Overview of Cnidarians

Stalked Jellyfish (Lucernaria quadricornis), White Sea, Karelia, Russia
Andrey Nekrasov/Getty Images

A cnidarian is an invertebrate in the Phylum Cnidaria. This phylum includes corals, sea anemones, sea jellies (jellyfish), sea pens, and hydras.

Pronunciation: Nid-air-ee-an

Also Known As: Coelenterate, Coelenterata

Characteristics of Cnidarians

Cnidarians exhibit radial symmetry, which means their body parts are arranged symmetrically around a central axis. So, if you drew a line from any point at the edge of a cnidarian through the center and to the other side, you'd have two roughly equal halves.

Cnidarians also have tentacles. These tentacles have stinging structures called cnidocytes, which bear nematocysts. Cnidarians got their name from these stinging structures. The word cnidarian comes from the Greek word knide (nettle)

The presence of nematocysts is a key feature of cnidarians. Cnidarians can use their tentacles for defense or for capturing prey. 

Although they can sting, not all cnidarians pose a threat to humans. Some, like the box jellyfish, have very potent toxins in their tentacles, but others, like moon jellies, have toxins that don't have enough power to sting us.

Cnidarians have two body layers called the epidermis and gastrodermis. Sandwiched in between is a jelly-like substance called mesoglea.

Examples of Cnidarians 

As a large group comprised of thousands of species, cnidarians can be pretty diverse in their form. Overall, though, they have two main body plans: polypoid, in which the mouth faces up (e.g., anemones) and medusoid, in which the mouth faces down (e.g., jellyfish). Cnidarians may go through stages in their life cycle in which they experience each of these body plans.

There are several major groups of cnidarians:

  • Anthozoa: sea anemones, sea pens, and corals. These animals have a polypoid body plan and attach to a substrate, such as other animals, rocks or algae.
  • Hydrozoa: hydrozoans, also known as hydromedusae or hydroids. These organisms alternate between polyp and medusa stages and are usually colonial organisms. Siphonophores, which include Portuguese man-of-war and by-the-wind sailors, are examples of animals in the Class Hydrozoa. Most cnidarians are marine organisms, but there are some hydrozoan species that live in fresh water.
  • Scyphozoa or Scyphomedusae: true jellyfish are in the Class Scyphozoa. These animals are known for their bell shape with dangling oral arms. Some jellyfish have tentacles also. The lion's mane jellyfish is the largest species, with tentacles that may stretch more than 100 feet.
  • Cubozoa: box jellyfish. These animals have a cube-shaped bell, with tentacles dangling from each corner. The sea wasp, a type of box jellyfish, is said to be the most venomous marine animal.
  • Staurozoa: stalked jellyfish or Stauromedusae. These strange-looking, trumpet-shaped animals aren't free-swimming like regular jellyfish. Instead, they attach to rocks or seaweed and are typically found in cold water.
  • Myxozoa:  parasitic microorganisms that evolved from jellyfish There has been debate over the years over where these animals should be classified - the latest research places them in the Cnidaria phylum, and an important piece of evidence is that these creatures have nematocysts. Myxozoa species can affect fish, worms, amphibians, reptiles, and even mammals. One economic impact is that they can affect farmed fish such as salmon.

Smallest and Largest Cnidarians

The smallest cnidarian is a hydra with the scientific name Psammohydra nanna. This animal is less than half a millimeter in size. 

The largest non-colonial cnidarian is the lion's mane jellyfish. As mentioned above, the tentacles are thought to stretch more than 100 feet. The bell of this jellyfish can be over 8 feet across.

Of colonial cnidarians, the longest is the giant siphonophore, which can grow to over 130 feet.


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Kennedy, Jennifer. "Overview of Cnidarians." ThoughtCo, Aug. 26, 2020, thoughtco.com/cnidarian-definition-3863683. Kennedy, Jennifer. (2020, August 26). Overview of Cnidarians. Retrieved from https://www.thoughtco.com/cnidarian-definition-3863683 Kennedy, Jennifer. "Overview of Cnidarians." ThoughtCo. https://www.thoughtco.com/cnidarian-definition-3863683 (accessed March 21, 2023).