Asteroidea

Asteroidea, the starfishes, a class of the Echinodermata. The body is flattened, and is either pentagonal or has a varying number (usually five) of radiating arms. These are hollow and contain prolongations of the digestive and reproductive organs; the ventral side has a furrow containing the tube feet. The anus, when present, is dorsal, as is also the opening of the water vascular vessels. The skeleton consists of a series of many calcareous plates, the importance of which varies considerably in different families. The larva is a small free-swimming animal, provided with a series of arms, and known as a Bipinnaria or Brachiolaria. The most recent classification is into the Phanerozonata, those with large marginal plates (e.g. Astropecten), and the Cryptozonata, those with these plates absent or rudimentary (e.g. Asterias). The first starfish occur in the Cambrian period.