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Corals belong to the Cnidaria or Coelenterata group (or phylum) of animals, which also includes corals, hydras, , Portuguese men-of-war, sea anemones, sea pens, sea whips, and sea fans. Corals form the class Anthozoa within this phylum.
They typically have stone-like, 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 significant proportion of their food. They live in 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.
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