Patricia Day, age 12, of Lynn, Mass., for her question:

HOW DOES A BARNACLE FORM?

A barnacle is a salt water shellfish that fastens itself to wharf piles, ship bottoms, rocks and even whales. Marine engineers spend much time experimenting to find ways to prevent barnacles from attaching themselves to ships and wharves.

Barnacles pass through three stages of life. When first hatched, they are small and have six pairs of legs and one eye. They can swim from birth. In this stage, they look like young water fleas. In the second stage, they have 12 pairs of legs, two additional eyes and two large feelers. In the third stage they still have 24 legs but they lose their eyes.

Then as adults, barnacles attach themselves to some object for the rest of their lives. A hard, limestone box forms around the barnacle. The box has a lid that can be closed in case of danger.

The barnacle feeds itself by waving its legs through the open lid. Its bristly legs strain out microscopic plants and animals which the barnacle eats. Its legs also bring water with dissolved oxygen into the shell.