A marine biologist has discovered a world in a grain of sand.

In this case, the world is a colony of tiny animals called bryozoans that attach themselves to a grain of sand underwater and live out their lives -- feeding, growing, reproducing -- on a single sand grain.

Bryozoans, which constitute a phylum of their own, are sometimes called moss animals because they live most of their lives clustered together in thick patches fastened underwater to solid surfaces such as rocks and boat hulls. Each individual waves its tentacles about to gather food to stuff into its mouth.

The largest bryozoans are no more than a millimeter or two long. The ones on sand grains are much smaller. Some individuals take on specialized shapes and functions. For example, some become broomlike and sweep the colony clean, while others have jawlike structures adapted for defense. Still others form themselves into the walls of brood chambers for the young or the pumping systems that send water through networks of passages penetrating the colony.

The sand-grain colonies were discovered by Judith Winston of the American Museum of Natural History in New York. She and a Danish colleague were looking for bryozoan larvae, which swim freely for a few hours before settling down on a hard surface. They were using a microscope to examine sand from the sea bottom off the Atlantic coast of Florida.

They found larvae, but they also discovered 33 species of bryozoans living on sand grains. Of these, 24 species were known from their colonies on larger objects, and nine were new to science.