The starfish is one of the most curious of sea creatures. Among its queer relatives are the prickly sea-urchins, the sea cucumbers, and the sand dollars. There are more than 6,000 of these relatives and they are called “echinoderms,” which means spiny-skinned.

The starfish and its relatives all have well-developed nervous and digestive systems. This system follows the same five-armed arrangement which occurs in all echinoderms.

The starfish are sometimes divided into three groups. There are the brittle stars, which break-off their long snaky rays if they are caught. Their arms may extend 20 to 25 centimeters. There are the feather Stars, whose waving rays resemble little plumes. And there are the ordinary sea stars which usually measure about 13 centimeters.

The tough, leathery skin of a starfish is covered with very short spines. In the center of their bodies, on both the upper and undersides, are button-shaped disks. Through these disks they draw in or expel sea water. The disks on the under sides act as mouths. The eyes are at the tips of their arms and are protected by a circle of spines.

Along the underside of their arms are grooves, and along these grooves are arranged little tubelike sucker feet. These are used both for moving about and as organs of smell. Sea stars cannot travel very fast with their little tube feet, but they can do something more remarkable. They can open an oyster! They attach the sucking disks of their feet to either half of the oyster shell and pull at it until the oyster finally opens. Then the starfish turns its stomach inside out, brings it through its mouth, and wraps it about the oyster.

Starfish can also eat by taking food into their mouths in the ordinary way. They can, also replace broken arms. They may even grow a whole new body from one arm!