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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