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You probably already know this, but starfish aren’t actually fish. They don’t have gills or any other organ that specializes in extracting oxygen from the water. And their surface area to volume ratio is so low that they can easily diffuse gases in and out via their skin. So what exactly is a starfish? 

Echinoderms

These weird star-shaped animals fall under the phylum Echinodermata. Echinoderms, as the animals in this phylum, are called, all have this star shape. Some scientists call them the “hedgehogs” of the marine world because they are also spikey. Aside from the starfish, other familiar echinoderms are sea urchins, sea cucumbers, and sand dollars.

How Starfish Move

The most common questions people ask about starfish are, 1) how do they walk and 2) how do they grow back after being chopped in half. Unlike humans, starfish don’t have muscles. What they do have are hundreds of tiny tubular feet that are propelled by hydraulics. The pumping in and out of these tubes creates movement. Think about those dancing tubular balloon characters outside of car dealerships. Starfish feet moves sort of like those things, but more controlled.

Where Starfish Regenerate From

As for the being cut in half question, any part of the starfish that contains the center “disc region” will grow back new limbs. That means if you cut off the leg of a starfish, the leg won’t be able to grow into a brand new starfish. However, the starfish with the severed leg will grow a new leg.

Starfish in Research

If you remember planaria (flatworms) from high school biology, then you know that these worms can regenerate when they’re cut in half as well. This type of animal is of great interest to scientists because it would be fantastic if we figured out how to regrow limbs!

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