Usually they have five arms, but they can have many more as on the common sunstar (Crossaster papposus) and the purple seastar (Solaster endeca). A starfish unlucky enough to lose an arm (or a leg) can also keep plodding on until a new one grows.
Starfish are interesting creatures and children especially like finding them on the beach or in rock pools. They seem to stay very still and don’t appear to get up to any antics. However, looks are deceiving, starfishes are voracious predators, especially on sea shells.
They attach their sucker feet to each half of the shell and tug it bit by bit until it gives way and opens up. It is a tug of war, but eventually the shellfish gets tired, runs short of oxygen and gives up the fight. The starfish then flips its stomach into the shell and starts to digest the fish from the outside. Starfish have two stomachs-one to digest the food and the other to shoot out and engulf and paralyze their prey.
It is amazing the distance they can cross without hardly seeming to move at all. Like other echinoderms starfish have a hydraulic system which they use to move around. They suck in water through small openings on the surface. A set of radial canals leads off this to the arms. Muscular bulbs then spurt the water out to the suction legs. The suction legs can also grip tightly and can either open shells or cling to steep rock walls. Starfish are the strongmen of the invertebrate kingdom as well as being some of the giants.