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...
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