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Today I was going to write about Christmas tree worms, but when I opened my zoology textbook, it fell open to a page titled "LEECHES." The section began, "Leeches are predominately a freshwater group found in lakes, streams, ponds, marshes and open ditches. They also occur in all seas."

All seas? We have leeches? Is this one more creepy-crawly thing to watch out for when I'm snorkeling or diving?

I've had only two encounters with leeches in my life, and both were memorable. The first occurred when I was a child in the Midwest, playing in the creek behind my house. I stepped out of the water and found a leech clinging to my big toenail.

Screaming hysterically, I ran to the house for my mother, who also screamed hysterically. Fortunately, my uncle was there. "Girls," he mumbled. Then he plucked the creature from my nail and flushed it down the toilet.

Many years later, I went on a dive trip to the Great Barrier Reef and, after a week of diving, took a tour of Australia's nearby tropical rain forest. A charming Crocodile Dundee-type guide took a group of us on a hike where he caught snakes with his bare hands and found us several tree possums. He also marched us through streams crawling with leeches.

One woman, a dermatologist, spotted a leech on her sandaled foot and shrieked. Checking her toes, she found several more, and soon we were all searching our bodies for leeches.

"Stop that," the guide scolded. "You wouldn't behave like this over mosquito bites, and those are far worse. They itch and carry diseases."

He stepped from the water and looked down at his bare feet. They were covered with leeches. "See?" he said, grinning. "No worries." And off he strode.

I wore sneakers that day and, by some miracle, didn't connect with a leech.

Even so, the image of all those black bloodsuckers on the doctor's and guide's feet is a picture I won't soon forget.

But the man was right. Leeches don't cause a reaction, don't carry diseases and don't stick around long. A hungry leech drinks about 10 times its weight in blood and then drops off.

To suck blood, a leech attaches to a person or animal with both a front and a rear sucker. Then, with saw-toothed jaws, it makes a wound and gorges on blood.

Saw-toothed jaws sound painful, but they aren't because leech saliva contains an anesthetic. This isn't compassion on the leech's part, but rather is a way of stealing blood without getting noticed.

While feeding, leeches also release an anticoagulant and a substance that dilates tiny blood vessels. These two agents cause leech wounds to bleed a bit longer than normal after the creature falls off, but other than that, most people have no reaction to a leech bite.

Conversely, mosquito anticoagulant causes an almost immediate release of histamine, causing an itchy, red bump -- not to mention the concern about malaria, dengue and a host of other diseases mosquitoes can carry.

No matter how harmless leeches may be, however, most of us still would rather not have them sucking our blood. But there's good news: There are no freshwater leeches in the islands, and the marine species around here attach only to turtles, sharks and other fish.

Of the several marine experts I asked, not one had ever heard of a marine leech attaching itself to a human.

Well, that's a relief. Now I can look up Christmas tree worms.