile polyps provokes an up-regulation of protein in the endoplasmic reticulum.