http://animals.nationalgeographic.com/animals/invertebrates/starfish/

Sea stars are purely marine animals, even using [2] sea water instead of blood to pump nutrients throughout their bodies. Marine scientists have undertaken the difficult task of replacing the beloved starfish’s common name with sea star because, well, the starfish is not a fish. It’s an echinoderm, closely related to sea urchins and sand dollars.

There are some 2,000 species of sea star living in all the world’s oceans, from [1] tropical habitats to the cold seafloor. The five-arm varieties are the most common, hence their name, but species with 10, 20, and even 40 arms exist.

They have bony, calcified skin, which protects them from most predators, and many wear striking colors that camouflage them or scare off potential attackers. Purely marine animals, there are no freshwater sea stars, and only a few live in brackish water.

Beyond their distinctive shape, sea stars are famous for their ability to regenerate limbs, and in some cases, entire bodies. They accomplish this by housing most or all of their vital organs in their arms. Some require the central body to be intact to regenerate, but a few species can grow an entirely new sea star just from a portion of a severed limb.

Most sea stars also have the remarkable ability to consume prey outside their bodies. Using tiny, suction-cupped tube feet, they pry open clams or oysters, and their sack-like cardiac stomach emerges from their mouth and oozes inside the shell. The stomach then envelops the prey to digest it, and finally withdraws back into the body.