Wally Bodge, age 11, of Belmont, N,C., for his question:
What exactly are sea urchins?
He belongs to the sea, and the term "urchin" refers to his prickles. Sometimes his remains wash up on a beach and all we see is a crusty hollow ball, adorned with neat rows of pits and buttons. His prickles have gone: so have the tough skin and the soft insides that belonged to the living sea urchin. All that remains is the limy skeleton he wore under his prick¬ly skin.
Most well developed animals have a left and right side and four limbs of some kind. The sea urchin belongs to a group of five sided animals. Ages ago when the world was young, they separated from the main stream of animals and developed their unusual body design.
This phylum of echinoderms, or spiny skinned creatures. includes the starfish, which is not a fish; the sea cucumber, which is not a vegetable; and the sea lily, which is not a flower.
The 7S0 known species of sea urchins and sand dollars are echinoids of the class Echinoidea. They are many colors and ranging from a couple of inches to more than a foot wide. But under each prickly round ball there is a body based on a five sided design.
This pattern can be traced on a sea urchin's skeleton. The rows of bumpy pits on the outside of the crusty hollow ball are arranged like five orange sections. In the living sea urchin, the limy skeleton was clothed with a tough, muscular skin that was embedded with his small forest of prickly spikes. The soft organs and systems of tubes inside his spherical skeleton had the same five sided design.
The living sea urchin pokes thin tubes through the tiny pits in his skeleton. They are called tube feet, because he uses them for walking. But he also uses them as feelers and also as fingers to stuff morsels of food into his little round mouth. Naturally the main duty of the prick¬led is to discourage hungry enemies. But when needed, his muscular skin moves them back and forth to help the tube feet when the sea urchin walking along the sandy sea floor.
The points of his five neat orange sections come together in the centers of what are normally the upper and lower parts of his remarkable body. On the lower side, on which he normally walks, they center around the small round mouth, which is set with five very tough little teeth.
Starfish and certain other echinoderms are meat eaters, preying on clams and oysters. The sea urchins eat scraps of decaying plants and ani¬mals. They are useful scavengers, because they help to keep the sea clean. However, they also are very fond of fresh seaweed. In some parts of the world they are regarded as destructive pests, because they devour large helpings from the kelp beds. In these regions the kelp seaweed is harves¬ted to make fertilizers and also food for humans.