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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 length oft 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 ends
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 minutes.
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