Some creepy creatures float in old glass jars inside an ordinary-looking building in Beltsville. In one jar there's a 30-foot-long worm that looks like a huge strand of gray fettuccine pasta. Another jar holds a cluster of worms that resembles a big tangle of yellow-brown spaghetti.

Knowing where these pickled parasites came from makes them seem even creepier. The fettuccine-like strand is a tapeworm that was living inside a Minnesota man. The spaghetti-like blob is a bunch of roundworms found dangling from the nostril of a very unfortunate dog from Chevy Chase.

These are two of the more than 20 million specimens kept at the U.S. National Parasite Collection in suburban Maryland. It is open to school groups by appointment.

Parasites are animals that live in, or on, a plant or other animal. Tapeworms, roundworms, flukes, ticks, mites, leeches and fleas are all parasites. So are head lice.

Parasites sip their host's blood or soak up its stomach juices -- sometimes sickening the host, sometimes just wiggling harmlessly inside it.

But there's more to parasites than freaky freeloading, says Eric P. Hoberg, the scientist in charge of Beltsville's parasites. It's the world's largest parasite collection, with about 105,000 lots, or batches. "Some of them are actually quite beautiful!" says Hoberg, who works for the U.S. Department of Agriculture. "And so many of them have a great story behind them."

Hoberg grabs a jar off a shelf to prove his point. It contains tapeworms from crabeater seals, collected in the 1940s by famed explorer Richard E. Byrd on a trip to the Antarctic. On another shelf Hoberg finds worms that were removed from Baltimore fishermen who got the not-so-bright idea to eat raw baitfish.

The 113-year-old collection Hoberg watches over has shelf upon shelf of parasite-filled jars, tubes and glass microscope slides.

Why study parasites?

Parasites can kill animals and people. Deadly parasites aren't common in developed countries such as the United States, but they cause widespread illness in parts of Africa, Asia and South America. For instance, more than 300 million people worldwide have malaria, a parasitic disease passed on through the bite of a certain type of mosquito. Every year malaria kills about a million people, most of them children.

Parasites also are an example of how animals adapt to survive. (See graphic, at right.) They offer clues to their host's diet, behavior and habitat -- helping scientists understand entire ecosystems as far back as prehistoric times.

As many as 1,500 lots of parasites are added to the Beltsville collection each year. When a new species is discovered, Hoberg and the scientists he works with often get to study and name it.

Hoberg also does research. Lately he has been studying a roundworm found in Arctic musk oxen.

Could warming Arctic temperatures be helping them multiply? And could rising temperatures around the globe allow other parasites to spread, possibly causing disease? Those are questions, Hoberg says, that research on parasites might one day answer.

-- Fern Shen

The U.S. National Parasite Collection has more than 20 million specimens, including 100-year-old spiny-headed worms, below, from the intestines of a whale.