Think of them as pet rocks for the new millennium.

Urban areas around the country are suddenly besieged by plastic animals: cows in New York, pigs in Cincinnati, catfish in New Orleans, flamingos in Miami and (of course) buffalos in Buffalo.

Here in Boston, we have cod.

The cause of all of this? Last year, Chicago, pinching an idea that originated in Switzerland, placed decorated cows around the city. It was a gimmick that got a lot of national press. Chicago eventually auctioned off the cows, with $3.5 million in proceeds going to local charities.

This year they're everywhere. In some kind of miracle of coincidences, American cities have simultaneously come up with what they all believe to be an original and unique idea.

In Boston, the idea was the brainchild of Jeffrey Keough of the Massachusetts College of Art and Jill Medvedow of the Institute of Contemporary Art. With funding from Fourth-of-July fireworks maven David Mugar and support from the mayor's office, the two came up  with the Cavalcade of Cod.

They created fish, each one 5 feet, 5 inches in length, big enough to give artists a large, blank canvas on which they can let their imaginations run riot. The cod are fiberglass, by the way, not plastic, and there are two kinds: one is the traditional swimming cod, the other is seated. Sponsors pay $3,500 for the cod itself and another $1,000 to the artist who decorates it. The cod will all be auctioned off on Dec. 8. As in Chicago, the proceeds will go to charity.

The cod are hard to miss. Over 80 have been mounted on pedestals and now dot the streets and sidewalks of downtown, the waterfront and Back Bay.

Dunkin' Donuts, for example, commissioned the ``Chocolate Frosted nCod,'' which looks a lot like a chocolate frosted doughnut, replete with rainbow jimmies. Fancy clothier Louis, Boston sponsored ``Fishion Cod,'' a cod dressed in old newspaper.

Herb Chambers Auto Dealers has sponsored a number of cod, most of them made up to look like cars (the ``Codillac'' and the ``Hot Rod Cod''). Artist Jennifer Schminke has painted red fins on her cod and called it (are you ready?), ``The Ruby Flippers from the Wizard of Cod.''

You get the idea. There are a lot of puns (yes, there is a ``Pahkd Cod'' with a bright orange traffic ticket as well as a coin-encrusted, ``In Cod We Trust''). The cods are sometimes silly, usually entertaining and generally provoke a smile, if not outright laughter.

But are they art?

The cod are controversial. They're frivolous, some say, inappropriate  for our most important public spaces. Public art, the argument runs, should be serious.

Perhaps. That attitude certainly seems in vogue in the committee-driven processes that have been behind some of Boston's  most recent efforts at public art. Unfortunately, this predilection for grimness seems to be giving us a lot of art that is - how does one put this delicately? - not particularly good.

In recent years we have seen the installation of three major pieces of public art: the Holocaust Memorial near Faneuil Hall, the Irish Famine Memorial at Downtown Crossing and the Firefighters Memorial on the Commonwealth Avenue Mall.

Each tries to tackle a serious and worthy subject. But to this eye, at least, their success at doing so is questionable.

The Holocaust Memorial is supposed to be ``evocative and rich in metaphor.'' Yet it undermines its message with steam billowing around it, fake embers glowing red and a series of tall glass pillars that look a lot like the stage setting for the fictional rock band Spinal Tap.

The Firefighters Memorial is an ersatz rip-off of Maya Lin's Vietnam War Memorial in Washington, D.C. And the Irish Famine Memorial is so obvious in its point (starving Irish on one side, well-fed on theother) that viewers feel like they're being hit over the head with a hammer.

All three of these pieces are very earnest and strive to be very important.

The cod, however, are not important. They're whimsical.  Light-hearted and accessible, they are the kind of thing that appeals  to both children and adults. They are humorous while still being thought provoking.

They have one other virtue as well: They're not permanent. The organizers of the cavalcade doubt the fish will be back next year.  Like pet rocks, they're a novelty that is best done once and then abandoned. But Keough, Medvedow and Mugar have done the city a favor: They've taught a button-down place like Boston that, every once in a while, it's OK to have fun.

 Tom Keane writes every Friday for the Herald. He can be reached at tomkeane@tomkeane.com.