AND WE'RE BACK with part three of our brave new sea hunt:

As we mentioned in the beginning of these Courses, there are lots of marine plants given more familiar "green" names: seaweeds called sea wrack or sea lettuce, sea kale or sea cole, sea rocket and sea purslane and that lovely Irish moss we wrote about not long ago. But some are actually animalsmisnamed for plants, such as the sea truffle and the sea quince, both of which are mollusks.

The sea cucumber -- from the French beche de mer -- is a shell-less, sausage-shaped and altogether rather rude-looking gastropod that can grow anywhere from six inches to six feet long, and can be white, red or brownish-black. (If you believe in sympathetic magic, you should obviously put this on your aphrodisiac list as well as the sea urchin.) It's also called the sea slug, which is easy to understand, but kind of hard on the imagination; and the sea rat, which isn't easy either way. It's covered with little but grudging tentacle-y nodules -- it's vaguely related to both the starfish and the urchin -- and altogether kind of fibrous which, given its shape, isn't surprising.

"It" is really "they," as there are hundreds of varieties of sea cucumber -- some, like the old joke about bananas, all outsides and insides and nothing left worth eating. The kind most often seen in Chinese restaurants, called "trepang," takes more than its share of preparation.

According to the folks at New Fortune in Gaithersburg (in the Walnut Hill Shopping Center on Route 355 just north of Shady Grove Road; 301/548-8886), they come from the grocery very hard and shrunken, having been partly cleaned and boiled after catching to avoid spoiling and then dried. Before any cooking, they have to be soaked overnight or even longer in several changes of water; then they are boiled for nearly as long before they wind up jellyish and quite expanded. Then they have to be cleaned -- "scrubbed hard" emphasizes New Fortune's Fong Tai, both inside and out (remember, it's an animal, however lowly), then combined with other foods.

"By itself, it doesn't have much flavor," says Fong, and after all that boiling, who can blame it? It does, however, release a natural sort of gelatin, so it's most frequently used to make a thick, gelatinous soup or combined with other foods and sauces. It's thought by the Chinese to be very healthful, however, so they often combine it with other vegetables, which is one way it's prepared at New Fortune. There it's also offered stir-fried with abalone, which helps to reconstitute its marine flavor a little.

At the famously busy Fortune in Falls Church (5900 Leesburg Pike; 703/998-8888), it is listed on the menu in similar ways, sauteed with seasonal greens or with abalone and baby mustard greens; but the staffs at both restaurants are willing to make it in any other way you happen to have learned to like it.

This is very like the sort of sea cucumber caught around New Zealand; you can get it already reboiled and vacuum-packed, and Fong Tai recognized it; but frankly, it looks like a gray-fleshed black sponge and tastes about like it, too, unless you know how to deal with it. So don't try this at home. Trust me, I know.

But here's the really good news: There is another and far more flavorful type of sea cucumber which grows in Europe, especially around the rocks and shallows of Spain and France. According to Jose Ramon Andres, executive chef of Cafe Atlantico (405 Eighth St. NW; 202/393-0812), this sort of sea cucumber ("pepino del mar" in Spanish but "espardenyas" in Catalan, referring to a small native shoe) is "the most amazing thing you will ever taste," with a texture that is a cross between fish and squid and with a flavor like scampi (or, to my mind, between rock lobster and a good delicate sea urchin).

Like monkfish, sea cucumber has a sort of Cinderella history: Up until only a few years ago, it was considered a nuisance by Catalonian fisherman, who had to clean the rough-textured and clingy creatures out of their nets when they fouled them. But some smart restaurant along the coast north of Barcelona started serving them, and the news gradually spread. And now, says Andres, they are the most expensive seafood in Spain: At the height of the summer season, a six-ounce portion, which cooks down to nearly nothing, may cost $40 or $50. He himself traveled several hours just to eat some. Even now, pretty much the entire harvest is consumed in the immediate region; the markets in Madrid never get them (unless they buy them from French suppliers who get them off the southern coast).

These espardenyas look rather more like melons than cucumbers, and when you open them up and clean them out, they look like a drawing of a pumpkin, with little lines that curve out from the top and back in at bottom. In fact, it's only those little tiny strips of meat that you eat, which is why the stuff is so expensive.

Even Andres has been having trouble getting Spanish sea cucumber, though he hasn't given up; but recently he's found some from Maine, which are similar, but offer even less meat: From a box of more than 30 pounds, he ended up with less than five pounds of cleaned meat, at a price, not counting labor, of about $30 a pound -- not much less expensive than they are in Spain. He's been experimenting with different recipes, generally as part of the chef's tasting menu; ask when you go in. Believe me, you'll never look at slugs the same way again.