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The body form of the crown-of-thorns starfish is fundamentally the same as that of a typical starfish, with a central disk and radiating arms. Its special traits, however, include being disc-shaped, multiple-armed, flexible, prehensile, and heavily spined, and having a large ratio of stomach surface to body mass. Its prehensile ability arises from the two rows of numerous tube feet that extend to the tip of each arm. In being multiple-armed, it has lost the five-fold symmetry (pentamerism) typical of starfish, although it begins with this symmetry in its life cycle. Adult crown-of-thorns starfish normally range in size from 25 to 35 cm (9.8 to 13.8 in). They have up to 21 arms. Although the body of the crown of thorns has a stiff appearance, it is able to bend and twist to fit around the contours of the corals on which it feeds. The underside of each arm has a series of closely fitting plates which form a groove and extend in rows to the mouth.They are usually of subdued colours, pale brown to grey-green, but they may be garish with bright warning colours in some parts of their wide range. The long, sharp spines on the sides of the starfish's arms and upper (aboral) surface are poisonous and resemble thorns, they create a crown-like shape, giving the creature its name. The spines are stiff and very sharp, and readily pierce through soft surfaces. Despite the battery of sharp spines on the aboral surface and blunt spines on the oral surface, the crown-of-thorns starfish's general body surface is membranous and soft. When the starfish is removed from the water, the body surface ruptures and the body fluid leaks out, so the body collapses and flattens. The spines bend over and flatten, as well. They recover their shape when reimmersed, if they are still alive.
The Crown-of-Thorns starfish prefers to live in sheltered areas such as lagoons, and in deeper water along reef fronts, where there is less disturbance by currents, tides, weather etc.