Tracy Allen, age 11, of San Diego, California, for her question:

How do starfish reproduce?

Chances are, a starfish never meets his parents and even the parents do not have to meet each other. These strange little creatures have no family life to speak of, yet under favorable conditions they manage to multiply at a great rate. The sex cells are cast adrift to meet and fertilize each other in the water. Most of the developing embryos are devoured by the hungry sea, but enough of the infant starfish survive to grow up and repeat their odd reproductive cycle.

Many of these fascinating creatures are shaped like five pointed stars, though none of them are true fishes. They are classified as echinoderms along with sea lilies and sea cucumbers, sea urchins and other spiny skinned creatures that belong to the sea. The bodies of most animals are built on a two sided plan. The body plan of the echinoderms is patterned on five or multiples of five. There are male and female starfish and the reproductive cells are in what you might call the arm pits.

The common five armed starfish share shallow waters with oysters and other edible shellfish. Where there is one, there are bound to be hundreds more both male and female. In summer, when the water reaches just the right temperature, the adults launch their reproductive cells. The sperm and egg cells enter the water through tiny pores between their arms. This is the one and only contribution that the parents do make to what we would call family life.

During the brief busy season, countless numbers of male and female cells becloud the water. Each egg cell drifts helplessly around and soon perishes unless fertilized. The smaller sperm cell has a tail like flagellum and swims frantically around to find an egg cell. When this occurs, the two single cells unite and form a fertilized egg called a zygote. The dynamic forces of reproduction immediately spark a fantastic series of changes.

The zygote divides into two cells, the two cells divide into four, then into eight, 16 and so on. Before the first day is done, this amazing unit of life is a blastula, a hollow ball with two layers of cells. The cells on the outside are ciliated with tiny hairs and the blastula waves them to travel through the water. As the embryo continues to develop, the hollow inside is remodeled to form the typical hollow stomach of the echinoderm. Eventually, the skin becomes the tough body of the starfish.

But this doesn't happen until his days as a free swimming larva are over. The existing echinoderms are the only animals with a five sided body plan. But perhaps their ancestors were everyday two sided creatures. At any rate, this is what the development of larva seems to suggest. During this phase of life, the developing starfish is a two sided creature. The five side pattern emerges only after the larva settles down to a life on the ocean floor.