Starfish, more accurately called sea stars, are not really fish but echinoderms, which means "spiny skin" in Greek. True to their name, starfish really are covered in a spiny skin on their top sides, which is made up of two different parts: dermal branchiae and pedicellaria. Their undersides are covered with tube feet, or podia.
Meet the Starfish
Two different types of starfish exist: those belonging to the taxonomic class Asteroideas, which are true sea stars and sun stars, and those belonging to the taxonomic class Ophiuroideas, which are basket and brittle stars. Over 2,000 species are found worldwide, all of which live in marine habitats. While most sea stars have five arms, they can have more and they may range in size from under 1/2 inch wide to more than 4 feet. They come in all shades, but usually are brightly colored.
Part of the skin covering starfish's topsides is made up of dermal branchiae. These are gill-like formations that are used to absorb oxygen from the water. Like most animals, their bodies require oxygen to function properly, but it's harder to get hold of when you're an aquatic organism. Essentially, dermal branchiae act like lungs and allow these creatures to breathe underwater.
Pedicellaria are small pincer organs that are part of the skin covering the top of starfish. They're able to grab and hold things that are touching the surface of their skin. These organs have two main purposes: to clean or remove unwanted objects from their skin, such as sand or barnacles, and to capture food that then can be transferred to their mouths. However, this is not the most common method that sea stars use to hunt for dinner.
The tube feet, or podia, on the undersides of starfish are used as a form of locomotion, as these creatures don't have any fins or flippers with which to move themselves around. They use their water vascular system to expand and contract their tube feet, which allows them to propel themselves. Suckers on the end of these podia can be used to grab prey, or adhere themselves to rocks or other surfaces. They also may have some sharp spines between their tube feet, to help protect them from getting damaged.
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