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Cnidarians, also known as coelenterates, diverse group of aquatic, invertebrate animals armed with microscopic stinging structures. Cnidarians make up the phylum Cnidaria, which encompasses more than 9,000 species, including corals, hydras, jellyfishes, Portuguese man-of-war, and sea anemones. Cnidarians live in all oceans, and a few species inhabit fresh water.
Cnidarians have many shapes and they range in size from microscopic hydrozoans to jellyfishes that are 2 m (7 ft) in diameter with tentacles 30 m (100 ft) long. Although they have various physical characteristics, all cnidarians exhibit radial symmetry—that is, similar body parts radiate from a central mouth. Six to ten tentacles surround a cnidarian’s mouth to aid in the capture and ingestion of the animals they feed on.
Cnidarians have a saclike body with a single mouth opening. The body wall is composed of two sheets of cells—an inner layer (the endoderm) and an outer layer (the ectoderm). A gelatinous mesoglea layer holds these two cell layers together. Cnidarians are invertebrates (animals that lack a backbone), but the ectoderm of some cnidarians, including hard corals and some hydrozoans, may form a skeleton-like structure externally. The ectoderm of other cnidarians, such as some soft corals, forms an internal skeleton-like structure. The ectoderm and endoderm layers contain contractile fibers that enable the animal to move about. Invertebrate zoologists believe these fibers are primitive versions of the muscle cells found in more complex animals.
Cnidarians lack internal organs and they do not have digestive, circulatory, or respiratory systems. Secretions from endoderm cells digest food within the central body cavity and endoderm cells also distribute nutrients and dissolved oxygen to all parts of the body. Lacking an anus, cnidarians discharge waste matter through the mouth opening.
During their life cycle cnidarians may form two types of body structures, the polyp and the medusa. Some cnidarians alternate between polyp and medusa at different stages of their life cycles, some cnidarians live solely as polyps, and others only as medusae.
A polyp has a cylindrical shape. The mouth, surrounded by tentacles, is located at one end of the polyp, and the opposite end of the polyp typically attaches to a rock or other object on the sea bottom. Polyps may be solitary, as in some types of hydras, or they may attach to other polyps to form a colony, as corals do. In some cnidarians, such as those from the genus Obelia, the polyps of a single colony may perform specialized functions. Some polyps in a colony capture food, others reproduce, and others protect the colony.
The medusa body resembles a saucer or umbrella. The mouth is located in the center of the undersurface of the saucer-like body, surrounded by tentacles. Medusae are solitary and most swim freely in the water. The gelatinous mesoglea layer in medusae is quite thick, which is why cnidarians with the medusae body form are commonly referred to as jellyfish.
The name cnidarian derives from the Greek word knidos, meaning “nettle,” referring to the microscopic stinging structures known as nematocysts found on cnidarian tentacles. A cnidarian will stun a prey animal with its nematocysts and then use its tentacles to bring the prey to its mouth to swallow.
Each nematocyst contains a coiled tubule that shoots out like a harpoon with explosive force. There are about 30 kinds of nematocysts that work in different ways to stun prey. The tubules of some nematocysts wrap around prey. The tubules of other nematocysts stick to prey, while other tubules pierce the surface of small animals. The nematocysts of some cnidarians contain poison that can paralyze or kill prey.
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