Cnidaria

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Cnidaria

(nīdâr`ēə) or

Coelenterata

(səlĕntərä`tə), phylum of invertebrate animals comprising the sea anemonessea anemone
, any of the relatively large, predominantly solitary polyps (see polyp and medusa) of the class Anthozoa, phylum Cnidaria. Unlike the closely related corals, these organisms do not have a skeleton.
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, coralscoral,
small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral.
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, jellyfishjellyfish,
common name for the free-swimming stage (see polyp and medusa), of certain invertebrate animals of the phylum Cnidaria (the coelenterates). The body of a jellyfish is shaped like a bell or umbrella, with a clear, jellylike material filling most of the space between
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, and hydroids. Cnidarians are radially symmetrical (see symmetry, biologicalsymmetry, biological,
similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed. Symmetry in body shapes is related to the lifestyles of organisms.
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). The mouth, located at the center of one end of the body, opens into a gastrovascular cavity, which is used for digestion and distribution of food; an anus is lacking. Cnidarians are further characterized by having a body wall composed of three layers: an outer epidermis, an inner gastrodermis, and a middle mesogloea. Tentacles encircle the mouth and are used in part for food capture. Specialized stinging structures, called nematocysts, are a characteristic of the phylum and are borne in the tentacles and often in other body parts. These contain a coiled fiber that can be extruded suddenly. Some nematocysts contain toxic substances and are defense mechanisms, while others are adhesive, helping to anchor the animal or to entangle prey.

Two body forms and two lifestyles are characteristic of the Cnidaria (see polyp and medusapolyp and medusa,
names for the two body forms, one nonmotile and one typically free swimming, found in the aquatic invertebrate phylum Cnidaria (the coelenterates). Some animals of this group are always polyps, some are always medusae, and some exhibit both a polyp and a medusa
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). The sessile hydroid, or polyp, form is more or less cylindrical, attached to its substratum at its aboral (opposite the mouth) end, with the mouth and surrounding tentacles at the upper, oral, free end. Colonies of hydroids comprise several different types of individuals: some function in feeding, some in defense, and some in reproduction. The motile jellyfish, or medusoid form, is flattened, with the tentacles usually located at the body margin. The medusoid's convex aboral surface is oriented upward, and the concave oral surface is oriented downward.

With few exceptions, the cnidarians are marine. There are over 9,000 known living species; fossil records of cnidarians date back to the Ordovician era. Cnidarians are carnivorous, the major part of their diet consisting of crustaceans. Animals in this phylum have no specialized excretory or respiratory organs but do have a nervous system. Both sexual and asexual reproduction occur. There are three classes of cnidarians.

Class Hydrozoa

The Hydrozoa include solitary or colonial cnidarians, which have a noncellular mesoglea, lack tentacles within the gastrovascular cavity, and have no gullet. As a rule, the hydroid stage predominates in the life cycle, although in some the jellyfish stage is larger. The order Hydroida includes the many small, colonial hydroids so often seen clinging to wharves and submerged objects along the seacoasts everywhere, economically important because they foul surfaces. The order also includes solitary hydroids, some reaching several inches in height. One, in the genus Branchiocerianthus, is said to reach 8 or 9 ft (244–274.5 cm) in length. The common freshwater genus Hydra also belongs to this order, as does the freshwater jellyfish, genus Craspedacusta, and the commonly studied hydroid jellyfish, genus Gonionemus. There are also pelagic hydroid colonies, unusual in having one very large hydroid member, which lives with its mouth downward and its aboral surface upward, like a jellyfish. The aboral end is equipped with a projecting sail. Velella, the purple sailor, is an example. The order Milleporina includes colonial organisms that form a massive, porous exoskeleton, somewhat resembling corals. They are sometimes abundant in tropical seas and may contribute to coral reef formation. The order Siphonophora includes often large, floating colonies made up of members of varying form and function. Typical is Physalia, the Portuguese man-of-war. Its colorful float is a gas-filled member of the colony and attains lengths up to 1 ft (30 cm). Other members of the colony hang downward from the lower surface of the float; some of these have very powerful nematocysts able to cause severe physiological reaction in swimmers coming in contact with them. These organisms are able to kill sizable fish with their tentacles.

Class Scyphozoa

Cnidarians of class Scyphozoa have a predominant jellyfish stage. They are characterized by a cellular mesoglea and tentacles in their gastrovascular cavity. All of the largest jellyfish belong to this class. The common Aurelia aurita is seen in bays and harbors, sometimes in large numbers. It is pallid, unlike some of the more colorful species in the genus Cyanea. Stalked jellyfish, the Stauromedusae, are unusual members of the Scyphozoa; they are found attached to seaweed, especially in cooler marine habitats. The order Rhizostomea includes jellyfish in which the original mouth has closed, and which have many subsidiary mouths found in frilled oral arms. Cassiopaeia is a well-known example, living in warmer, shallow waters, where it is often found lying on the bottom upside down, exposing its green algal symbionts to the sun.

Class Anthozoa

Class Anthozoa includes Cnidaria that have no jellyfish stage. This is the largest class of cnidarians, containing over 6,000 species. A gullet extends for a short distance into the gastrovascular cavity, and septa are present, which increase the surface for digestion and absorption. Anthozoa are flower animals, including a great many beautiful and colorful organisms, e.g., the sea anemonesea anemone
, any of the relatively large, predominantly solitary polyps (see polyp and medusa) of the class Anthozoa, phylum Cnidaria. Unlike the closely related corals, these organisms do not have a skeleton.
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, sea pansysea pansy,
fleshy, leaf-shaped colony of marine organisms belonging to the genus Renilla in the same phylum as the jellyfish. The colony consists of a stalk formed by a large organism called a primary polyp (see polyp and medusa) that is thrust into soft bottom material;
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, sea fansea fan,
colonial marine animal forming erect, flattened, branching colonies in tropical and subtropical waters. Colonies may be several feet high and are often colorful, with purples, reds, and yellows predominating.
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, and coralcoral,
small, sedentary marine animal, related to the sea anemone but characterized by a skeleton of horny or calcareous material. The skeleton itself is also called coral.
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. Anthozoans are colonial or solitary organisms.

Subclass Alcyonaria

Subclass Alcyonaria includes almost universally colonial organisms in which each of the polyps, or hydroid members, has eight feathery tentacles. Most of them produce a skeleton, and many make some contributions to coral reefscoral reefs,
limestone formations produced by living organisms, found in shallow, tropical marine waters. In most reefs, the predominant organisms are stony corals, colonial cnidarians that secrete an exoskeleton of calcium carbonate (limestone).
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. While some are found in temperate seas, they are especially common in subtropical to tropical regions. The organ pipe coral (Tubipora), a soft coral (Alcyonium), the Indo-Pacific blue coral (Heliopora), and the sea penssea pen,
long, slender colonial organism of the same phylum as the jellyfish. Sea pen colonies are formed by several genera of the order Pennatulacea. The colony consists of a stalk formed by an organism called a primary polyp (see polyp and medusa) and short branches formed by
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, which have a stalk extending into the bottom mud or sand, are some typical alcyonarian corals. Horny corals, of the order Gorgonacea, are perhaps the best known. These form branching, upright colonies and have a skeleton that is partly composed of a horny material called gorgonin. These are the sea whipssea whip,
erect colony of marine animals of the phylum Cnidaria, with whiplike branches. The skeleton consists of a horny axis, overlaid with small calcareous bodies called spicules.
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 and sea fans so characteristic of shallow tropical waters.

Subclass Zoantharia

The subclass Zoantharia includes both solitary and colonial forms, in which the polyp has more than eight tentacles. The solitary sea anemones belong here, in the order Actiniaria, characterized by the lack of a skeleton. The stony corals so important in forming coral reefs belong to the order Madreporaria; they are especially characterized by their calcium carbonate exoskeleton, marked by many cups for the polyps, each of which contains stony septa dividing the gastrovascular cavity into compartments. The shape of coral skeletons depends on the pattern of growth of the colony. For example, in brain corals the polyps are arranged linearly; in the eyed coral (Oculina) the polyps are separated from each other by spaces, giving the skeleton a pitted appearance. The burrowing anemone, Cerianthus, lives in burrows in the sand and has a greatly elongated body. It is characteristic of the order Ceriantharia.

Cnidaria

[nī′dar·ē·ə]
(invertebrate zoology)
A phylum of the Radiata whose members typically bear tentacles and possess intrinsic nematocysts. Also known as Coelenterata.