John Rowland, age 12, Cuyahoga Falls, Ohio
How does a starfish move?
A little starfish, beached by the tide, is a very sorry sight indeed. But he is not nearly so helpless as he looks. He can crawl in any direction he chooses, he can stand on tiptoe or pull himself through a hole no wider than one of his arms. If he lands on his back, he can flip himself over like a pancake and he can open a stubborn oyster with ease ‑ which is more than we can do. If he loses one or more of his arms he can grow new ones ‑ which is also more than we can do.
Actually, the starfish is one of Nature's small miracles, perfectly fitted to almost any problem. he may meet. His upperside is his back,, studded with rows of little buttons which make it tough for a larger sea dweller to digest him. Near the center, between two arms, is a round hole covered with a porous disc ‑ the sieve plate. Sea water enters this sieve to circulate through the starfish’s stomach, which fills the center and extends down the tapered arms.
It is this circulating water which the starfish uses to walk. The water connects with tiny tube feet, call mullae, along the underside of his body. His mouth is at the canter of the underside and it is usually clamped shut with five little teeth. Grooves radiate from the center along each of the pointed arms. The tube feet are inside these grooves and when the starfish is not in motion they are withdrawn out of sight.
When he decides to go for a stroll, the tube feet are extended like little glass rods, each ending in a sucker. He can control the amount of water in his suckers by letting in more or less through the sieve plate in his back. When full of water, the suckers cling to a solid surface; when empty, they let go.
The starfish uses this hydraulic actions or water powers to walk along. He can lead the way with any one of his five arms. The lead arm stretches forward, its suckers swell up with water and grip the ground. Then side muscles contract and pull the rest of the starfish after it. The little suckers in the lead arm then let go and the arm stretches out to take another step forward.
True, this is not a very fast way to travel. But the starfish is in no hurry. Chances are he is hunting for an oyster, which is his favorite food. His victim cannot give chase because he cannot move at all so the starfish has all the time in the world. He folds himself over the two halves of the oyster like a fist. Then he extends has sucker feet and uses his hydraulic pressure to pry the two halves of the oyster apart. This gives some idea of the strength of his tube feet ‑ not only can they pull him along, but they can open an oyster.