A coral polyp is a tubular sac-like animal with a central mouth surrounded by a ring of tentacles. The end opposite the tentacles, called the base, is attached to the substrate.
Depending on the species, coral polyps may measure less than an inch to several inches in diameter (a few millimeters to several centimeters). One of the largest corals, Fungia (mushroom coral), is a solitary coral that can extend 10 in. (25 cm) in diameter. Colonial coral polyps are much smaller and average 0.04 to 0.12 in. (1-3 mm) in diameter.
Coral colonies also vary in size. Some corals form only small colonies. Others may form colonies several feet (a few meters) high. Star coral (Montastrea annularis) colonies reach an average height of 10 to 13 ft. (3-4m).
Natural pigments in coral tissue produce a range of colors including white, red, orange, yellow, green, blue, and purple. Colored calcareous spicules (needle-shaped structures) give some octocorallians their colors. Algae that live within the tissues of some corals may make the coral appear brown, green, or orange.
Tentacles are for defense and for moving food to the mouth. Depending on the subclass, a coral polyp's tentacles are arranged in multiples of six or eight. The tentacles contain microscopic stinging capsules called nematocysts. A nematocyst is a bulbous double-walled structure containing a spirally folded, venom-filled thread with a minute barb at its tip. A tiny sensor projects outside the nematocyst. When the sensor is stimulated physically or chemically, the capsule explodes and ejects the thread with considerable force and speed. The barb penetrates the victim's skin and injects a potent venom.