Starfish and Urchins - Spiny Skins

These organisms are great. They are also very beautiful. Starfish, urchins, and sea cucumbers are all grouped as echinoderms. Echinoderm? What does that mean? "Echino" means spiny and "derm" means skin. You may have heard of your epidermis. That's your outside skin. The creatures in this group all have some type of spiny structure on the outside.

Images of Echinoderms

Tube Feet

What makes them so special? Can you say that you have tube feet? They can. If you ever turn a starfish over you will see hundreds of little tubes on each arm. Those tubes attach to an object, suck in, and attach to help the creature move. Those tube feet are also quite handy when it's time to eat. Starfish are hunters, moving around the rocks looking for food. Sometimes they come across a bivalve (mussel or clam). The starfish doesn't want to eat the shell. It wants to eat the organism inside. They use those little tubes to attach to the shell. Since the suction is so strong, that can slowly open the bivalve and eat the soft part of the mussel inside.

And a Vascular System

There are small suction cups on the end of each of the tube feet. The other big idea that is not so easy to see is that starfish have something called a water based vascular system. You have blood that transports nutrients all over your body. Echinoderms circulate water throughout their bodies. The system not only transports molecules, but also works with muscles to walk and move. The canals of their vascular system are found throughout their entire body.

We already talked about starfish moving around and hunting down bivalves for dinner. In the echinoderms, you'll also discover that everybody seems to eat in a certain way because of their body types. Starfish move around. They have a lot of strength. They are able to go after living creatures. Urchins are a little different. They don't move around so well. They also don't have arms to help them grab food. You'll find that urchins are the grazers of the echinoderms. Like cows eating grass, they eat little pieces of food and algae. Finally, you have your sea cucumbers. Round, squishy and not very impressive. They sit on the bottom of the ocean and eat. More specifically, they eat whatever you find down there.

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