The word starfish refers to about 1,800 species of marine animals that are star-shaped. The common term starfish is confusing, though. Starfish aren't fish - finned, tailed animals with backbones - they are echinoderms, which are marine invertebrates. So scientists prefer to call these animals sea stars.
Sea stars come in all sizes, shapes and colors. Their most noticeable characteristic is their arms, which form their distinctive star shape. Many sea star species have 5 arms, and these species most resemble a traditional star shape. Some species, like the sun star, can have up to 40 arms radiating out from their central disk (the usually circular area at the center of the sea star's arms).
All sea stars are in the Class Asteroidea. Asteroidea have a water vascular system, rather than blood. A sea star draws seawater into its body via a madreporite (a porous plate, or sieve plate), and moves it through a series of canals. The water provides structure to the sea star's body, and is used for propulsion by moving the animal's tube feet.
Although sea stars don't have gills, tails or scales like fish do, they have eyes - one at the end of each of their arms. These are not complex eyes, but eye spots that can sense light and dark.
Sea stars may reproduce sexually, by releasing sperm and eggs (gametes) into the water, or asexually, through regeneration.
Learn more about sea star feeding, reproduction and habitats.