Susie Adams, age 10, of Winston Salem, N.C., for her question:
HOW CAN WE TELL MALE FROM FEMALE EARTHWORMS?
This is impossible because every earthworm is both male and female. There are other simple animals of this sort, and in some cases one alone can produce offspring. This is not so in the world of worms. Two parents must mate to produce batches of baby worms but there is no difference between the mother and father worms.
A pair of male and female rabbits must mate to produce baby bunnies, and this familiar system prevails among the higher animals. In the world of earthworms, things are different. Each parent produces male and female cells in different sections of its body. Each has a store of sperm and egg cells, so any pair can mate to multiply.
Mating usually occurs on a moist evening and takes several hours. The parents entwine to swap a supply of sperm cells. These are stored separately from the egg cells. Fertilization is helped along by the clitellum, a puffy pink cuff around the body of each parent.
The clitellum oozes slippery mucus and slides forward, carrying sperm cells to fertilize a few cells in the egg pouch. The embryos begin to develop inside a cocoon. This slides forward and slips over the worm's head. The cocoon snaps shut, and a pea size egg case is left alone on the moist ground.
After mating, the parents go their separate ways. Later, each will produce a number of egg sacs until all their stored eggs are fertilized and packaged. Each egg sac contains several embryos, though usually only one or two survive.
Development inside the egg sac depends a lot on the weather and may take from two weeks to five months. The babes are miniature copies of their parents, about one sixth of an inch long. They crawl around in search of decaying vegetation and become mature after six to 18 months. With luck the survivors may live about six years, and captive earthworms often live 10 years.
Though the adults never know their own offspring, each parent is both mother and father. It provides female cells for its own eggs and sperm cells for the eggs laid by its partner. However, since the offspring never know their parents, they do not have to solve these highly complex family relationships.