Cnidarians
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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.
Reproduction in cnidarians varies among the different species. They may reproduce by means of asexual reproduction, sexual reproduction, or both. Polyps generally perform asexual reproduction by budding, in which an outgrowth from the body wall separates to form a new polyp or medusa. Medusae primarily reproduce sexually—they produce gametes (sex cells), and a gamete (sperm) from a male medusa fuses with a gamete (egg) from a female medusa to form a zygote. The zygote develops into a larva, which in turn develops into a polyp or medusa. The medusae of some cnidarians may also form polyps by budding.
Scientists divide cnidarians into four classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. They base this division partly on whether the polyp or medusa is more conspicuous during an animal’s life cycle.
Scientific classification: The phylum Cnidaria is made up of the classes Hydrozoa (predominately polyps), Scyphozoa (primarily medusae), Cubozoa (primarily medusae), and Anthozoa (polyps only).
See Types of Cnidarians; Polyp and Medusa Structures
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.
Reproduction in cnidarians varies among the different species. They may reproduce by means of asexual reproduction, sexual reproduction, or both. Polyps generally perform asexual reproduction by budding, in which an outgrowth from the body wall separates to form a new polyp or medusa. Medusae primarily reproduce sexually—they produce gametes (sex cells), and a gamete (sperm) from a male medusa fuses with a gamete (egg) from a female medusa to form a zygote. The zygote develops into a larva, which in turn develops into a polyp or medusa. The medusae of some cnidarians may also form polyps by budding.
Scientists divide cnidarians into four classes: Hydrozoa, Scyphozoa, Cubozoa, and Anthozoa. They base this division partly on whether the polyp or medusa is more conspicuous during an animal’s life cycle.
Scientific classification: The phylum Cnidaria is made up of the classes Hydrozoa (predominately polyps), Scyphozoa (primarily medusae), Cubozoa (primarily medusae), and Anthozoa (polyps only).
See Types of Cnidarians; Polyp and Medusa Structures
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