Showing posts with label Cnidaria. Show all posts
Showing posts with label Cnidaria. Show all posts

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

Types of Invertebrates

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Zoologists (scientists who study animals) classify invertebrates into about 30 major groups, known as phyla. These phyla vary enormously in the number of species they contain. Arthropods (phylum Arthropoda) are the invertebrate phylum with the most species—more than one million known species and countless more awaiting discovery. The mollusks (phylum Mollusca) make up the second largest group of invertebrates, with at least 50,000 species. Among the simplest invertebrates are the sponges (phylum Porifera). Other major invertebrate phyla include the cnidarians (phylum Cnidaria), echinoderms (phylum Echinodermata), and several different groups of worms, including flatworms (phylum Platyhelminthes), roundworms (phylum Nematoda), and annelids (phylum Annelida).

Invertebrates Phylum:

Placozoa -> Microscopic marine animal. Flattened body composed of two outer layers of flagellated cells enclosing loose cells. Reproduces asexually and possibly sexually. – Examples: Single species identified as Trichoplax adherens.

Porifera -> Simple, multicellular animals with tissues but no distinct organs. Commonly known as sponges, they typically attach to rocks, shells, or coral. Most sponges have an irregular shape supported by a skeleton composed of calcareous crystals, silicon spicules, or spongin fibers. Most sponges have an internal water canal system that moves water through the body, providing a constant supply of food particles and oxygen to all cells and removing wastes. Sponges reproduce sexually. They regenerate lost or injured body parts. – Examples: Sponges

Cnidaria -> Aquatic radially symmetrical animals with tentacles encircling the mouth at one end of the body. Cnidarians appear in two forms during their life cycle, the sessile, cylindrical polyp and the free-swimming medusa that looks like a jellyfish. Most feed on zooplankton, although some eat larger invertebrates. Cnidarians use tentacles to capture prey, immobilizing the prey with stinging cells called nematocysts. Cnidarians reproduce both asexually (by budding) and sexually. Some species are hermaphroditic, with both eggs and sperm produced in each individual. Most regenerate lost or injured body parts. – Examples: Coral, hydra, jellyfish, Portuguese man-of-war, sea anemone

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