Cnidarian

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Cnidarian (pronounced: /nɪˈdɛəriən/)

Cnidarians (from the Greek "cnidos" meaning "stinging nettle") are a phylum of aquatic invertebrates that includes over 10,000 species such as jellyfish, coral, and sea anemones. They are characterized by their unique cells called cnidocytes, which contain specialized organelles known as nematocysts used for capturing prey.

Etymology

The term "Cnidarian" is derived from the Ancient Greek word "κνίδη" (knídē), which translates to "nettle". This is in reference to the stinging cells (cnidocytes) that are a defining characteristic of this group of organisms.

Characteristics

Cnidarians are predominantly marine species, although some, like the Hydra, are found in freshwater environments. They exhibit two basic body forms: the free-swimming medusa and the sessile polyp. Both forms are radially symmetrical, a characteristic that sets them apart from most other invertebrates.

Cnidarians possess a simple, diffuse nerve net rather than a central nervous system. They also have a simple digestive system with a single opening that serves as both the mouth and anus.

Classification

The phylum Cnidaria is divided into four main classes:

  • Anthozoa: This class includes sea anemones and corals. They exist only in the polyp form.
  • Scyphozoa: These are the true jellyfish. They predominantly exist in the medusa form.
  • Cubozoa: Known as box jellies or sea wasps. They are similar to Scyphozoa but have a more cube-shaped medusa.
  • Hydrozoa: This class includes the Hydra and Portuguese man o' war. They can exist in both polyp and medusa forms.

Related Terms

  • Cnidocyte: A type of cell unique to, and characteristic of, cnidarians.
  • Nematocyst: A specialized organelle within cnidocytes that contains a barbed or venomous coiled thread.
  • Medusa: One of the two basic body forms of cnidarians, typically free-swimming and bell or umbrella-shaped.
  • Polyp: The other basic body form of cnidarians, typically sessile and cylindrical in shape.

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