The polyp tissues
The sac-like body cavity of the coral polyp is the coelenteron, which has a single opening to the outside. The coelenteron of one polyp is linked to those of adjacent polyps by tubes through which water circulates and nutrients are transported. The coelenteron serves many functions including digestion and the circulation of fluids for respiration and nutrition. The mouth leads to a short tube, the pharynx, which opens into the body cavity. In most corals it is short; in others (notably Goniopora and Alveopora) it is extraordinarily extendable, allowing the mouth and tentacles to protrude far beyond the skeleton to aid food capture. The coelenteron is a complex structure, made so by the skeletal structures, all of which lie outside it, but which are enveloped by it. The coelenteron is partitioned by vertical mesenteries, arranged in a radial fashion according to the position of the septa. These mesenteries give the gastrodermis a large surface area for digestion, photosynthesis and respiration, and also contain the reproductive organs. A series of coiled filaments, the mesenteric filaments, are packed along the inner margins of the mesenteries. These further extend the surface area of the mesenteries and are extruded through the mouth in response to stress.
As with all coelenterates, the body wall is primarily composed of two cell layers, the ectodermis on the outside and the gastrodermis on the inside. These layers are separated by the mesoglea, which is initially non-cellular but which may contain a wide range of cells after initial growth. In corals with small corallites the mesoglea is microscopically thin while in others, notably the big mussids, it may be several millimetres thick and is of tough construction.
Extended polyps have an anemone-like appearance. The mouth is usually slit-like and may be surrounded by an oral cone. The tissue between the cone or mouth may be raised into an oral disc.
Tentacles are tubular and have the same two tissue layers as the rest of the polyp so that the cavity inside them is part of the coelenteron. Tentacles are smooth in corals that feed on detritus but most have stinging cells for defence or food capture. These cells, the nematocysts, are microscopic in size, but in most corals are grouped into wart-like nematocyst batteries, which are clearly visible underwater. Nematocysts also occur on vesicles of Physogyra and Plerogyra, which are sac-like structures composed of body wall that are inflated with water when tentacles are retracted during the day. Other cells of the ectodermis secrete slimy mucous which coats the polyp and which is moved around by microscopic cilia. The mucous is used to remove sediment from the polyp surface and is also used by detritus feeders to capture food.
Cnidaria are the simplest organisms to have discrete nervous, muscular and reproductive systems and in corals all these are well developed. A simple nerve net composed of both ectodermal and gastrodermal cells permeates the body wall, with connections to a variety of specialised cells responsible for sensing mechanical and chemical stimuli as well as light. A muscular system, consisting of specialised cells of both ectodermal and gastrodermal origin, allows polyps to extend and retract in response to signals from the nerve net. These signals are transmitted from polyp to polyp, as seen in the progressive retraction of polyps when part of a colony is mechanically disturbed.
Reproductive organs develop within the mesoglea of the mesenteries. This happens on an annual cycle in most species, after which the organs disappear, to re-form the following year. Some corals, notably Fungia and Porites, have separate male and female sexes, but most are hermaphroditic. In either case, the gonads are arranged around the base of the pharynx in radial symmetry. Some hermaphrodite corals have male and female gonads on different mesenteries, in others the testes are above the ovaries on the same mesenteries and in others the testes and ovaries grow together.
The gastrodermis has an array of specialised cells for digestion, part of which occurs in the body cavity and part inside the digestive cells themselves. Nutrients are readily moved among polyps so that neighbouring polyps have a similar rate of growth and thus do not compete for space. The gastrodermis also contains the zooxanthellae, the unicellular symbiotic algae which are essential to the growth and survival of most zooxanthellate corals.