Sharon Pankratz, age 11, of Visalia, California, for her question:
How do sand dollars form?
The sand dollar you find on the beach is a sort of bony skeleton. When alive, the modest little sea dweller was covered with a thick skin, plus a forest of tall or short prickles. He is a first cousin of the prickly sea urchin and both of them are second cousins to the starfishes. All of these small creatures form from eggs that are cast adrift in the salty sea.
Sand dollars and sea urchins are classed as echinoids, a name which refers to their prickly skins. The sea urchins are as round as door knobs and some measure 12 inches wide. The sand dollars are pancake style sea urchins, ranging from half an inch to three inches wide. Though these simple little sea dwellers are well armed against most of their enemies, they prefer to live more or less in hiding.
Adult sand dollars squat on the floor of the sea, often partly covered with sand. Life begins anew when the summer sun warms their watery world. This is the mating season. However, there is no dating and the parents do not even meet each other.. Instead, they strew millions of male and female cells into the hazardous sea. Unless pairs of cells meet and unite in a short time, they perish.
When a male and a female cell unite, they form a fertilized egg and the miraculous process of a new life begins. The egg cell divides, then divides again and again. This process is repeated until the budding embryo becomes an infant sand dollar larva. You would never guess that this frisky youngster is related to the stodgy sand dollars squatting down there on the floor of the sea.
He is a born swimmer, What's more, he has a head and a tail end plus two opposite sides. This is most remarkable, for his parents are five sided creatures, related to the five armed starfishes. But for a while, the sand dollar larva lives the life of a two sided animal. Later he gives up his freedom and sinks down to the bottom. There his body goes through a drastic remodeling. Eventually, he becomes a prickly flat five sided echinoid, just like his parents.
For a long time, biologists were puzzled by the way in which a sand dollar forms. Now they suspect that his strange life story tells an even stranger story of the past. Perhaps the sand dollar ancestors were swimmers. Biologists suspect that many millions of years ago, the adults resembled the larva sand dollars of modern times. Gradually the family gave up traveling and became stay at homes.
This theory is logical. We know that many animals change their habits and even their life style in order to survive. We know that life in the hungry sea is very risky and only a very small number of sand dollar larvae survive long enough to become adults. It seems reasonable that their remote ancestors gave up swimming in order to survive.