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The Cnidarians: Jellyfish, Sea Anemones, Hydrozoans, and Corals

Updated on September 26, 2014

The Cnidaria Phylum of the Animalia Kingdom Includes Jellyfish, Sea Anemones, Hydrozoans, & Corals

I love watching Jellyfish swim, unless I'm in the water with them! The way jellyfish move through the water is beautiful, almost magical. The bodies of jellyfish are beautiful too, whether clear or brightly colored!

Sea Anemones are pretty and interesting to watch as well! Have you seen their tentacles swaying gently around, as the sea anemone stays rooted on it's rock or ocean floor? They seem so innocent, just standing around, yet they're silently waiting for their "dinner" to swim into their arms! Hey, they've got to eat too! Be sure and check out the video father down this page of a sea anemone catching it's prey! You might be surprised at how quickly his tentacles react and just what they do when a fish swims up! Suddenly sea anemones don't look so much like a plant anymore!

Did you know that hydrozoans, such as the Portuguese man-of-war, are really colonial organisms? That means each "one" is really several individuals living together! Hydras, another type of hydrozoan, look a great deal like plants, but can actually move around by doing somersaults - a process called tumbling!

Corals, delicate corals. You probably already know they live in reefs. They look like plants, but are really tiny, spineless, fragile animals. Did you know corals are nocturnal? At night, they poke their tentacles out to feed!

Jellyfish, Sea Anemones, Hydrozoans, and Corals are all part of the animal kingdom phylum Cnidaria. Many Cnidarians may look like plants, but don't be fooled by their appearances; they're animals!

Photo of a moon jelly (above) by Hans Hillewaert
Article © 2012 - 2014 Janiece Tobey. All rights reserved.

Characteristics of Cnidarians - What do Cnidarians have in common?

  • Cnidarians have radial symmetry. Radial symmetry means that at least part of their body has a round shape with things branching out from or located around a central point.

  • Cnidarians have specialized tissues that do specific jobs. For example, the stinging cells are specialized for the job of stinging and the cells which contract to allow movement are specialized for that job. There are also mucus-secreting cells, nerve cells, and so on. Even though cnidarians have tissues, they do not have organs.

  • The gastrovascular cavity of Cnidarians is an internal sac used for digestion.

  • Unlike the digestive tact of humans, the gastrovascular cavity of cnidarians has only one opening, their mouth. Because there is only one opening, the animal not only eats with his mouth, but also releases waste from it!

  • Cnidarians have tentacles that surround their mouths.

  • Located on the tentacles of Cnidarians are stinging cells called, cnidocytes. Inside each cnidocyte is a nematocyst - a small barbed threadlike tube that forcefully shoots out and stings or paralyzes prey or enemies.

  • The body wall of a cnidarian consists of three layers. Two of these layers are cells (ectoderm and endoderm). The 3rd layer, which is in the middle of the other two, is mesoglea (a jelly-like substance - like in sponges.).

  • Cnidarians have two body forms. They can either have a Medusa shape, or a Polyp shape. Many types of Cnidarians include both body forms in their life cycle.

  • Medusas are jelly-like and free-floating organisms. They can have a bell shape, or be more like a thin upside down saucer. Medusas consist of an umbrella-shaped body, a gastrovascular cavity, a mouth on the inside of the umbrella shape, and tentacles that hang from the body and surround the mouth.

True Jellyfish

True Jellyfish
True Jellyfish | Source

Box Jellyfish

Box Jellyfish
Box Jellyfish | Source
  • True Jellyfish (in class Scyphozoa) and Box Jellyfish (in class Cubozoa) have the medusa shape.

  • Polyps attach to the sea floor, often in large colonies. Polyps consists of a basal disc that attaches to a rock or other item in the ocean, a cylindrical body stalk, the gastrovascular cavity which is located inside the stalk, a mouth on the top of the polyp, and tentacles which surround the mouth.

    Corals, hydras, and sea anemones are all polyps for at least part of their life cycle.


Hydra | Source

Venus Fly Trap Anemone

Venus Fly Trap Anemone
Venus Fly Trap Anemone | Source

The Phylum Cnidaria video below lasts about 8 minutes and 30 seconds, not counting the credits at the end. It shows numerous cnidarians, providing facts about each of the types (Hydras, Jellyfish, Corals, and Sea Anemones). Of special interest is the demonstration on how cnidarians sting!

The Phylum Cnidaria

Meet The Cnidarians!



Sea Anemones,

and Coral!

Jellyfish - Class Scyphozoa, and Class Cubozoa

Moon Jelly

Moon Jelly Image Credit: Hans Hillewaert

Lion's Mane Jelly

Lion's Mane Jelly image credit: Dan Hershman

The Lion's Mane Jelly (above) is the largest jellyfish in the world! The diameter of their bell shape can be 8 feet across! Their tentacles can be 120 feet long! That's longer than a blue whale! For more about the Lion's Mane Jellyfish, visit: Lion's Mane Jellyfish

  • Jellyfish eat tiny plants, shrimp, crabs, and other species of jellyfish!

  • The mouth of a jellyfish is up under it's umbrella shape.

  • Want to know what eats jellyfish? Sea turtles do! So does the Bearded Goby fish. Even some humans eat jellyfish! Feeling hungry?

    Read more about gobis: "Finally! Something that eats jellyfish!"

  • The tentacles of jellyfish can sting. In some species, the sting is quite poisonous! Even dead jellyfish can sting!

  • The animals we usually see and identify as jellyfish have a medusa form. Yet most types of jellyfish also have a polyp form at some point in their life cycle.

Jellyfish Feeding

Swimming in Jellyfish Lake

Can you believe this lady (in the video above) is swimming with the jellyfish?

Sea Anemones - Class Anthozoa

Sea Anemones

Sea Anemones Image credit: Giacomo Merculiano

  • Sea Anemones are soft bodied organisms that have a polyp shape.
  • Sea Anemones eat fish and other ocean life.
  • Sea Anemones can pull their tentacles inside their bodies and form a tight ball for protection.
  • They reproduce asexually by pulling themselves into two!

Sea Anemone Catching and Eating A Fish

More Information On Sea Anemones and Other Cnidarians

Sea Anemones & Other Cnidarians
Sea Anemones & Other Cnidarians
This is an Amazon DVD on Sea Anemones and Other Cnidarians you can rent or purchase!
Sea Anemones (Ocean Life)
Sea Anemones (Ocean Life)
Information for children about sea anemones.

The Hydrozoans - Class Hydrozoa of the phylum Cnidaria

Hydra - as seen through a microscope

Hydra Image: Oinari-san

  • Hydrozoans, like the hydra pictured above, have a polyp shape. Most hydrozoans, except the hydra, also have a medusa stage in their life cycle.
  • Although hydras attach to rocks or plants in the water. The sticky area where they attach is called a basal disk.
  • Sometimes hydras lessen the amount of sticky stuff holding them to a surface and can move around a little, by gliding or tumbling. Tumbling is like doing somersaults!
  • Many hydrozoans live together in colonies. Interestingly, even though the individual cells of the colonies don't depend upon one another as they do in multicelluar organisms, the cells can still exhibit specialization.
  • The Portuguese Man of War is a type of hydrozoan that is made up of many polyps and medusas.

Portuguese Man O' War - a type of hydrozoan made up of many medusa organisms and polyp organisms connected together

photo of Portuguese Man O War image by Langmurf

Hydra Budding - Animation

Live Hydra

Corals - Class Anthozoa


Coral image: Toby Hudson

  • Corals are tiny polyp animals living together in colonies. The colonies are called reefs.

  • Corals secrete a hard exoskeleton made out of calcium carbonate. The exoskeleton of one becomes cemented to the exoskeleton of neighboring corals. In this way, the reef grows, with new corals living on the dead skeletons of former corals.

  • For more information about coral, visit: Coral Facts

Coral Reef Ecosystem

Comments? Questions? - Did you enjoy finding out more about jellyfish, coral, sea anemones, and hydrozoans?

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    • profile image


      8 years ago

      Loving this, your teaching style draws one in from the intro and your pictures work in perfect haromony. I just saw an article on the Moon Jellyfish but your picture shows it in all its glory. A year or so ago I saw a feature on the Box Jellyfish, such a powerful sting that destroys tissue painfully and many don't survive and might not realize they were stung. Beautifully done in every way, I'm sure the Cnidaria are all very pleased!

    • CrossCreations profile image

      Carolan Ross 

      8 years ago from St. Louis, MO

      An amazing adventure into the underwater world, beautifully done!

    • ajgodinho profile image

      Anthony Godinho 

      9 years ago from Ontario, Canada

      Not only informative, but beautiful biology lens...blessed!

    • profile image

      Kenken99 LM 

      9 years ago

      Awesome lens!

    • profile image


      9 years ago

      Thank you for the education on Cnidarians! They really are beautiful underwater creatures. I love coral, have always been fascinated by its beauty.

    • profile image


      9 years ago

      Wonderful, educational lens with beautiful photography. Learned a lot. Blessed.

    • LisaDH profile image


      9 years ago

      I had no idea jellyfish and corals were related. Very informative!

    • Scarlettohairy profile image

      Peggy Hazelwood 

      9 years ago from Desert Southwest, U.S.A.

      Wow, I didn't know much about these beautiful creatures. Love the photos!

    • profile image


      9 years ago

      Fantastic! I love your lenses, they are always so well done.

    • profile image


      9 years ago

      Loved the pictures and very well presented

    • profile image


      9 years ago

      Beautiful! For such simple creatures, they are fascinating and complex. Love it!

    • Ramkitten2000 profile image

      Deb Kingsbury 

      9 years ago from Flagstaff, Arizona

      Jellyfish are so beautiful ... when they're in the water, where they should be. On the beach, just blobs. I used to see them a lot, growing up on the ocean in New England. I gave them a respectful distance, though, because even if the stings of those particular jellyfish weren't poisonous, they sure hurt! This is a beautiful and interesting lens.

    • profile image


      9 years ago

      Very educational! Blessed by a SquidAngel!

    • flicker lm profile image

      flicker lm 

      9 years ago

      Nicely done! Loved the photos! And the video of the Sea Anemone catching a fish was fascinating.


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