cnidarian
or
coelenterate
Cnidarian body forms. A cnidarian may display either the sessile polyp form or the free-swimming …
(credit: © Merriam-Webster Inc.)
Any of about 9,000 species of mostly marine aquatic
invertebrates, constituting the phylum Cnidaria (or Coelenterata), that are unique in possessing specialized stinging cells (cnidocytes) borne on the tentacles. Cnidocytes contain fluid-filled capsules (nematocysts) with a harpoonlike coiled thread used for stinging, paralyzing, and capturing prey. Cnidarians have no well-defined separate respiratory, circulatory, or excretory organs; their tissues, composed of two cell layers, surround a cavity known as a coelenteron (gastrovascular cavity), which is the basic internal organ. Tentacles surrounding the mouth are used to capture and ingest food. Cnidarians are carnivorous, feeding mostly on
zooplankton but also on small crustaceans, fish eggs, worms, smaller cnidarians, and even small fish. Cnidarians range in size from nearly microscopic to more than 100 ft (30 m) long and more than a ton (910 kg) in weight. There are two basic body forms: the
polyp (e.g.,
coral) and the
medusa (e.g.,
jellyfish).
See also hydra;
Portuguese man-of-war;
sea anemone.
coelenterate1. any invertebrate of the phylum Cnidaria (formerly Coelenterata), having a saclike body with a single opening (mouth), which occurs in polyp and medusa forms. Coelenterates include the hydra, jellyfishes, sea anemones, and corals
2. (loosely) any invertebrate of the phyla Cnidaria or Ctenophora
)