Most starfish have five appendages, sometimes called arms. The most easily recognizable starfish are those that have the archetypal shape of a five-pointed star, which is how they got their name. A few species have more than five appendages, but always in multiples of five; for example, some have 10 arms, some have 15 and one species has 50 arms. The arms radiate outward from a main body, called a central disc. Starfish have one surface that faces upward, or dorsally, and one that faces downward, or ventrally. The ventral surface is where the feeding organs are.
The starfish feeds from the bottom, through a mouth that leads to a two-chambered stomach. It excretes from an orifice on the dorsal surface. Starfish eat a variety of foods, depending on the species and location. Some eat mollusks like clams, oysters and barnacles. Some eat particles of dead plant and animal material. Some graze on coral. All starfish are found near coastlines, especially around coral reefs.
Most starfish reproduce sexually, but post-coital reproductive behaviors vary a great deal. Some species simply release the fertilize ova to grow among plankton, others deposit eggs in sheltered areas and still others actually tend the eggs until they hatch. A few starfish reproduced asexually, by growing from a severed arm. If an arm is severed without a portion of the central disc, it cannot regenerate into a new starfish.
Starfish depend on their extremely tough hides for protection, but many species have augmented that defense with abrasive surfaces, painful gripping surfaces and even spines. A few species of starfish have the ability to sting and inject painful venom. One particular species to look out for that is common throughout tropical waters around the world is the crown-of-thorns starfish, which is covered in 3-inch spines that are sharp enough to cut diving gloves, with a venom that can cause intense pain, vomiting and even temporary paralysis.