Cnidaria

The idea

Cnidarians have tissue-level organisation, with specialised cells forming tentacles that capture prey, but a single opening serves for both food intake and waste removal.

Hydra hunting

A Hydra stretches its thin arms into the water and catches a small prey.

What kind of body structure helps it catch prey?

• Specialised cells forming tentacles
• Pores that let water in
• A hard backbone

Cnidarian body

Open each tab.

Better, but with a limit

Specialised cells make a big change from sponges, because tentacles can capture prey. But there is still a single opening for both food and waste.

Cnidarians have tissue-level organisation, with specialised cells forming tentacles that capture prey, but a single opening serves for both food intake and waste removal.

Notes

Cnidarians have tissue-level organisation and tentacles to capture prey, but only a single opening for food intake and waste removal.

Check yourself

Which are cnidarians?

What level of organisation do cnidarians have?

What are tentacles used for?

What is the limit of the cnidarian body mentioned in the book?

• Sponges and earthworms. Sponges are poriferans, and earthworms are annelids.
• Hydra, jellyfish and corals — correct. Yes!
• Snails and octopuses. Snails and octopuses are molluscs.
• No tissues. Sponges are multicellular without tissues. Cnidarians have tissue-level organisation.
• Organ system level. Organ system level is shown by annelids and molluscs, not by cnidarians.
• Tissue level — correct. Yes!
• Capturing prey — correct. Yes!
• Letting water flow through pores. Pores that let water flow through belong to sponges.
• Supporting the body as a skeleton. The book says tentacles are used for capturing prey.
• They have no tissues. They have tissue-level organisation.
• One opening serves for both food intake and waste removal — correct. Yes!
• They have no way to catch prey. They catch prey using tentacles.