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While snorkeling with my 10-year-old niece recently, we stopped to look at a rock-boring sea urchin. Each of these creatures, I told her, has a mouth with five teeth on its underside. When the urchin decides to settle down, it scrapes and scrapes on a rock with its teeth until it makes a cave or tunnel. And there the urchin usually spends its life, eating tiny bits of seaweed it catches on its spines.

To show my niece that rock urchins aren't dangerous, we lightly touched the tips of its spines.

Later that day, when she returned from snorkeling with her uncle, I asked her what she saw. This girl knows how to make her aunt happy. "Rock urchins," she said. "Lots and lots of rock urchins."

Fish are OK, but my first love has always been marine invertebrates because these life forms are so different from us. Sea urchins are a good example. They look like creatures from another planet.

I met my first sea urchin in 1981 in a Mexican marine preserve. It was my first time snorkeling, and I was thrilled.

"What did you like best?" my companion asked.

"I liked those weird black plants."

"What plants?"

"The round ones."

He looked puzzled.

"You know, the ones that look like pincushions."

He laughed. "Those aren't plants -- they're animals."

I thought he was joking, but he soon convinced me that those black, cactuslike things were creatures called sea urchins. But all he knew about them was that it's bad to step on one.

A couple of years later, we moved to Hawaii and learned a lot about these creatures the Hawaiians call wana (pronounced VAH-nah).

In science talk, sea urchins are known as echinoids after their class name, Echinoidea. This word means "hedgehog" in Greek and refers to the moveable spines that cover most sea urchin bodies.

Ocean-goers know the spines of some sea urchins all too well because they're sharp and can easily puncture skin.

Wana move around while they graze, but they aren't out to get you. Their spines, however, are a good defense because once you step on one, you're careful to never do it again.

The accepted treatment for wana punctures is to break off the spines sticking out from the skin and leave the rest alone. This isn't a popular remedy because people want something done about this painful, visible injury. But wana spines are brittle and barbed and can't be removed without damaging the surrounding tissue.

Wana spines that are stuck in human tissue eventually dissolve on their own. The black wana bear a dark pigment and can "tattoo" the skin. Because of this, it often seems like the spines remain in your hand or foot a lot longer than they actually do. Your body eventually absorbs this harmless dye.

If a wana spine is in a joint or looks like it's getting infected, see a doctor.

Not all sea urchins have sharp spines. The one called slate pencil has red, paddlelike blades. Another, the tough, wave-resistant shingle urchin, has broad spines that overlap like shingles.

Sea urchins eat drifting algae or scrape it off rocks. They also catch drifting animals with pincers that, when alerted by an intruder, stand up from the surface of the urchin's body. These tiny jaws-on-stalks prevent animals like barnacles and corals from making their home on the urchin's back. They also provide another food source.

The list of facts about sea urchins is so long, I will never know everything about them. But I do know that those round, spiny things in the ocean aren't plants. They're space aliens.