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9.3: Cnidarians

Difficulty Level: At Grade Created by: CK-12
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Are corals animal, vegetable, or mineral?

Some corals may look like rocks. But they are alive! And some corals may look like plants, but they are actually animals. Just like all other animals, they eat food to get energy.


Cnidarians , in the phylum Cnidaria , include organisms such as the jellyfish and sea anemones. These animals are found in shallow ocean water. You might know that these animals can give you a painful sting if you step on them. That’s because cnidarians have stinging cells known as nematocysts . Cnidarians use nematocysts to catch their food. When touched, the nematocysts release a thread of poison that can be used to paralyze prey.

Body plan

The body plan of cnidarians is unique because these organisms show radial symmetry. This means that they have a circular body plan, and any cut through the center of the animal leaves two equal halves.

The cnidarians have two basic body forms:

  1. Polyp : The polyp is a cup-shaped body with the mouth facing upward, such as a sea anemone.
  2. Medusa : The medusa is a bell-shaped body with the mouth and tentacles facing downward, such as a jellyfish.

Unlike the sponges, the cnidarians are made up of true tissues. The inside of a cnidarian is called the gastrovascular cavity , a large space that helps the organism digest and move nutrients around the body. The cnidarians also have nerve tissue organized into a net-like structure, known as a nerve-net. Cnidarians do not have true organs, however.

Cnidarian Colonies

Some types of cnidarians are also known to form colonies. Two examples are described below.

  1. The Portuguese Man o' War ( Figure below ) looks like a single organism but is actually a colony of polyps. One polyp is filled with air to help the colony float, while several feeding polyps hang below with tentacles. The tentacles are full of nematocysts. The Portuguese Man o' War is known to cause extremely painful stings to swimmers and surfers who accidentally brush up against it in the water.

The Portuguese Man o' War can deliver nasty stings with its tentacles.

  1. Coral reefs ( Figure below ) look like big rocks, but they are actually alive. They are built from cnidarians called corals. The corals are sessile polyps that can use their tentacles to feed on ocean creatures that pass by. Their skeletons are made up of calcium carbonate, which is also known as limestone. Over long periods of time, their skeletons build on each other to produce large structures known as coral reefs. Coral reefs are important habitats for many different types of ocean life.

Corals are colonial cnidarians

Corals are colonial cnidarians.


  • cnidarians : Invertebrate animals including jellyfish and corals; they are characterized by radial symmetry and stinging cells called nematocysts.
  • gastrovascular cavity : Internal space that is the site of digestion and distribution of nutrients.
  • medusa : Bell-shaped body plan of some cnidarians with mouth and tentacles facing downward.
  • nematocyst : Stinging cells characteristic of the cnidarians.
  • polyp : Cup-shaped body plan of some cnidarians with mouth and tentacles facing upward.


  • Cnidarians have radial symmetry and true tissues.
  • Some cnidarians form colonies, such as corals.


Use the resource below to answer the questions that follow.

  1. How do cnidarians move?
  2. Why was movement a useful innovation for cnidarians? Be as complete as possible in your answer.
  3. What is a nematocyst? For what purpose(s) are they used?
  4. In what sorts of habitats are cnidarians found?
  1. How do polyps differ from medusas? Be thorough and complete in your answer.
  2. When moon jellies gather together to mate, what kind of behavior are they displaying?
  3. Do all cnidarians have a polyp and medusa phase?


  1. What are some examples of cnidarians?
  2. How is a jellyfish different from a Portuguese Man o’ War?

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Difficulty Level:

At Grade


7 , 8

Date Created:

Nov 29, 2012

Last Modified:

Nov 04, 2014
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