Although fish and starfish both belong to the kingdom Animalia -- which is the largest division of taxonomic classification -- starfish lack a spine, and therefore belong to a different phylum than true fish. For this reason, biologists refer to them as "sea stars." Starfish are distinguishable from similar animals, such as sea urchins and brittle stars, by such characteristics as the way the plates of their endoskeletons fit together.
The kingdom Animalia comprises all organisms that scientists classify as animals. Organisms belonging to this kingdom share the following characteristics: They have more than one cell and lack rigid cell walls; they are heterotrophic (i.e., they obtain their energy through consumption); they usually reproduce sexually; they are capable of moving at some point in their lives; and they can respond rapidly to external stimuli.
Starfish belong to the phylum Echinodermata, which is the largest phylum without any members that live on land or in fresh water. Echinoderms have five-point radial symmetry as adults, but are bilaterally symmetrical as larvae. They have an endoskeleton consisting of calcareous plates that often include spines. Echinoderms also have a unique water-vascular system that allows them to move, eat and attach to surfaces. Sea urchins, sand dollars and brittle stars are other examples of echinoderms.
The class Asteroidea consists of starfish, or sea stars. Most starfish are predaceous (i.e., they hunt and eat other organisms). Some species in this class can have up to 50 arms, but most have only five. Sea stars have separable plates in their endoskeleton, allowing for some mobility. Tube feet, which are part of the starfish's water-vascular system, occur in rows along the bottom of each of their arms.
Within the class Asteroidea, starfish are divisible by type into the orders Brisingida, Forcipulatida, Notomyotida, Paxillosida, Spinulosida, Valvatida and Velatida. These orders contain one to fourteen families. Within these families, starfish are further divisible into genera and species. As of 2011, there are roughly 2,000 known species of starfish, each of which bears a scientific name consisting of its genus and species.
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