Asteroids can range from less than 2 cm to over one m in diameter, although the majority are 12 to 24 cm. Arms extend from the body from a central disk and can be short or long. A majority have 5 arms, although some can have up to 40. Calcareous ossicles make up the internal skeleton.
The water vascular system of the sea stars open up at the madreporite, a perforated opening in the central part of the animal. Internally, the madreporite leads to a stone canal, made up of skeletal deposits. The stone canal is attached to a ring canal which leads to each of the five (or more) radial canals. Tiedemann’s bodies and polian vescicles are pouches on the ring canal whose function may be osmoregulation or hydraulic regulation within the water vascular system. Each radial canal ends in a terminal tube foot, which has a sensory function.
Each radial canal has a series of lateral canals that terminates at a tube foot. Each tube foot is made of an ampulla, podium, and usually a sucker.
The oral surface, under the central disc, is where the mouth is located. The hemal system parallels the water vascular system and probably distributes nutrients from the digestive tract. Hemal channels extend to the gonads.
Larvae are bilaterally symmetrical and adults are radially symmetrical.
Other Physical Features: ectothermic ; heterothermic ; bilateral symmetry ; radial symmetry