What are cnidarians?
Cnidarians are simple animals that live in water, mostly in the sea. Hydra lives in fresh water. Jellyfish, sea anemones and corals live in the sea.
All cnidarians share these features:
- The body is shaped like a bag or an umbrella.
- There are tentacles around the mouth.
- The tentacles carry stinging cells (cnidocytes) to catch food.
- The body wall has two layers of cells with jelly between them.
- There is one inside space, the gut cavity, with one opening.
Hydra: body and food
Hydra is about as long as a grain of rice. It has a tube-shaped body. The bottom sticks to a plant or stone. The top has a mouth surrounded by tentacles.
How it feeds: A small animal such as a water flea brushes a tentacle. Stinging cells fire thin threads. Some threads hold the prey and some give poison that makes it still. The tentacles bend and push the food into the mouth. Food is digested in the gut cavity. Leftovers go out of the same mouth.
Hydra has no heart, lungs or blood. Oxygen goes in through the thin skin. Carbon dioxide goes out the same way.
Two layers of the body
The body wall has an outer layer that touches the water and an inner layer that lines the gut. Between them is a thin jelly-like layer. Animals built like this are called two-layered. Higher animals like earthworms have three layers.
Regeneration and reproduction in Hydra
Regeneration means growing back lost parts. If a Hydra is cut into pieces, each piece with enough cells can grow into a whole new Hydra. The cells divide and shape themselves into a new mouth, tentacles and foot.
Hydra also makes new Hydras by budding. A small bump grows on the side, gets its own tentacles, and then breaks off. When food is scarce, Hydra can also make eggs and sperm.
Corals and other cnidarians
A coral polyp looks like a tiny Hydra with a hard cup of limestone around it. Thousands of polyps stay joined together as a colony. When polyps die, their cups stay. New polyps grow on top. Over thousands of years this builds a coral reef.
Reefs are home to many fish and protect coasts from waves. Corals are hurt by warm water, dirty water and digging. Jellyfish swim as bell shapes. Sea anemones stay fixed like flowers.
Try it
In the 3D: press Feed and watch the prey go into the mouth. Then press Cut it and count the new Hydras. How many pieces became how many animals?
At home: next time you see a picture of a coral reef, count how many different shapes of coral you can spot. Draw a polyp with its ring of tentacles.
Key formulas and definitions
- Cnidarian body = 2 cell layers + 1 gut cavity + 1 opening + tentacles with stinging cells
- Hydra: feeds with stinging cells; regenerates; reproduces by budding
- Many polyps together = a coral colony; many colonies over time = a reef
- Forms: polyp (fixed, like Hydra) and medusa (free-swimming, like jellyfish)
Worked examples
1. A Hydra is cut into 3 pieces and all pieces survive. How many Hydras will there be after some days, and what is this called?
There will be 3 Hydras. Each piece grows back the missing parts. This is called regeneration.
2. Why does Hydra not need lungs or a heart?
Its body is very thin and every cell is close to the water. Oxygen can enter, and carbon dioxide leave, directly through the skin. Food spreads easily from the gut cavity.
3. A coral reef grows 1 cm every year. How tall will it be after 50 years if the growth stays the same?
50 x 1 cm = 50 cm.
Common mistakes
- Saying Hydra has a separate mouth and anus. It has only one opening for both.
- Thinking coral is a rock or a plant. Coral is made of living animals (polyps).
- Calling every sea animal that stings a fish. Jellyfish are not fish; they are cnidarians.
- Saying cnidarians have three layers. They have two layers and a jelly between them.