Starfish, or sea stars, are echinoderms in the class Asteroidea, characterized by their distinctive bodies, which look rather like stars. Despite what the familiar name suggests, starfish are not fish. Many people have a soft spot for starfish, thanks to their unusual shape and the bright array of colors that these marine creatures come in. Starfish are also popular with people since they are easy to see and observe, preferring the intertidal zone near the seashore. If you want to see some starfish for yourself and you live near the ocean, a tidepooling trip should yield a broad assortment.

There are around 1,800 species of starfish, ranging from the sunflower star, a species with a multitude of arms, to the bat star, a starfish with five connected arms which looks a little like a pentagonal coaster. Starfish come in an array of colors, shapes, and sizes, and they eat a wide assortment of marine life. They are also popular food sources for many marine animals and some humans as well.

The physical structure of a starfish is quite interesting. These animals exhibit radial symmetry, meaning that they are symmetrical around a central axis, rather than bilaterial symmetry, a form of biological symmetry which may be more familiar to you, given that many organisms including humans are bilaterally symmetrical, having two distinct halves. Starfish are also capable of regenerating arms which have broken off, and in some cases a new starfish can generate from a severed arm. Some species use this trait as a defensive tactic, dropping their arms when threatened.

The body of a starfish is covered in flexible protective plates. If you have ever touched a starfish, you may have noticed that the animals feel slightly leathery. The bottom of a starfish is covered in tiny tube feet which the animals use to move around and to hunt. The tube feet also help starfish glue themselves to rock, another way to protect the animals from predators.

As if this wasn't enough, the starfish has a few tricks up its sleeve. These animals are capable of everting their stomachs, which means that they can literally turn their stomachs inside out to wrap them around prey! This biological adaptation allows starfish to eat animals much larger than them, and it is quite fascinating to watch, in the opinion of this wiseGEEK writer.

Starfish species range in size from around half an inch (a little over one centimeter) to three feet (one meter) in diameter, and they can have between five and 25 arms. Because starfish are such well-known sea creatures, they are commonly on display at aquariums and touch pools, for the benefit of those who cannot get to the ocean.