The Phylum Echinodermata contains over 6,000 species in 6 classes having 35 orders. The word echinoderm means "spiny skin" and this phylum includes urchins, brittle stars, starfish, cucumbers, and feather stars. These creatures all have an internal calcium skeleton and mostly travel using tube feet. Their vascular system pumps water, not blood. By pumping water into or out of their tube feet, the animal is capable of securing itself to most surfaces. Their mouth area usually contacts the substrate, except for cucumbers. They are scavengers and/or filter/suspension feeders.
Actually, the word starfish is a misnomer; as these are of course invertebrate, not fish. However, their star-shaped body lends certain realism to the term. True starfish comprise the Class Asteroidea, and there are about 1600 species. However, only a few are truly suited for the aquarium. They can be found to depths of 20,000 feet. The majorities of starfishes, now more properly referred to as sea stars, are carnivorous or omnivorous and can eat small fishes, algae, mollusks and coral polyps. In fact, they don’t have to fit their prey into their mouth, as they are capable of expelling their stomach and digesting whatever they are resting upon, such as what the Crown-of-Thorns does to coral polyps.
Mostly nocturnal, these bottom dwelling multi-arm echinoderms are excellent scavengers and some of those mentioned below are extremely easy to maintain, yet some are not recommended for reef systems. A good rule of thumb for reef keepers is to avoid any starfish with knobby backs such as those in the genus Protoreaster. And keep in mind; all starfish are very vulnerable to sudden salinity changes.
Two classes are of interest in this grouping, Class Crinoidea, Sea Lilies/Feather Stars and Class Asteroidea that is said to encompass ‘True’ Starfish containing many ‘orders’ that also includes basket and brittle stars.