Anthozoa

Anthozoa is a class of marine invertebrates which includes the sea anemones, stony corals as well as soft corals. adult anthozoans are nearly all attached to the seabed, while their larvae can disperse as component of the plankton. The basic point of the adult is the polyp; this consists of a cylindrical column topped by a disc with a central mouth surrounded by tentacles. Sea anemones are mostly solitary, but the majority of corals are colonial, being formed by the budding of new polyps from an original, founding individual. Colonies are strengthened by calcium carbonate as well as other materials and work various massive, plate-like, bushy or leafy forms.

Anthozoa is covered within the phylum Cnidaria, which also includes the jellyfish, box jellies & parasitic Myxozoa and Polypodiozoa. The two leading subclasses of Anthozoa are the Hexacorallia, members of which name six-fold symmetry and includes the stony corals, sea anemones, tube anemones and zoanthids; and the Octocorallia, which have eight-fold symmetry and includes the soft corals and gorgonians sea pens, sea fans and sea whips, and sea pansies. The smaller subclass, Ceriantharia, consists of the tube-dwelling anemones. Some additional sort are also pointed as incertae sedis until their exact taxonomic position can be ascertained.

Anthozoans are carnivores, catching prey with their tentacles. Many classification supplement their power to direct or build needs by making use of photosynthetic single-celled algae that cost within their tissues. These species exist in shallow water and numerous are reef-builders. Other species lack the zooxanthellae and, having no need for well-lit areas, typically live in deep-water locations.

Unlike other members of this phylum, anthozoans do non have a medusa stage in their development. Instead, they release sperm and eggs into the water. After fertilisation, the planula larvae form component of the plankton. When fully developed, the larvae decide on the seabed and attach to the substrate, undergoing metamorphosis into polyps. Some anthozoans can also reproduce asexually through budding or by breaking in pieces.

Anatomy

The basic body form of an anthozoan is the polyp. This consists of a tubular column topped by a flattened area, the oral disc, with a central mouth; a whorl of tentacles surrounds the mouth. In solitary individuals, the base of the polyp is the foot or pedal disc, which adheres to the substrate, while in colonial polyps, the base links to other polyps in the colony.

The mouth leads into a tubular pharynx which descends for some distance into the body before opening into the coelenteron, otherwise required as the gastrovascular cavity, that occupies the interior of the body. Internal tensions pull the mouth into a slit-shape, and the ends of the slit lead into two grooves in the pharynx wall called siphonoglyphs. The coelenteron is subdivided by a number of vertical partitions, known as mesenteries or septa. Some of these advance from the body wall as far as the pharynx and are known as "complete septa" while others do non extend so far and are "incomplete". The septa also attach to the oral and pedal discs.

The body wall consists of an epidermal layer, a jellylike mesogloea layer and an inner gastrodermis; the septa are infoldings of the body wall and consist of a layer of mesogloea sandwiched between two layers of gastrodermis. In some taxa, sphincter muscles in the mesogloeaover the oral disc and act to keep the polyp fully retracted. The tentacles contain extensions of the coelenteron and have sheets of longitudinal muscles in their walls. The oral disc has radial muscles in the epidermis, but most of the muscles in the column are gastrodermal, and put strong retractor muscles beside the septa. The number and arrangement of the septa, as living as the arrangement of these retractor muscles, are important in anthozoan classification.

The tentacles are armed with nematocysts, venom-containing cells which can be fired harpoon-fashion to snare and subdue prey. These need to be replaced after firing, a process that takes about forty-eight hours. Some sea anemones have a circle of acrorhagi external the tentacles; these long projections are armed with nematocysts and act as weapons. Another form of weapon is the similarly-armed acontia threadlike defensive organs which can be extruded through apertures in the column wall. Some stony corals employ nematocyst-laden "sweeper tentacles" as a defence against the intrusion of other individuals.

Many anthozoans are colonial and consist of house polyps with a common origin joined together by living material. The simplest arrangement is where a stolon runs along the substrate in a two dimensional lattice with polyps budding off at intervals. Alternatively, polyps may bud off from a sheet of living tissue, the coenosarc, which joins the polyps and anchors the colony to the substrate. The coenosarc may consist of a thin membrane from which the polyps project, as in most stony corals, or a thick fleshy mass in which the polyps are immersed except their oral discs, as in the soft corals.

The skeleton of a stony coral in the design Scleractinia is secreted by the epidermis of the lower part of the polyp; this forms a corallite, a cup-shaped hollow portrayed of calcium carbonate, in which the polyp sits. In colonial corals, coming after or as a sum of. growth of the polyp by budding, new corallites are formed, with the surface of the skeleton being covered by a layer of coenosarc. These colonies follow a range of massive, branching, leaf-like and encrusting forms. Soft corals in the subclass Octocorallia are also colonial and have a skeleton formed of mesogloeal tissue, often reinforced with calcareous spicules or horny material, and some have rod-like maintain internally. Other anthozoans, such as sea anemones, are naked; these rely on a hydrostatic skeleton for support. Some of these species have a sticky epidermis to which sand grains and shell fragments adhere, and zoanthids incorporate these substances into their mesogloea.