Asteroidea - Sea Star

The name Starfish has been replaced by the less misleading name "sea star." After all a sea star is not a fish. On your journey to the tide pools you are bound to see many of these great creatures.

The class Asteroidea contains about 1,500 living species. The body of a sea star has radial symentry and allows the creature to meet the environment 360 degrees.

Bat star - Patiria miniata

Sea stars usually have five arms but it is not uncommon for you to find one with more or less. If the sea star looses an arm it immediately begins growing a new one. This is why you may find some varieties of sea stars with some long arms and some nubs. However, the ability to reproduce body parts varies greatly among different species. At the tip of each arm there is a small red or orange light sensitive eye spot.

Ochre seastar - Pisaster ochraceous

The body is dorsoventrally flattened. At the end of each arm, on the "oral side," are a series of tubular feet. The sea star moves by a water vascular system as part of each of these tiny tube feet.

The water vascular system of echinoderms is a series of fluid filled canals that utilizes hydraulic pressure for movement. This fluid in the water vascular system is similar to sea water but also contains cells, proteins and some unique ions. This fluid is moved by the action of cilia.

Water enters the water vascular system through the madreporite. The madreporite serves the dual function of pressure reception and ensuring that an equal fluid pressure is maintained between the environment and the inside of the water vascular system. (Brusca and Brusca, 1990)

The tube feet adhere with tiny suction cups to a surface and pull itself along. The tiny tube feet of the sea star are used for location, attachment and respiration. On the "aboral" top side, the sea star has a series of tiny pedicellaria.

The tiny pedicellaria of a sea star looks like a claw when viewed under a microscope. It can remove unwanted particles from the seastar's body. If food happens to land on the sea stars aboral side, it can also use these pedicellaria to transport the food to the arms and then into its mouth.

The pedicellaria is composed of three ossicles: one forming the base and two in the form of jaws. In some, the ossicles forming the jaws can be crossing each other like a pair of blades in scissors.These tiny pinchers can help transfer food that lands on the sea star's back to its' mouth. and also serve to groom the sea star and keep its' aboral side clean.

If you put the sea star upside down on your arm, it will immediately use these pedicellaria to adhere to the hair on your arm. And likewise if you put the sea star right side up on your arm, you will feel the tiny tube feet as it moves.

On the oral (bottom) side the mouth is located in the center of a central disk. Five ambulacral grooves also radiate from the central disk and extend outward through a series of calcareous plates that make up the endoskeleton. The tube feet bear a small suction cup at their tips. They serve in locomotion, attachment and respiration.

The sea star has two stomachs, the pyloric and the cardiac. The cardiac stomach can be inverted to the outside of the sea star thus enabling it to get into small openings. The other stomach, the pyloric, can produce digestive enzymes and store digested food.

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