Sea Stars: Asteroidea

Sea stars have spiny skin and at least five arms large enough to hold digestive and reproductive organs. Some species of sea stars have as many as thirty arms. Sea stars are 0.4 inch (1 centimeter) to almost 3 feet (91 centimeters) across. The bottom of each arm is covered with rows of tube feet along a groove. Depending on the species, tube feet have suckers that help the sea star stick to hard surfaces or assist in prying open the shells of its prey.

The skeleton of sea stars consists of small plates that act as a firm but flexible skeleton. The upper and lower body surfaces also are covered with pinchers that range from simple spines to hooks. The upper surface is covered with many small, clear sacs used for exchanging oxygen, that is, for breathing. Sea stars have a nerve net but no brain. Even so, they are advanced enough to change on the basis of previous experiences and to stop behaviors, usually feeding behaviors, that continue to be unsuccessful.