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They typically have stonelike, horny or leathery skeletons which may be internal or external. Their bodies are in the form of a polyp, a hollow bag with a mouth and tentacles at the upper end and attached to the surface at its base. The tentacles carry stinging nematocyst cells.

The tentacles kill and capture food which is brought in through the mouth. Digestion occurs in the lower section of the body, and any waste matter is ejected through the mouth.

Some corals also have algae living inside their cells which contribute a signicant proportion of their food. They live in symbiosis with the coral.

The male coral produces sperm, the female eggs, and these also leave through the mouth. They unite to form a planula larva which swims for days or weeks before starting a new colony. Corals can also reproduce by budding.

Corals are colonial animals. The polyps are connected by a horizontal sheet of tissue. The skeleton is a cup with the polyp inside. Stony corals deposit calcium carbonate to form their skeleton (the same material which forms fur inside kettles in hard water areas). As old polyps die their cups are left behind and the base of the colony grows thicker.

Coral Reefs

There are four types of reef:

• fringing reefs around islands
• barrier reefs several kilometers offshore near a landmass (such as the Great Barrier reef of Australia)
• atolls are hollow rings and form on submerged volcanoes (such as the islands of Bermuda)
• patch reefs are small circular or irregular reefs rising from the floor of a lagoon or within an atoll.

Corals form an important ecological resource. They grow in clear water which contains very little food, so no other form of life can survive there without the corals. The algae which live inside the corals form the base of a food chain which supports a wide diversity of plants and animals. The nutrients are then recycled within the reef, so it forms an ecosystem.

Coral reefs provide an economic benefit as a major tourist attraction.