Corals, hydroids, sea anemones and jellyfish all belong to the phylum Cnidaria. Jellyfish represent the free-swimming medusa form, while the others represent the sedentary polyp form. Some cnidarians have both polyp and medusa stages in their life cycle.
Corals are sometimes broken off a reef in storms and washed up. These are the skeletons made by colonies of coral polyps. A coral polyp is like a tiny sea anemone with a limy skeleton surrounding it. Colonies of polyps build up coral reefs with these skeletons. Reef-building corals require high light levels and warm water (over 18C) and have algae living symbiotically inside their tissues.
Fossil stony coral
Goniastrea (Cnidaria: Scleractinia)
The patterns in this coral rock show where the living polyps were. This specimen is probably from the last interglacial period but the genus still exists today.
(Cnidaria: Hydrozoa: Hydroida)
These delicate structures, often attached to seagrasses, are related to corals and jellyfish. They are colonies of tiny polyps attached to branched stems. Each polyp has a mouth surrounded by tentacles like a tiny sea anemone. Small planktonic animals are caught by stinging cells on the tentacles and eaten.
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