Fascinating Facts About Cnidarians

Corals, Jellyfish, Sea Anemones, Sea Pens and Hydrozoans

Purple-striped jelly fish dancing underwater
Purple-striped jelly fish dancing underwater. Elfi Kluck/Photographer's Choice/Getty Images

The Cnidaria is the phylum of animals that contains corals, jellyfish (sea jellies), sea anemones, sea pens and hydrozoans. Cnidarian species are diverse, but these animals share many similar characteristics, which you can learn about below.

Cnidarians are also known as coelenterates - a reference to the name for their digestive cavity, which you'll learn more about below.

Cnidarian Body Types

First, a bit about the body plan of cnidarians. There are two types, called polypoid and medusoid. Polypoid cnidarians have tentacles and a mouth that face up (think of an anemone or coral). These animals are attached to a substrate or colony of other animals. Medusoid types are those like jellyfish - the "body" is on top and tentacles and mouth hang down.

Characteristics of Cnidarians

  • Are radially symmetrical - their body parts are arranged around a central point
  • Are composed of two layers of cells - the epidermis, or outer layer, and the gastrodermis (also called the endodermis), which lines the gut. In between there is the mesoglea, which is a jelly-like substance. This mesoglea is most obvious in jellyfish.
  • Have a digestive cavity (the coelenteron) which is their stomach, gullet and intestine. This has one opening, which serves and both the mouth and anus. So, a cnidarian eats and expels wastes from the same spot.
  • Ouch! (or not) - Cnidarians have stinging cells called cnidocytes,which are used for feeding and defense. The cnidocyte contains a nematocyst, which is a stinging structure made up of a hollow thread that has barbs inside. If the trigger at the end of the cnidocyte is activated, the thread unfurls outward, turning inside out, and then the thread wraps around or stabs into the tissue of the prey, injecting a toxin. Not all cnidarians have stings that are painful to humans, but some do, and some may even be fatal. The name 'cnidaria' comes from the Greek word for nettle, which is a type of plant with stingers.

    Cnidarian Classification

    • Kingdom: Animal
    • Phylum: Cnidaria
    • Classes: Anthozoa (sea anemones, corals); Cubozoa (box jellyfish); Hydrozoa (hydrozoans, also known as hydromedusae or hydroids); Scyphozoa or Scyphomedusae (jellyfish); and the Staurozoa (stalked jellyfish).

    Examples of Cnidarians

    Here are some cnidarians featured on this site:

    Habitat and Distribution

    With thousands of species, cnidarians are diverse in their habitat and are distributed in all the world's oceans, in polar, temperate and tropical waters. They are found in a variety of water depths and closeness to shore - depending on the species, they may live anywhere from shallow, coastal habitats to the deep sea.


    Cnidarians are carnivores and use their tentacles to feed on plankton and other small organisms in the water. Some cnidarians, such as corals, are inhabited by algae (e.g., zooxanthellae), which do photosynthesis, a process that provides carbon to the host cnidarian.


    Different cnidarians reproduce in different ways. Cnidarians can reproduce asexually by budding (another organism grows off the main organism, such as in anemones), or sexually, in which spawning occurs - sperm and eggs are released by male and female organisms into the water column, and free-swimming larvae are produced.

    Cnidarians and Humans

    There are many ways cnidarians may interact with humans - cnidarians may be sought-after in recreational activities, such as scuba divers going to reefs to look at corals. Swimmers and divers may also need to beware of certain cnidarians because of their powerful stings. Some cnidarians, such as jellyfish, are even eaten. Different cnidarian species may also be collected for trade for aquariums and jewelry.


    • Coulombe, Deborah A. 1984. The Seaside Naturalist. Simon & Schuster.
    • Fautin, Daphne G. and Sandra L. Romano. 1997. Cnidaria. Sea anemones, corals, jellyfish, sea pens, hydra. Version 24 April 1997. The Tree of Life Web Project, http://tolweb.org/. Accessed October 12, 2011.
    • University of California Museum of Paleontology. Cnidaria. Accessed October 12, 2011.