The class Asteroidea belongs to the phylum Echinodermata.
The larvae of echinoderms have bilateral symmetry, but during metamorphosis this is replaced with radial symmetry, typically pentameric.
Starfish are included in the subphylum Asterozoa, the characteristics of which include a flattened, star-shaped body as adults consisting of a central disc and multiple radiating arms.
Several species have specialized feeding behaviours including eversion of their stomachs and suspension feeding.
They have complex life cycles and can reproduce both sexually and asexually.
Luidia ciliaris has seven arms, members of the Solasteridae have ten to fifteen while the Antarctic Labidiaster annulatus can have up to fifty.
It is not unusual in species that typically have five arms for some individuals to possess six or more through abnormal development.
Most can regenerate damaged parts or lost arms and they can shed arms as a means of defence.
The Asteroidea occupy several significant ecological roles.
In Forcipulatida, such as Asterias and Pisaster, they occur in pompom-like tufts at the base of each spine, whereas in the Goniasteridae, such as Hippasteria phrygiana, the pedicellariae are scattered over the body surface.