Corals may look like a strange species of plants or fungi to you right. Well actually corals are animals which are along with its anemones, hydras, and jellyfishes; they belong to the phylum Cnidaria of invertebrates, which are characterized by having radially symmetrical bodies with saclike internal cavities.
In the colonial reef-building stony corals millions of these small animals generate a hard outer casing to defend them from damage and predators. When a polyp or coral dies, their outer skeletons steadily build up to form a coral reef.
