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Introduction: One of the most fascinating invertebrate groups is the phylum Echinodermata. Meaning "spiny skin." members of this group have spines of some sort covering their bodies. These spines may be very pronounced, like in the sea urchin, or very reduced, as in the sand dollar (see images below). All echinoderms are marine, meaning they all live in salt water and about 6,000 different species have been identified.

All members of this group also exhibit radial symmetry. This means they have a central axis and be divided into equal pieces like cutting a pie. Specifically, echinoderms have pentamerous radial symmetry. They can all be cut evenly into 5 equal parts, regardless of how many arms they may have. Notice the "starfish" imprint on the sand dollar above? All members of this phylum show this trait.

Echinoderms use tube feet for locomotion and a very sophisticated hydrolic system to make it work. This system is called a water-vascular system and it starts with an opening on the dorsal side of the animal. This opening is called the madreporite. When looking at a starfish, many people often mistake it for an eye because it is usually orange in color (the "eyes" of a starfish are located at the end of each arm). Water is pumped in through this opening into a series of tubes that go all around the animal's body. By regulating where the water goes, the echinoderm can force certain tube feet out while contracting others. This prevents it from getting tired and allows it to move across the ocean floor.

One particularly peculiar member of the Echinodermata is called a sea cucumber. Looking like a giant slug (see image left), this creature uses its tube feet to move across the ocean floor in search of prey. When danger is near, or when someone picks them up, these animals have a very unique defense mechanism. They are able to throw their stomachs up out of their mouths. Chances are, if you were holding a sea cucumber and it did this, you would drop it.

Sea cucumbers are actually a prized food source in some parts of the world.

Directions for this Lesson: Answer the practice questions and then watch the video to learn more about this amazing group of invertebrates.

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