Washington — In search of one of the rarest animals on Earth, we scramble down a hill made slippery by new snow, cross a bridge, and walk single file along the bank of Rock Creek. We proceed with caution, even though the wildlife we are stalking is about as large and ferocious as a cocktail shrimp.
''Now get prepared for this,'' says Jaren Horsley, invertebrate specialist and assistant to the director of the National Zoo, ''you know, like you were approaching the Grand Canyon.''
And then, suddenly, we are there: face to face with the lair of the Hay's Spring scud. It is a hole in the side of a hill, about a foot wide, seeping water.
''Exciting, isn't it?'' Mr. Horsley says.
This sinkhole on the outskirts of Washington's National Zoo is the Hay's Spring scud's only known home. A tiny crustacean that looks like a sand flea wearing CB antennas, the scud is one of the smallest and most obscure endangered species listed by the US government.
Perhaps only in America would such a creature be the subject of official concern. The US Endangered Species Act, now celebrating its 10th birthday, is the most far-reaching law in the world for protecting rare plants and animals.
Over its decade of existence the Species Act has sparked its share of controversy. (Remember the snail darter and Tellico Dam?) But many experts now consider the act one of the most successful US environmental laws, a shield for everything from the bald eagle to humbler organisms such as the scud and the turgid-blossom pearly mussel.
''The act has focused national attention on the problem,'' says Ron Lambertson, associate director of the US Fish and Wildlife Service. ''Before it was passed, people used to say, 'Who cares about one more species?' They don't say that anymore.''
The scud's life style has changed little since it was marked ''endangered'' in 1982. Its sinkhole is perhaps more closely watched for signs of dangerous pollution. Scientists, who are about the only people interested in the animal, must now get Interior Department permission to collect scud specimens.
But for a number of other species, the protective arm of the Endangered Species Act has made a difference:
* The peregrine falcon, the speedy F-15 of birds, was extinct east of the Mississippi River by the early 1970s. Since 1975, government wildlife agencies and environmental groups have bred 1,000 peregrines in captivity, then introduced them to the wild.
''It's probably our best success story,'' Mr. Lambertson says. ''There are nesting pairs in all historic peregrine habitat areas. We even have a pair living on the Chesapeake Bay Bridge.''
* Fifteen years ago the American alligator population, a vital part of the Southern marsh grass ecosystem, was rapidly being converted into handbags and flashy shoes. After the alligator was listed as ''endangered,'' and making those handbags became illegal, things turned around.
Now there are so many alligators in the South ''they are becoming sort of a pest,'' admits Tom Troy of the National Audubon Society. ''They come up into people's yards and eat dogs and cats.''
* In 1941 only 15 whooping cranes were left alive in the world. Fish and Wildlife Service recovery efforts, which have included the use of sandhill cranes as foster parents, have since pulled the majestic white bird back from the brink. A bumper crop of eggs last year pushed the whooping crane population to 148.
Overall, 783 species are currently counted by the Fish and Wildlife Service as endangered or threatened. Along with well-known, majestic mammals and birds, the list includes a host of less prominent members: fish (unarmored three-spine Stickleback), plants (Tennessee purple coneflower) - even snails and a few insects.
The Species Act, signed into law by President Nixon on Dec. 28, 1973, gives the government an array of legal tools to help save these threatened organisms. It prohibits the hunting or digging up of endangered species, and requires the United States to develop plans for saving these animals and plants.
Federal money cannot be used for construction projects that rip up crucial habitats. The government can buy land needed to protect threatened species.
By themselves, these tools won't solve the worldwide problem of dwindling animal and plant types. Right now, the National Wildlife Federation says, one species a year is disappearing off the face of the earth.
But the Endangered Species Act has, at the very least, probably ''slowed down the rate of destruction'' in the US, naturalist Paul Ehrlich writes. It has set an example for other nations. For those individual organisms it defends, it has been a lifesaver.
''The act has been amazingly successful in balancing the competing interests of environment and development,'' says Dr. Robert Davison, a conservation expert for the National Wildlife Federation.
Of course, the law's path has not always been strewn with roses. In the late 1970s, it temporarily halted construction of Tellico Dam in Tennessee, which threatened a three-inch fish called the snail darter. The design of Maine's Dickey-Lincoln dam had to be altered to save the furbish lousewort, a variety of snapdragon.
These events caused so much controversy that Congress in 1978 created a Cabinet-level board, nicknamed the ''God Committee,'' with the power to veto projects that threaten species.
The specter, however, of tiny animals with strange names stopping the construction of many dams, shopping centers, and other projects has not materialized. Last year Congress quietly reauthorized the Endangered Species Act for another three years, with little discussion. Even industry representatives, while not thrilled with the law, contented themselves with minor grumbling.
''I don't think anybody's arguing with the intrinsic features of the act,'' says Bob Slocum, a private forest manager for the National Forest Products Association. ''Our concerns focus on how the act is applied.''
If nothing else, the decade-old law has made ''endangered species'' a household phrase in the US. Environmentalists, however, worry that too many people are interested only in glamorous endangered species - eagles, bears, whales.
After all, there aren't many ''Save the Scuds'' bumper stickers on US roads. But nonfurry life forms such as mussels and cactus can be more crucial to our ecology than many mammals. The Interior Department estimates that the extinction of one plant can lead to the disappearance of 30 other species, including insects and higher animals.
And plants can disappear faster than you might think. In 1978, half the US population of one rare cactus was ripped up, stuffed in 15 suitcases, and shipped to a German collector, according to Paul Ehrlich.
Yet when the Reagan administration took office, a new Interior Department assistant secretary told his troops they would now concentrate on mammals when looking for new endangered species to list. Lower organisms were to receive less attention. New listings dropped sharply, to only four in 1981.
That order has since been rescinded. Last year, 24 species were added to the endangered list.
Ironically, the scud - an animal neither furry, nor cute, nor majestic - was one of the first species named ''endangered'' under President Reagan.
''There's a big debate - are you getting down to an absurd level when you protect something like this?'' asks Jaren Horsley of the National Zoo, back at the scud's humble sinkhole. ''Well, we really like it, anyway. It's nice to have your own endangered species in your own backyard.''