Cnidarians: Types & Examples

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  • 0:00 Introduction To Cnidarians
  • 0:49 Body Structure of Cnidarians
  • 3:03 Examples of Cnidarians
  • 6:30 Lesson Summary
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Lesson Transcript
Instructor: Marta Toran

Marta has taught high school and middle school Science and has a Master's degree in Science Education.

Corals and jellyfish belong to a group of invertebrate animals called Cnidarians. In this lesson, you will learn about the classification and biology of these fascinating organisms and their relatives, including one of the most lethal creatures known to mankind: the Australian box jellyfish.

Introduction to Cnidarians

You might not know them as cnidarians, but you are probably more with jellyfish, corals and sea anemones, which all belong to this group of animals. They are simple organisms, which have been around for millions of years, and have remained relatively unchanged.

Although in show business they are often only given guest or minor roles in shows featuring sarcastic sponges or movies starring adorable clownfish, in real life cnidarians are extraordinary organisms that play a critical role in marine ecosystems. They are beautiful, graceful and sometimes extremely dangerous animals, but what makes them truly fascinating is that they can be so simple and yet so diverse (there are over 10,000 species of them!)

Body Structure of Cnidarians

Cnidarians are invertebrate animals (meaning they have no backbone, like insects, worms and sponges,) which live mainly in the ocean at a wide range of depths and temperatures. Their name comes from the Greek word for stinging nettles, 'cnidos', because they have thousands of stinger cells on their tentacles. Each stinger cell releases a harpoon-like structure, which injects poison into their prey, which can hurt, paralyze or even kill them.

Cnidarians can have two basic body forms: polypoid and medusoid. Anemones and corals generally exist as polypoids (the mouth and tentacles of the organism facing up, the other side attached to a rock or other surface), while true jellyfish have medusoid forms (free swimmers with the mouth and tentacles hanging downwards).

Polypoid and Medusoid Body Forms

Some cnidarians go through both stages in their life cycle. A particularly interesting case is that of the Immortal Jellyfish (Turritopsis nutricula) pictured here, which reaches sexual maturity as a medusoid and is then able to revert back to its immature polyp stage after it reproduces. Basically, it would be the equivalent of becoming an adolescent, having a baby and then reverting back to being a preteen and going through puberty all over again, and then again and again, thus staying forever young. This 'turning back the clock' can go on for many generations, in theory making the jellyfish 'immortal', although in reality competition, predation or disease would eventually kill it.

Immortal Jellyfish
Immortal Jellyfish (Turritopsis nutricula)

In addition to the two possible body forms, all Cnidarians share certain common features:

  • They have nematocysts (stinging cells)
  • They have radial symmetry (cutting planes through the center creates identical segments; they have top and bottom but no 'sides')
  • They are carnivores (they feed on smaller invertebrates)
  • They have no organs or organ systems (just tissues containing specialized cells grouped together)
  • They have no brains but have nerve impulses running through their body and are able to detect signals from the environment

Examples of Cnidarians

Cnidarians are classified into four main classes according to their body forms and characteristics, all of which you're looking at on screen:

Cnidarian Classification


Anthozoa (flower-like animals) are strictly polypoids that attach to solid surfaces such as rocks, shells and sometimes other living organisms. This class includes true corals, anemones and sea pens. Corals are polyp colonies which feed on plankton they trap with their tentacles. They form reefs, which are among the world's most productive ecosystems because they host photosynthetic algae. Fish and other invertebrates also live among the coral, hiding from predators among the coral's stinging tentacles in exchange for food scraps. Anemones may look like beautiful, exotic underwater flowers, but they are vicious predators that use their tentacles to lure and trap small fish and shrimp passing by.


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