Although many corals resemble plants, they are actually members of the animal phylum Cnidaria. The tentacles are tipped with stinging cells called nematocysts.
Corals use the nematocysts to defend themselves and to capture prey.
There is no skeleton inside the polyp itself. Instead, the polyps sit on top of an external skeleton that is made from the polyp's secretions. As their names might suggest, these two types of corals have very different skeletal structures, but there are other differences too. The soft corals have a flexible skeleton, made of a protein called gorgonin. Their skeleton also contains calcium carbonate, but only in small clumps called spicules.