William Feller, age 11, of Millersville, MN for his question:
What is a sea urchin?
Long ago, when people believed in fairyland, an urchin was supposed to be a mischievous elf who often turned himself into a hedgehog. Later, the word urchin was used to describe any small boy bent on mischief. A sea urchin lives in the sea, but he is neither an elf nor a boy. However, he does look somewhat like a small hedgehog.
The prickly little fellow is related to the starfish, which is not really a fish, and the serpent star, which is not really a serpent. He is also kin to the sea cucumber, which is not a plant, and the sea lily, which is not a flower. This misunderstood group of oddities belong in the animal phylum Echinodermata the spiny skinned ones.
The sea urchin lives on the floors of shallow seas and sometimes in tidal pools. On the beach, you are more likely to find a skeleton of a living sea urchin and the two look vastly different. The skeleton is a boney white little globe of calcium.
The inside of the strange little globe is hollow. The outside is adorned with a lacy pattern of buttons and holes. There are five double rows of holes arranged like the sections of an orange. There are also many little bumps and buttons of various sizes. At the top arid bottom of the round skeleton there are two larger holes. One is the mouth hole and you may still see its five little teeth.
The living sea urchin dwells inside this skeleton and the outside of the round globe is covered with muscles skin and prickles. The prickles are set in the little buttons and they can be moved this way and that by muscles in the skin. Sometimes the sea urchin uses his prickles to dig himself a shallow hole in the sea floor, but their main purpose is to keep him from being eaten. They are not his principal means of travel but they help.
There are several varieties of sea urchin, but none of them are more than a few inches wide, including prickles, The short seined urchin looks like an overgrown burr and his spikes are sometimes tipped with white. The needle seined urchin has long needle fine and needle sharp spines which may be poisonous. The slate pencil urchin has thick, clumsy spines which look rather like old time slate pencils.
No matter what his spines are like, the sea urchin travels on tube feet. He has a great number of these slender tubes, each ending in a little sucker. They are put out and withdrawn through those holes in the skeleton. When he wishes to travel, the sea urchine tube feet extend beyond his prickles. They work by hydraulic action, or water power. When filled with water, they are firm and the little suckers stick to the ground. When empty, they relax and the little suckers let go. The sea urchin travels by filling and emptying the tube feet on first one side, then another.