Do you what are the Echinoderms? Starfishes, sea urchins, sea cucumbers and sea lilies all belong to the phylum echinodermata.
What are the echinoderms
Echinoderm means spiny skinned and all these animals have spiky or leathery small plates. In the arthropods the skeleton is external, en-closing the body in a case, but in the echinoderms it is internal and lies just below the skin.
The spines are extensions from the plates and echinoderms are able to move them slightly as they are hinged at the base. The member of this group that you are probably most familiar with in the star fish.
If you pick one up from the beach it will fell quite hard and rigid and this is because it is probably dead. The plates of the starfishās skeleton are not joined together and so when alive it can bend its body and move its arms easily.
How does a Starfish move?
The starfishes and sea urchins creep slowly around on hundreds of hydraulically operated tube feet. If you flip a living starfish over you will see rows of these tiny feet running the light of each arm. (If you leave the starfish upside down, time it to see how long it takes to turn itself the right way up again.
Some can do it in two minutes, others take over an hour.) Each tube foot is connected to a muscular sac embedded inside the arm of the starfish. All the feet are joined up by a system of tubes which draws in water from the sea.
The whole arrangement is called the water vascular system. The sac contracts and pushes out the foot under pressure.
A suction disc at the end grips the sea floor, the foot contracts forcing the water back into the sac, and the starfish is drawn along a fraction of an inch. Each foot would have little effect on its own bout when the feet are used in relays the starfish has a speed of 2 inches a minute.
How does a Starfish open a Mussel with its feet?
Have you ever tried to prise apart he shell of a mussel or an oyster? It is practically impossible to do by just pulling, so how does the starfish manage it using only its tiny feet the secret is that the starfish again uses its feet in relays. It hunches itself over the tightly closed shell and attaches its tube feet to either side.
By resting some feet and pulling with others the starfish is able to exert a strong pull over a long period, and gradually the bivalve weakens until it gives up the test of strength. Once the valves open a fraction the starfish turns its stomach inside out and pushes it into the shell to digest he contents.
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