Biology: Echinoderms

The "Spiny Skinned" Animals

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An echinoderm is a radially symmetrical invertebrate that lives on the ocean floor. Echinoderm means "spiny skinned".

The skin of most of these animals is supported by a spiny internal skeleton, or endoskeleton, made of plates that contain calcium.
Adult echinoderms have a unique kind of radial symmetry in which body parts, usually in multiples of five, are arranged like spokes on a wheel.

Have an  internal fluid system called a water vascular system.
Consists of fluid-filled tubes within the echinoderm's body.

Echinoderms do not have a head end where sense organs and nerve tissue are found.
They are adapted to respond to food, mates, or predators coming from any direction.

Most echinoderms are either male or female. Eggs are usually fertilized right in the seawater, after the female releases her eggs and the male releases his sperm. The fertilized eggs develop into tiny, swimming larvae that eventually undergo metamorphosis and become adult echinoderms.

Sea Stars

Source: National Geographic Kids

Sea stars are predators that eat mollusks, crabs, and even other echinoderms.

The sea stars grasps a clam with all five arms. Then it pulls on the tightly closed shells with its tube feet. When the shells open, the sea star forces its stomach out through its mouth and into the opening between the clam's shells. Digestive chemicals break down the clam's tissues, and the sea star sucks the partially digested body of its prey.

If a sea star loses an arm, it can grow a replacement. A few species of sea stars can even grow a whole animal from a single arm.

Some sea stars reproduce by splitting into many parts.