Laura Correll, age 13, of Charlotte, N.C., for her question:
HOW DOES A STARFISH MOVE?
Most animals have a right and a left side and advance with the face forward. The starfish has a five sided body, and any of his five arms can act as the leader. His movements are operated by a remarkable system of water power, plus multitudes of miniature muscles and suckers. His strolling speed is about two inches a minute—but when rushed he can dash along at six feet in 60 seconds.
A true fish has two sides, a head end and a tail end. This disqualifies the starfish as a genuine fish, for he has five sides with a head in the middle. The true fish advances by following his nose. The starfish has a choice of five different directions. If he happens to be a species with more than five arms, he may advance in any of 50 directions.
Though any one of the arms may act as leader, some species have a favorite leading arm.. It lifts up and sets down to point the way. The other arms follow along, curving forward as they lift to advance and curving backward as they lag behind. This progress over the sea floor is very graceful. The internal plumbing system that makes it work is downright astonishing.
On the top side, near the center, the starfish has a sieve plate that lets water into a circular tube. This ring canal has valves to the radial canals that run down the center of each arm. Each radial canal sprouts many pairs of small tubes with muscular elbows called ampullas. From here, the plumbing system angles to the underside of the starfish. There it becomes a multitude of pliable pipes called the tube feet—each of which ends in a sticky little sucker.
The ampullas act like valves to let the water into and out of the tube feet. When filled with water, the tube feet are stiff and firm enough to act like armies of tiny legs. As an arm moves forward, the tube feet set down and their suckers grip the ground.
The arm remains securely in place until time comes to take the next step. Then the tiny muscles relax to let some of the water out of the tube feet. With less water pressure they relax their grip. The lagging arm can be lifted and pointed forward to get a grip in the next step forward.
The starfish’s hydraulic system of controlled water power has enormous strength. When he straddles his arms over an oyster, he can exert enough pull to part the two shells—and dine on the meat inside. And you know how hard it is to pry open an oyster.