The phylum Cnidaria contains around 10,000 species which are found only in aquatic (mostly marine) habitats. The phylum is divided into 2 subphyla; Anthozoa and Medusozoa which reflect the 2 major body forms – the polyp (as exhibited by Anthozoan corals and sea anemones) and the medusa (typical of jellies and hydroids). All have a radial symmetry with mouths surrounded by tentacles that bear cnidocytes. Both forms also have a single orifice and body cavity that are used for digestion and respiration. Nearly all cnidaria are predatory, and their nematocysts can paralyse and kill prey much larger than they are. Small sensory hairs near the nematocysts are sensitive to vibrations in the water and any prey swimming past can trigger the nematocyst which shoots out the barb. This penetrates the prey’s outer covering and injects it with venom, then the prey is then moved to the mouth by a tentacle. Many Cnidaria live in colonies made up of large numbers of individuals joined together in some way. These individuals (called zooids) can either be directly connected by tissues or share a common exoskeleton made from chiton or calcium carbonate. Although there are a vast array of species, the relative number that are kept ornamentally is small. Increasingly, many ornamental species are now propagated in captivity. Advances in this field are even allowing captive grown corals to be used to repopulate degraded wild reefs. Click or touch a thumbnail title text to explore a particular branch of these fascinating organisms.

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