True starfish (not to be confused with brittle stars) are part of the Echinoderm class Asteroidea. They are all marine, and have pentaradial symmetry—meaning five-sided radial. Most starfish have five arms, but these arms “split” in some species, resulting in more arms (but always in a multiple of five.) These arms have no set length—they can even be merely stubs, like in the pillow (or cushion) star. Asteroids have a Water Vascular System, which they pump full with sea water through a bony pore called a madreporite (white spot seen in picture) on their aboral surface (top surface). They use their WVS for locomotion, food and waste transportation, and even respiration.
Starfish are incredible creatures. They have no eyes, ears, or nose—but they get by using extraordinarily sensible elongated sucker-like muscles, or tube feet, that line their arms; some tube feet even have simple light receptors that act as primitive eyes. They can regenerate an arm (or two or three) if lost. This can be considered a form of asexual reproduction. They are predators, and can regurgitate their “stomach” and surround/digest their prey (usually clams and other brachiopods.)