Cnidarians

Cnidarians

Cnidaria is a phylum containing some 11,000 species of simple animals found exclusively in aquatic, mostly marine, environments. Cnidarians get their name from cnidocytes, which are special cells that carry stinging organelles. The corals, which are important reefbuilders, belong here, as do the sea anemones, jellyfish, sea pens, sea pansies and sea wasps. The names Coelenterata and Coelentera were once applied to the group, but those names included the Ctenophores,comb jellies, and have been abandoned. Cnidarians are very evident in the fossil records, when they first appeared in the Precambrian era.The basic body shape of a cnidarian consists of a sac with a gastrovascular cavity, with a single opening that functions as both mouth and anus. Ithas radial symmetry, meaning that whatever way it is cut along its central axis, the resulting halves would always be mirror images of each other. Their rmovement is coordinated by a decentralized nerve net and simple receptor.There are four main classes of Cnidaria Class Anthozoa ,anemones, corals, etc. Hydrozoa , Portuguese Man o' War, Obelia, etc, Scyphozoa , jellyfish, Cubozoa, box jellies. Several free-swimming Cubozoa and Scyphozoa have rhopalia, complex sensory structures that can include image-forming eyes with lenses and retinas, and a gravity-sensing statolith like the function in the otolith of the vertebrate inner ear. Tentacles surrounding the mouth contain nematocysts, special stinging cells, which they use to catch prey and defend themselves from predators. The ability to sting is what gives cnidarians their name.

Cnidarians are eumetazoan, possessing true tissues and organs. It has radial symmetry and is composed of two layers of tissue, known as the ectoderm and endoderm, or gastroderm, with a gelatinous mesoglea in between them containing only scattered cells. The organisms are considered to be diploblastic, but the mesoglea may be similar to the mesoderm in other animals.The ectoderm surrounds the cnidarian's; stomach., or gastrovascular cavity which functions as both mouth and anus; it is used both to ingest food and excrete waste. It also serves along with the mesogloea as a hydrostatic supporting skeleton. Firm skeletons are only found among polyps, which produce lime for that purpose.

The cnidarian does not possess a true circulatory system. Respiration takes place

through diffusion of oxygen directly through their tissues without specialised organs such as tracheae, gills or lungs. The gastrovascular system plays a role in the digestion and dispersion of food and the removal of metabolic waste: it surrounds the gastrovacular cavity as well as its extensions in the tentacles of polyps. The gastrovacular system serves two separate functions, digestion and transportion. Food particles are gathered by the feeding muscles of the gastroderm.Members of Cnidaria have life-cycles that alternate between asexual polyps and sexual, free-swimming forms called medusae. Medusae have a hat or bell-shaped appearance and mostly swim passively with the current. Their tentacles hang freely below their bodies. However, they can actively swim by means of co-ordinated muscle contractions against the water contained in their gastrovacular cavity. Polyps, in contrast, are anchored to the substrate by their basal discs, although a few species can move in curious slow-motion somersaults. By nature they display their tentacles upwards, away from the substrate. Polyps often live in large colonies.

In reality there is a vast variation within the life-cycles of cnidarians. Within the group Anthozoa, the medusal stage is virtually non-existent; the larva, once fusing with the substratum and developing into the polyp stage, grows benthic or sessile, meaning it no longer metamorphosises into the medusal stage. Among the Scyphozoa and Cubozoa, the medusae are the dominant form in the life-cycle, while the polyps are in turn reduced or absent Medusae are extremely varied and range in size from a few millimeters to over 30 metres,with tentacles. The Hydrozoa are intermediate, with significant medusoid and polyp forms.

Cnidarians take their name from a specialised cell, the cnidocyte (nettle cell). Tentacles surrounding the mouth contain nematocysts, specialized stinging cells. The nematocysts are the cnidarians main form of offence or defense and function by a chemical or physical trigger that causes the specialized cell to eject a barbed and poisoned hook that can stick into, ensnare, or entangle prey or predators, killing or at least paralysing its victim.

Another important cell type is the interstitial cell, pluripotent cells that can transform into other cell types such as spermatazoa, adenocytes or nerve cell, though not into epithelial or feeding muscle cells; the latter two can only be produced by cells of the same type. These give many cnidaria an extra capacity for regeneration. In particular the genus hydra serves as a model for the research of pattern formation processes.

The movement of cnidaria is controlled by a decentralised net of true nerve cells. Concentrations of nerve cells are found in the mouth area ofpolyps ,the hypostome, on the tentacles and stem ,pedunculus, and with jellies a ring of nerves is often found around the screen . It was long acceptedthat cnidaria were diploblastic. New research indicates that cnidaria seem to possess a mesoderm in addition to the ecto- and endoderm, from which themusculature of the medusa develops, among others. Up until now taxonomists have classified cnidaria in