Cnidarians
The phylum Cnidaria includes both solitary and colonial organisms that have radial and/or bilateral symmetry. Typical cnidarians alternate each generation between a fixed polyp stage and a free living medusoid stage. Most cnidarians are considered carnivores because of their ability to actually catch food with their stinging cells called nematocysts. Some groups, particularly the reef-corals employ photosynthetic algae (zooxanthellae) within their tissues in a symbiotic relationship to aid in supplying food needed for their rapid growth. Cnidaria contains over 10,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are mostly one cell thick. They have two basic body forms: swimming medusae and sessilepolyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single orifice and body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by a decentralized nerve net and simple receptors. Several free-swimming Cubozoa and Scyphozoa possess balance-sensingstatocysts, and some have simple eyes. Not all cnidarians reproduce sexually. Many have complex lifecycles with asexualpolyp stages and sexual medusae, but some omit either the polyp or the medusa stage.