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| Chocolate chip starfish|
Starfish or Sea Stars are echninoderms that comprise the taxonomic class Asteroidea. There are many diverse varieties of sea stars having arms ranging from 5 to the sunflower star with 15-26. They are perhaps best known for their star-like shape and their movement. Their tube feet provide a grace and strength that is controlled completely by a water vascular system, which is also used for circulation and respiration.
Sea Stars rely on their Water Vascular System for movement (Tube Feet), respiration (ampullae), and circulation. The Water Vascular System is filled with sea water and is mixed with protein, potassium and amoeboid cells. The Circular Ring Canal is connected to the Radial Canal which extends into each of the five arms of the Sea Star. On either side of the Radial Canal is a short Lateral Canal. Each of the Lateral Canals contains a valve, which than goes into a bulb called an ampulla or a tube foot. (Tube Feet all end with a suction cup-like feature on the bottom). The tube foot is forced to become longer because the ampulla reduces in size forcing the fluid into the tube foot. When the tube foot comes in contact with the surface below it, it suctions to the surface of the object by creating a vacuum. Than the foot's muscular fibers shorten the foot again and forces the fluid back into the ampulla.
The Sea Star has no intestine; instead the food is digested in the stomach by enzymes produced in by the digestive glands. The digestive system consists of a mouth, esophagus, cardiac stomach, pyloric stomachs, rectal saecae, and anus. The food will go into the mouth, than go through a small tract called the esophagus than into the two stomach were food is broken down by the stomach enzymes. The waste is than held in the rectal saecae, for only a short time, and than is excreted through the anus. 
Sea Stars are carnivorous that mostly prey on bivalves that they come across from very slowly moving across the ocean floor or a reef. They can also repair themselves if they get injured. If one of their arms were to get ripped off, they have the ability to reproduce a new arm. If it contains the central part of the body it has the ability to regrow a whole new Sea Star.[Pg.737 Prentice Hall Biology]
Sea Stars have radial symmetry because of their five arms. The Sea Star is held together and protected by its exoskeleton and that is were it gets its hard body. Sea Stars are covered with prickly structures that are sticking out from the skeletal plates. Each of the spines is surrounded by skin gills which function in respiration, along with the pellicallariae. 
Some species of Star Fish have more or fewer arms than the basic five arms. The mouth is located underneath the sea star, which is called the ventral surface. The topside of the sea star, with the spines on it, is called the dorsal side. The opening to the water vascular system is on the ventral surface and is called the madreporite, it filters the water going into the water vascular system. Some Sea Stars have the spiny surface for a means of protection and others are smooth and do not have any. This shows that the species of Sea Stars variety in shape, size, and color.
On each arm there is both a radial canal and an eye spot. The radial canal stretches across the length of each arm. On the radial canal there are tiny teeth called ampullae. On each arm there is also an eye stop that is located at the very end of each arm, but this eye can only see light and dark. This makes it so that it can see and react to movement.
Each Sea Star has its own unique coloring which they use as a camouflage or a warning to both prey or predators. 
The tube feet on Sea Stars are very important they help in both locomotion and feeding. In feeding it will grab onto the shell of the bivalve and prey it open, once it gets it open far enough it will stick it's stomach into the creature and pours out enzymes which partially digest the animal in it's own shell.[Pg. 376 Prentice Hall Biology]
Sea Stars are very important to its habitat. Since they are carnivorous they prey on bivalve controlling how many organisms are in the ocean. This way it will not become over populated or crowed.
But some Sea Stars can harm its habitat, like the Crown-of-Thorns starfish. This sea star has tiny spines all over each of its arms that are poisonous. They will feed mostly on coral and destroying it. The Crown-of-Thorns has been known to leave many places of coral destroyed.
Sea Stars always live in the ocean, but they can be found out of the water attached to rocks during low tide. They live on coral reefs and rocks. Sea stars will be found in both very shallow water or deep water, but mostly in the shallow water on the ocean floor or on coral reefs. They will show up at low tide on many rocks with a lot of other sea stars. Sea stars will also show up under the sand, but only at low tide. (Prentice Hall Biology Pg. 738)
Star Fish reproduce sexually, that is performed outside of the organism. While the females release their eggs the males release sperm into the water not very far from the females hoping that they will fertilize the female's eggs. Because of the Chemical Pheromone small groups of Sea Stars will reproduce around the same time because they can tell that other groups of Sea Stars are reproducing. Right after fertilization the larva will than turn into free swimming larva and is also bilateral. After the free swimming larva swims around the ocean for some time it swims straight to the bottom of the ocean and than it will developed into a radial symmetry organism.[pg. 736 Prentice Hall Biology] The larva, also called Bipinnaria, will feed on their yolk or by catching some plankton and than feeding on it. The crown-of-thorns reproduces the most every year by releasing 100 million eggs.
Sea Stars also reproduce asexually. Asexual reproduction occurs only when they get an arm ripped off that has some of the central body in it. Some sea stars their one of their arms will decide to walk off and leave the sea star; forcing it to perform asexual reproduction because it is the only organism reproducing and helping it reproduce.
When Star Fish reproduce they get into big groups so they can increase the chances. Each sea star is both male and female. Some species of sea stars will brood their young. This means that the male will spawn gametes will the females hold the eggs either their bodies or on the outside of their bodies. They will also attach their eggs to the sea floor; sea stars mainly do this in deep oceans or cold polar areas. All sea stars will perform complete metamorphosis. and they will all form on the bottom of the ocean floor. 
-  By Joanne Tilloston General Biology 2 Laboratory
-  By Wikipedia
-  By Australian Institute of Marine Science
- Prentice Hall Biology Book By Miller and Levine
-  By Jonathan Dale