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Two- and Three-Layered Animals (Cnidarians)

Cnidarians (hydra, jellyfish, sea anemones, corals) have a body of two cell layers, ectoderm outside and endoderm inside, with jelly between them, a single-opening gut, stinging cells and radial symmetry. Animals above them have a third layer, the mesoderm, and are bilaterally symmetrical.

🎬 Step-by-step story

  1. This is a hydra, a tiny pond animal. It has a foot at the bottom, a bag-shaped body, a mouth on top and tentacles all round.
  2. Slice it across. Only two layers of cells: ectoderm outside and endoderm inside, with jelly (mesoglea) between. One gut cavity inside has just one opening.
  3. Each tentacle carries stinging cells. A thread shoots out at the prey and holds it. The tentacle then carries the prey to the mouth.
  4. Look from the top: a hydra is round like a wheel. Cut it at any angle and you get two equal halves. This is radial symmetry.
  5. Animals above cnidarians have three layers. A new middle layer, the mesoderm, makes muscles and organs. They have a front and back and one mirror line.
  6. Free play: choose an animal and turn the cut. Hydra gives equal halves at every angle. The flatworm gives equal halves only for the one up-and-down cut.

Tip: drag the 3D scene to turn it. Use two fingers to zoom.

🤔 Common doubts, cleared

Is a hydra a plant because it sits on a rock?

No. It eats other animals with its tentacles, so it is an animal that is fixed for most of the time.

Is the jelly a layer of cells?

Not really. The mesoglea is a jelly with only a few scattered cells, so cnidarians are still two-layered.

Do stinging cells hurt humans?

Hydra stings are too weak to feel, but some jellyfish stings are very painful or even dangerous.

Why is radial symmetry good for hydra?

It sits or drifts, and food may touch it from any side, so it has tentacles all round.

Where is the mesoderm in the body?

It lies between the outer ectoderm and inner endoderm. It builds muscles and organs.

Why do only some cuts give equal halves in a flatworm?

It has a left and a right half that are mirror images. Any other cut separates unlike parts, such as head from tail.

Germ layers: two or three

An early embryo forms layers of cells, called germ layers. The outer one is the ectoderm (becomes skin and nerves), the inner one is the endoderm (becomes the gut lining) and some animals also have a middle one, the mesoderm (becomes muscles, blood and many organs).

The extra layer lets an animal build real organs and strong muscles, so it can grow bigger and move better.

Cnidarians: who are they?

Cnidarians live in water, mostly the sea: hydra (fresh water), jellyfish, sea anemones and corals. Their body is a bag with one opening, the mouth, ringed by tentacles. The inside is the gastrovascular cavity, where food is digested and shared. The same opening takes food in and sends waste out.

They have two body forms. The polyp is a tube fixed to a surface (hydra, coral, anemone). The medusa is a free-swimming umbrella (jellyfish). Some cnidarians show both forms in their life.

The hydra body, layer by layer

Hydra eats tiny water animals. It reproduces by budding: a small bud grows on the side, becomes a mini hydra and drops off. It can also make eggs and sperm.

Stinging cells (cnidocytes)

The name cnidarian comes from cnidocytes, special cells on the tentacles. Inside each is a coiled thread in a capsule. A touch makes the thread shoot out in a split second. Some threads stick, some wrap and some inject poison. This stops prey, and protects the animal from enemies. The tentacle then bends toward the mouth.

Symmetry: radial and bilateral

Radial symmetry: the body is like a wheel, with a top and bottom but no front and back. Any cut through the centre makes two equal halves. This suits animals that sit or drift and meet food from any side (hydra, jellyfish, anemone).

Bilateral symmetry: there is a left and a right, a front (head) and a back, a top and bottom. Only one cut, the up-and-down line down the middle, gives mirror halves. It suits moving animals, which have a head with eyes and a brain at the front. Flatworms and all animals above them are bilateral.

Try it

In the 3D: in the last step, choose the flatworm and slowly turn the cutting angle. Find the one angle where the halves are equal. Then choose hydra and try again.

At home: fold a paper in half. Draw a flower (radial) and a butterfly (bilateral). Fold each in different ways. Which one matches only for one fold?

Key formulas and definitions

Worked examples

1. Name three cnidarians and say which has a medusa form.

Hydra, sea anemone and jellyfish (also coral). The jellyfish is the medusa.

2. A flatworm is cut along its top-to-bottom midline. Are the halves equal? What if cut across the body from side to side?

Along the midline, yes: the two halves are mirror images. Across, no: the front half has the head and the back half does not.

3. Why does a hydra need stinging cells?

It has no head, eyes or jaws and cannot chase prey. Stinging cells on the tentacles stun and hold animals that touch them.

4. How many germ layers does a hydra have, and what is the jelly between them?

Two: ectoderm and endoderm. The jelly is the mesoglea, which is not a true cell layer.

Common mistakes

Practice quiz

1. How many germ layers does a hydra have?
2. Stinging cells of cnidarians are called:
3. Which is a free-swimming cnidarian form?
4. Which animal has bilateral symmetry?
5. The extra layer in three-layered animals is the:

Practice: answer these yourself

Type or choose your answer, then press Check. Use a hint if you are stuck; the full solution appears after you answer.

Frequently asked questions

What does diploblastic mean?

It means having two germ layers: ectoderm and endoderm. Cnidarians are diploblastic.

Do jellyfish have a brain?

No. They have a nerve net, a web of nerve cells, and some have simple eye-spots, but no brain.

Why are corals counted as animals, not rocks?

Coral is a colony of tiny polyps with tentacles and stinging cells. The hard rock-like part is the cup they build around themselves.

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