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The sea anemone. Plant or animal?

It may look like a plant, but it's not. Sea anemones are a group of water-dwelling, predatory animals in the phylum Cnidaria. A sea anemone is a polyp attached at the bottom to the surface beneath it. They can have anywhere from a few tens of tentacles to a few hundred tentacles. And they eat small fish and shrimp.


Cnidarians are invertebrates such as jellyfish and corals. They belong to the phylum Cnidaria. All cnidarians are aquatic. Most of them live in the ocean. Cnidarians are a little more complex than sponges. They have radial symmetry and tissues. There are more than 10,000 cnidarian species. They are very diverse, as shown in Figure below . See The First to Move at http://www.pbs.org/kcet/shapeoflife/episodes/move.html and Biology of Cnidarians at http://www.scivee.tv/node/2753 for an overview of these animals.

Cnidarian diversity: coral, anemone, hydrozoa

Cnidarian Diversity. Cnidarians show a lot of variability.

Structure and Function of Cnidarians

All cnidarians have something in common. It’s a nematocyst, like the one shown in Figure below . A nematocyst is a long, thin, coiled stinger. It has a barb that may inject poison. These tiny poison "darts" are propelled out of special cells. They are used to attack prey or defend against predators.

Cnidarian nematocyst development

Cnidarian Nematocyst. A cnidarian nematocyst is like a poison dart. It is ejected from a specialized cell.

There are two basic body plans in cnidarians. They are called the polyp and medusa. Both are shown in Figure below . The polyp has a tubular body and is usually sessile. The medusa (plural, medusae ) has a bell-shaped body and is typically motile. Some cnidarian species alternate between polyp and medusa forms. Other species exist in just one form or the other.

Cnidarian body plans

Cnidarian Body Plans. Cnidarians may exist in the polyp (left) or medusa (right) form.

The body of a cnidarian consists of two cell layers, ectoderm and endoderm. The cells surround a digestive cavity called the coelenteron (see Figure below ). Cnidarians have a simple digestive system. The single opening is surrounded by tentacles, which are used to capture prey. The tentacles are covered with nematocyst cells. Digestion takes place in the coelenteron. Nutrients are absorbed and gases exchanged through the cells lining this cavity. Fluid in the coelenteron creates a hydrostatic skeleton.

Cnidarians have a simple nervous system consisting of a nerve net that can detect touch. They may also have other sensory structures. For example, jellyfish have light-sensing structures and gravity-sensing structures. These senses give them a sense of up versus down. It also helps them balance.

Cnidarian Reproduction

Figure below shows a general cnidarian life cycle. Polyps usually reproduce asexually. One type of asexual reproduction in polyps leads to the formation of new medusae. Medusae usually reproduce sexually. Sexual reproduction forms a zygote. The zygote develops into a larva called a planula . The planula, in turn, develops into a polyp. There are many variations on the general life cycle. Obviously, species that exist only as polyps or medusae have a life cycle without the other form.

General cnidarian life cycle

General Cnidarian Life Cycle. Cnidarians may reproduce both asexually and sexually.

Ecology of Cnidarians

Cnidarians can be found in almost all ocean habitats. They may live in water that is shallow or deep, warm or cold. A few species live in freshwater. Some cnidarians live alone, while others live in colonies.

Corals form large colonies in shallow tropical water. They are confined to shallow water because they have a mutualistic relationship with algae that live inside them. The algae need sunlight for photosynthesis, so they must be relatively close to the surface of the water. Corals exist only as polyps. They catch plankton with their tentacles. Many secrete a calcium carbonate exoskeleton. Over time, this builds up to become a coral reef (see Figure below ). Coral reefs provide food and shelter to many ocean organisms. They also help protect shorelines from erosion by absorbing some of the energy of waves. Coral reefs are at risk of destruction today.

Great barrier reef

Great Barrier Reef. The Great Barrier Reef is a coral reef off the coast of Australia.

Unlike corals, jellyfish spend most of their lives as medusae. They live virtually everywhere in the ocean. They are typically carnivores. They prey on zooplankton, other invertebrates, and the eggs and larvae of fish.

KQED: Amazing Jellies

Jellyfish. They are otherworldly creatures that glow in the dark, without brains or bones, some more than 100 feet long. And there are many different types. Jellyfish are free-swimming members of the phylum Cnidaria. Jellyfish are found in every ocean, from the surface to the deep sea. To find out more about jellyfish, see http://www.kqed.org/quest/television/amazing-jellies--siphonophores2 .


  • Cnidarians include jellyfish and corals.
  • Cnidarians are aquatic invertebrates. They have tissues and radial symmetry. They also have tentacles with stingers.
  • There are two cnidarian body plans: the polyp and the medusa. They differ in several ways.
  • Many corals secrete an exoskeleton that builds up to become a coral reef.


Use this resource to answer the questions that follow.

  1. What is the general body plan of Cnidarians?
  2. Give four examples of Cnidarians.
  3. Describe the Cnidarian digestive cavity.
  4. Name the two forms of Cnidarians. Give examples of each.


1. What is a nematocyst? What is its function?

2. How do coral reefs form?

3. Compare and contrast cnidarian polyps and medusae.

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