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Lesson: Chapter 13 — The Living World

13.1. Invertebrates 2 of 6

Cnidaria — two layers and a sting

This is the phylum of diploblastic animals such as hydra, sea anemones and jellyfish.

Figure to be added

Figure 13.15 — several species of Cnidaria: hydra, sea anemone and jellyfish.
  • All are aquatic. Most live in sea water and a few in fresh water.
  • The multicellular body is built from two cell layers, which is why they are called diploblastic.
  • Inside the body is a cavity called the coelenteron, which acts as the alimentary canal.
  • The body shows radial symmetry.
  • All are predators. They strike, disable small animals and eat them. They bear stinging cells.
  • Asexual reproduction occurs by budding. Sexual reproduction occurs as well.

Two forms

The polyp form

Lives a sedentary life, fixed in one place.

The medusa form

Moves about, drifting through the water.

Figure to be added

Figure 13.16 — a section through a cnidarian: the polyp form and the medusa form, with the coelenteron, the cell layers and the tentacles marked.

Coral reefs

Coral polyps, which are cnidarians, build coral reefs. These perform an important service for the survival of marine life.

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Figure 13.17 — coral polyps.