They catch plankton with their tentacles. The algae need sunlight for photosynthesis, so they must be relatively close to the surface of the water. They are confined to shallow water because they have a mutualistic relationship with algae that live inside them. Some cnidarians live alone, while others live in colonies.Ĭorals form large colonies in shallow tropical water. They may live in water that is shallow or deep, warm or cold. It also helps them balance.Ĭnidarians can be found in almost all ocean habitats. These senses give them a sense of up versus down. For example, jellyfish have light-sensing structures and gravity-sensing structures. They may also have other sensory structures. Fluid in the coelenteron creates a hydrostatic skeleton.Ĭnidarians have a simple nervous system consisting of a nerve net that can detect touch. Nutrients are absorbed and gases exchanged through the cells lining this cavity. Digestion takes place in the coelenteron. The tentacles are covered with nematocyst cells. The single opening is surrounded by tentacles, which are used to capture prey. Cnidarians have a simple digestive system. The cellssurround a digestive cavity called the coelenteron (see Figure below). The body of a cnidarian consists of two cell layers, ectoderm and endoderm. Cnidarians may exist in the polyp (left) or medusa (right) form. Other species exist in just one form or the other.Ĭnidarian Body Plans. Some cnidarian species alternate between polyp and medusa forms. The medusa(plural, medusae) has a bell-shaped body and is typically motile. The polyp has a tubular body and is usually sessile. There are two basic body plans in cnidarians. A cnidarian nematocyst is like a poison dart. They are used to attack prey or defend against predators.Ĭnidarian Nematocyst. These tiny poison "darts" are propelled out of special cells. A nematocyst is a long, thin, coiled stinger. It’s a nematocyst, like the one shown in Figure below. \)Īll cnidarians have something in common.
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