The starfish 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 length 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 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 but when the feet are used in relays the starfish has a speed of 2 inches a minute.
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Have you ever tried to prise apart the shell of a mussel or an oyster? …