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Last week, while snorkeling at the Kaneohe Bay Sand Bar, my friends and I discovered hundreds, maybe thousands, of nickle-size holes dug in the fine sand.

My suspicion was that the diggers were worms. But when I carefully cleared the sand from around a hole, I found a cylindrical burrow but no animal. Later, as I searched for the identity of the hole-makers, this absence of a body was one of my best clues.

The animals that made the holes are acorn worms.

An acorn worm isn't much more than a thin, cylindrical bag with a head and collar resembling an acorn. But just because it's simple doesn't mean it's trivial. Acorn worms are considered evolutionarily advanced because they bear gill slits like sharks and rays.

Gill slits are a big deal because they aren't found in any other invertebrate. Therefore, the humble little acorn worm may be the link between animals with backbones and animals without.

There are about 70 species of acorn worms in the world, but people rarely see any of them. Some build and live in U-shaped burrows; others tend to burrow continuously, parallel to the surface. Still others hide beneath stones or seaweed.

Besides being buried or hidden, these worms are not often seen even when dug up. That's because the creature's body walls are so thin that the weight of the sand inside them bursts the walls when the worm is handled.

Acorn worms are full of sand because they are its vacuum cleaners.

In Hawaii, large acorn worms grow to 1 inch in diameter and 18 inches long and live in U-shaped burrows 30 feet or deeper. There, the big worms swallow large quantities of sand, digest any plant or animal material in it and then pass the sifted sand out behind. These fecal mounds look like coiled ropes of extremely fine sand. At a touch, the piles disperse like ash.

Other acorn worms in Hawaii prefer shallow water, such as that of the Kaneohe Bay Sand Bar. These worms are 1 to 8 inches long and dig straight burrows. After making a hole, the worm just keeps on tunneling, cleaning the sand as it goes along.

Acorn worms have formidable predators. In Hawaii a snail called the livid cone preys almost exclusively on acorn worms. This cone snail stabs the worm with a dartlike, venom-filled tooth that paralyzes the worm. The snail then eats at its leisure.

Another snail with a taste for acorn worms is Gould's auger. This snail can eat an acorn worm twice as long as itself, taking about 15 hours to do so.

Some lazy Gould's augers live in the fecal mounds of large acorn worms.

Each time the worm lifts it rear to defecate, the snail takes a bite. This isn't as bad as it sounds because acorn worms quickly regenerate lost flesh.

When disturbed, Hawaii's acorn worms emit a flash of light that confuses potential predators. But as I learned during my exploration of the Kaneohe Bay Sand Bar, an acorn worm's best defense is hiding.