Your average zoo does not have a red-brick Byzantine fortress for a reptile and amphibian house. Our National Zoo does, complete with Moorish zigzag striping, fancifully carved crocodiles and stone turtles supporting its columns.

This treasure of a building, a 1930s WPA project, has just been renovated and enhanced with pools, plants, rock ledges, logs and desert sands suited to the occupants.

Last Tuesday Mara Johnson's fifth-grade class from Meyer elementary paid a call. In the Herp Lab teaching area, curator Dale Marcellini asked

"Who's a little bit afraid of snakes?" Seven hands shot up from among 26 youngsters. "Who's real afraid?" Four hands. "I used to be," he said, "but you come to find most are harmless, easy to get a long with, easier than monkeys that bite."

First of the "safe" reptiles to be introduced is a football-sized red-footed tortoise. Marcellini carries it around the circle of youngsters for its shell to be patted and its elephantine legs examined. Next comes Charlie, a wriggling, scaly-backed, five- pound savanna monitor lizard ("Steady, Charles!"). Third (applause here), a very active corn smake, coiling its shining self around the curator's arm -- no ears, no ear slits and a silky, unslimy skin.

For this introduction a number of the youngsters stand up, poised to run. "It's all right to be cautious," Marcellini said to youngsters who choose not to touch. "And be sure to tie a good knot," he added, slipping the snake back into a rice sack, "so your mother will like you."

There is talk of skin-shedding and coldbloodedness, of jaws that unhinge and tongues that flick, and of how everyone present could outrun any animal in the building including the crocodiles, not to mention outthink them. "Reptiles are pretty dumb," says their admiring curator.

In small groups the youngsters tour Line A, a keeper's work area in back of the exhibits. They see a food chopper, floors you could eat off of, vitamins powdered for dusting onto crickets, a syringe for force-feeding listless lizards. And when a back door is opened onto an exhibit, we see what the public looks like to the inmates.

A second hour is spent circling the display area with guide Patsy Lozupone of the Friends of National Zoo. Now midway into a Closer Look program of six half-day Zoo visits, these youngsters look in an unhurried way, laugh, talk about what they see. Reunions are joyful -- for here lolls Charlie under a lamp by his pool, there lumbers a familiar tortoise, and there amongst the cornhusks hides their fascinator snake.

A close look is what it takes to find the leopard gecko whose tiny round scales blend into the sand pebbles.

Terry: "His mother couldn't find him!"

Guide: "No, but then she wouldn't be looking for him! She just lays the egg. He hatches out and he's on his own.")

It takes the closest scrutiny to compare this desert fellow's toes (short, for running in loose sand) and his stumpy tail (for storing fat) with the treeclimber toes and long balance-pole tail of the giant day gecko of the rain forest.

Finding some of the other residents also is a challenge, especially some of the green darters in amongst the greenery. Bryant awards first prize in camouflage to the Gaboon vipers lying all but invisible on dry leaves. Stacey is fascinated by tiny newborns -- Yarrow's spiny lizards -- smaller than a roach, and by the eggs under incubation, tortoise eggs bigger than Ping Pong balls and house gecko eggs like white berries. Terrence laughs at a green tree monitor playing king of the castle.

The appeal of the black tegu lizards is their activity, for these fellows are hiking briskly about, climbing over each othr, over rocks, into a tree; the appeal of the caiman group is their mesmerizing stillness. Here Dwight, who earlier declined to touch tortoise, lizard or snake, is the last to move on.

Everyone lingers by the yellow anaconda, which bunches its muscles and rumples slowly forward, no sidewinding like most snakes but with up-and-down movements.

Question, in front of the pythons: How do you lift one? Answer: With three people. "I like to carry the tail," said Marcellini.

The huge Aldabra land turtles, often seen inexorably mashing a plastic feed container against a wall -- they are fond of mashing -- line up for us as though for inspection.

It's time to go. Angela stops before a tank to say, "there's nobody there!" Even as she points, a large flattish form -- like a rubbery flap -- moves underwater, in the corner. Somebody is there! A soft-shelled African turtle, with skinny head and flippery feet.

"I saw him when I was a child in Langdon Elementary," says the teacher.

"Pigface," Marcellini calls him.

"Looks like a hat more than a pig," says Anthony.

"You mean a pancake," says Donna, catching, over her shoulder, a last look.