Yellow coral polyp
A coral polyp is one of the simplest animals lacking many features that other animals need to survive. It’s round with many fleshy tentacles around the mouth. The mouth opens into a simple stomach cavity. It has no intestines or anus, so waste is pushed back out through the mouth. A coral polyp has no brain, nervous system or eyes. There is no heart or blood supply.
Cross-section of a polyp
Hidden in the skin of the tentacles are special stinging cells. These protect the polyp and can be used to catch small prey. Each stinging cell can be individually fired. It fires a small poisonous barb attached to a thread. The thread remains attached to the cell and it entangles the small prey. The prey is drawn into the stomach cavity where it’s digested.
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The anemone is much larger than a coral polyp and does not make a limestone skeleton. Clown fish can live among the tentacles of large anemones. Their tentacles also have stinging cells. Clown fish don’t trigger the stinging cells so they are well protected from other predators.
Diagram of a stinging cell