Despite their name and their underwater habitat, starfish are not actually fish, but invertebrates known as echinoderms. Because of this, scientists have been trying to convince the public to call them sea stars instead of starfish. No matter what they're called, these ocean dwellers have some unique physical characteristics that help them thrive in their habitats.
The appendages, or arms, of a starfish give him his unique star shape. Most starfish have five arms, but some species have more, ranging from 20 to 40. The arms are covered in small suction cups on the bottom and spiny skin on top and serve several purposes. The suction cups help the starfish cling to the ocean floor and move him along as he uses water pressure within his own body to grab and release objects. The tough arms also help him open shellfish, which are a prime source of food. Female starfish release eggs from their appendages in mass quantities -- up to 2.5 million eggs per starfish. Each arm contains a duplicate of the major organs necessary for survival, allowing an injured starfish to regenerate, even if a majority of his body is eaten by predators.
Starfish skin is covered in tiny spines that look like small lumps and bumps to the naked eye. These spines help keep a starfish clean as he traverses the ocean floor by removing debris from his body and give him some protection against predators by making the skin difficult to penetrate. His skin also has small tufts that act like gills, filtering ocean water in and out of his body. Most starfish have skin that closely matches their environment to camouflage them so they can avoid predators.
The center of a starfish is not the control center, since the organs are located in the arms and the starfish does not possess a brain. Instead, it is the location of his mouth and madreporite, or pumping organ. This organ pumps water throughout the starfish's body, helping him to move. The mouth is not quite the same as the oral cavity on most animals, instead, the starfish pushes his stomach out through his mouth to consume his food. Once the food has liquified from the stomach acid, he pulls his stomach back inside his body.
Instead of eyes that see objects, a starfish has eyespots that sense light. These spots help the starfish navigate based on the amount of light or darkness in the area. He has one eyespot at the end of each arm that looks like a small dot that is part of his natural pattern. In addition to eyespots, a starfish uses chemical receptors that are not visible to the naked eye to navigate the ocean floor and to locate prey.