“;It's growing!”; one of the girls said. She was watching a wormlike creature she, her sister and mother found while digging in the sand at Kuliouou Beach Park on Maunalua Bay.

As if that wasn't weird enough, one half of the squishy thing was pinkish-purple and plump, its skin stretched tight, while the other half was yellow, wrinkled and flabby. “;I have never in all my years of digging in the sand seen anything like it,”; a reader said in a e-mail.

Fortunately, I have, because this description reminded me of some creature from the simulated reality in “;The Matrix.”; But no, it's just a humble acorn worm.

Or not so humble. These bags of guts and grit, with body walls so thin the worm can rupture if you pick it up, are closely related to us. Sort of. Acorn worms are a class of marine invertebrates with characteristics that suggest an evolutionary link to vertebrate animals.

The world's oceans host about 70 species of acorn worms. Most live on the sea floor in shallow water, but a few inhabit the deep ocean and some thrive around hydrothermal vents. The biggest of them is a worm well suited to sci-fi. Ranging from Brazil to North Carolina, it grows to 8 feet long.

Hawaii's most common acorn worm is Ptychodera flava, which ranges from 1 to 8 inches long. On the Web, go to calphotos.berkeley.edu and type in “;ptychodera flava.”;

Most Hawaii snorkelers and divers know acorn worms exist, not because they see the animals, but because they see the animals' feces. These dark mounds consist mostly of sand and pile up on the sea floor like ice cream from a soft-serve dispenser. These castings, as they're called, are flimsy. A wave of a hand or fin can collapse the pile and scatter the sand.

Acorn worms might not have much charisma, but they do good work. These industrious creatures clean the sand by sifting out the dead plant and animal material that sank to the bottom. The worms do this in two ways. One is by gulping sand. The worm burrows down head first, swallowing large amounts of sand as it goes and digesting any organic material there. The sand then exits through the anus along with the worm's digestive wastes. That's organic material, too, but less than what the worm took in.

Acorn worms also collect food on their sticky heads. As they move forward, organic particles adhere to the worm's mucus-coated skin. Tiny hairs called cilia beat the particles backward into the mouth, which lies beneath the collar. (The worm's smooth head and collar resemble an acorn.)

This collar does more than protect the mouth. The worm uses muscle contractions to move along, and the collar acts as a sort of anchor, preventing the creature's body from going backward.

Those beating hairs have a second job, too. Located all over the body, they beat not only food to the mouth, but sand over the body. This helps the worm scoot forward.

To most people, worms are about as low as you can go in both slime and spirit. “;I'm in no hurry to dig in the sand again any time soon,”; wrote my reader, who was revolted by her wormy find.

OK, acorn worms are slimy and squishy, but they're harmless and do a terrific job of cleaning up. I might not ever cuddle an acorn worm, but if I find one, I'll give it my thanks.