It was chilly, with leaves blowing into the water off the pier at the Chesapeake Bay Center for Environmental Activities. A splendidly unflappable white-haired volunteer, who knew youngsters and oysters and seemed to get along well with both, was wetly and actively introducing Ms. Langley's fifth grade, from Neabsco Elementary in Dale City, to the life of an estuary.

For this kind of field trip, activities at five stations along the pier involve a lot of hauling up lines a bumper rail around the pier's edge means that no one has to be held onto by shirttails), some sloshing, some seining, considerable close observation and note-tak ing and a tastetest: Children compare the estuary water they lick off their fingers with the contents of bottles marked seawater, fresh, salt and brackish.

The most popular station is No. 5, where pairs of boys and girls in armpit-high rubber waders head out into the water with a swine net between them to drag the shallows. Advice reaches them from the shore -- to hold the poles like a shovel, to keep the net on the bottom. The young call back bulletins, mostly on leaks in their footgear. The catch, on their return, picked from among weeds and leaves, is deposited in a bucket where the tiny swimmers are matched up with pictures in the Estuary Guide -- of naked goby and blenny silversides.

Lead lines go over the pier's side to measure depth; baited lines to catch crabs; a secchi disc to see how deep the light penetrates. At station No.2 youngsters drag a bot tom grabber; they drag it and haul it up, hand over hand. Each child feels the gritty muck as it's dumped out, then strains for descriptive words ("something beyond 'yucky,' please") to fill the worksheet.

Station No. 3 is the wettest, where it takes six bucketsfull of water sloshed through a plankton net to supply one petri dish of water for the microscope. Here the search is for microscopic floaters. In the course of the morning Neabsco Elementary captures one -- a specksize copapod, which scoots in a jerky manner -- a challenge to the young artists attempting its portrait. On their knees, heads down to papers on the dock, they draw lobster-monsters.

By noon these boys and girls have held crabs and touched the tops of jelly fish. They've handled muddy oysters and scrutinized in the oyster tray other members of the watery community -- barnacles, mud worms, fuzzy colonies of bryozoans that look like algae. And they've called goodbys from the pier to a scientist who's heading out in an open boat to her sampling station in the Bay where she will mess about in the wet, much as they have done -- collecting specimens. WHERE THE LAND MEETS THE SEA

Estuary Activity at Chesapeake Bay Center for Environmental Studies. Daily throughout the year for as many as 35 children, kindergarten through sixth grade. Also: Forest and Old Fields (fourth through sixth grades) and Insects and Spiders (K-3rd Grade). For reservations, call 261-4190.