Starfish are part of the phylum Echinoderm and are most closely related to sea cucumbers, brittle stars and sea urchins. They are in the kingdom Animalia, class Asteroidea, and can be any of a variety of different orders, since it is estimated that there are more than 2,000 different star-shaped sea critters known to man.
Starfish are not really fish at all, and scientists refer to them as sea stars. They are invertebrates with hundreds of tube feet that they use for help with movement and feeding. Instead of a backbone, the starfish has a hard plate under its skin that gives its body structure and support.