The Cnidarians: Jellyfish, Sea Anemones, Hydrozoans, and Corals
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.
- 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.
Venus Fly Trap Anemone
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!
Jellyfish - Class Scyphozoa, and Class Cubozoa
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.
Swimming in Jellyfish Lake
Can you believe this lady (in the video above) is swimming with the jellyfish?
Sea Anemones - Class Anthozoa
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
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
- Portuguese Man O' War (Virtual Zoo)
Information and photos of Portuguese Man of War
- Thousands of Portuguese Man-of-war wash up on South Padre Island, Texas
Look at all the Portuguese Man of War washed up on this beach!
Hydra Budding - Animation
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
A List of The Biology Pages In This Series
Homepage: Biology: Information, Videos, and Labs
Unit 1 on Cell Biology
Unit 2 on Genetics
Unit 3 on The History of Life on Earth
Unit 4 on Ecology
Unit 5 on Diversity
Unit 6 on All About Plants
Unit 7 on The Animal Kingdom: Invertebrates
Check back later for additional biology units!