Starfish

From the looks of it, you might think that a starfish has no eyes. But in fact, it has an eye on the end of each arm. Most starfish have five arms, so they also have five eyes, but some starfish have as many as twenty arms (or more), and as many eyes.

The very simple eyes of a starfish, are unable to form images. Each eye is a tiny spot of red pigment, that is sensitive to light. Nerves run from the pigment spots to the starfish's central nerve ring.

The signals from the eyespots, affect the animal's behavior, enabling it to avoid light that is too bright, and change its light preferences according to the time of day, water chemistry, and other factors. Since a starfish doesn't have a brain, it would not be able to make much use of images, even if its eyes were able to form them.

The underside of the starfish is covered with hundreds of tube feet, which it uses for walking around, for attaching tightly to rocks, and for holding on to prey. To move, each tube foot swings like a leg, lifting up and swinging forward, then planting itself on the ground, and pushing back.

At the tip of each tube foot, is a suction cup. These aren't used when walking on level ground, but can be used when walking up sheer surfaces. With so many little legs, starfish can reach enormous speeds -- 60 feet per hour. Some species, go even faster.

A starfish turns its stomach inside out to eat. When it finds something tasty (like a snail), it extrudes its stomach through a tiny hole. The stomach, is like a transparent bag, made out of thin rubber. It wraps around the prey, completely surrounding it, and the prey is digested.

If the prey is a tough morsel, like a hard shelled mussel, the starfish attaches its suction tube feet to the mussel's shells and simply pulls them apart. It can do this because, it is able to exert a huge, steady force, with a network of hydraulic tubes, that run through its entire body. Once the shells are open, the stomach goes in through the gap, and digests the mussel inside.



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