Starfishes and brittlestars are anatomically very similar and were formerly grouped together in a single class, the Stelleroidea, which included all star-shaped echinoderms. There are about 1,500 known species of starfishes, inhabiting the coasts of all known seas around the globe. Some live at low-tide level, or float on sea-weed at the surface of the sea; others burrow into the sea-floor. Next to holothurians, starfishes are the deepest-dwelling animals of the sea.
Starfishes usually creep about the sea-bed with the aid of their tube-feet, there are no swimming forms known. Those species which have suctorial tube-feet are able to climb and descend obstacles with their aid, and capture food. A number of genera have peculiar peg-like tube-feet , without suckers; they usually occur on submarine mud-banks, where they seem to use their tube-feet as oars or as stilts perhaps.
Although starfishes cannot see, since they have no eyes, they are able to detect light-fluctuations with the aid of a photosensitive eye-spot at the outer end of the ambulacral groove where it reaches the tip of the arm. Starfishes cannot be said to ‘ hear ‘, though they evidently detect the grosser vibrations often associated with sound. At the tip of the arm are some special tube-feet in some starfishes, which are held erect when the animal is active, and probably serve as taste-organs.
Starfishes are probably short-lived animals with a life-span of only a few years. Some Starfishes are known to be sexually mature after one year, though growth continues for about four years. Some kinds can regenerate lost arms. The brilliant colours of starfishes are due to biochromes which usually fade on preservation, and are chemically changed by alcohol. The colours include orange, yellow, green, blue, violet, brown, as well as variegated patterns. There are usually five arms, but many species have more than five.