Sea star: Encyclopedia II - Sea star - External Anatomy

Sea star - External Anatomy

Sea stars are composed of a central disc with (usually) five arms exhibiting pentaradial symmetry. The mouth is located underneath the sea star on the oral or ventral suface. The spiny upper surface covering the species is called the aboral or dorsal surface. On the aboral surface there is a structure called the madreporite which acts as a water filter and supplies the sea star's water vascular system with water to move.

Sea stars have a simple eye at the end of each arm. The eye is able to "see" only differences of light and dark, which is useful in detecting movement.

On the surface of the sea star, surrounding the spines, are small white objects known as pedicellariae. There are large numbers of these pedicellariae on the external body which serve to prevent encrusting organisms from colonising the sea star. The radial canal which is across each arm of the sea star has what are called ampullae which surround the radial canal. The ampullae are tooth-like structures. The aboral surface is also covered with papulae that are involved with the sea stars respiratory system.

Sea stars are often brightly colored, usually from reddish hues to violet, and unusual colors such as green and blue exist in some species, but come in muted colors as well. Patterns including mosaic-like tiles formed by ossicles, stripes, interconnecting net between spines, pustules with bright colors, mottles or spots. This mainly serves as camouflage or warning coloration displayed by many other marine animals as protection to the predator. Several types of toxins have been extracted from several species of sea stars and now being subjected into research worldwide for curing diseases or other uses such as pesticides.

Adapted from the Wikipedia article "External Anatomy", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki