When is a worm not a worm?

Answer #1: When it's a clam.

I grew up hearing about shipworms, the dreaded teredo worms that ate the logs that supported our wharves and float houses, and made our boats leaky. I'd seen their long holes in driftwood, and their white tubes cemented into the holes; long, snaking tubes for a tubeworm. Not until tonight did I learn that I had been completely mistaken about them all these years.

A teredo is a clam. An odd clam, to be sure; it's long (up to 2 feet) and snaky, like a worm, and the body's only protection is the calcareous tube it coats the tunnel with. But it does have the clam shell. On it's "head", or at least, the leading end. And it uses the double shell as teeth, to chew its way through the wood.

Its two shells, enclosing only the front end of the body, function as a tool, rather than a protective covering; their ridged and roughened surfaces are used for boring. (From bruceruiz.net)

Like other clams, it has two siphons. They are at the "tail" end. And the leading end is the foot. (It's a bivalve monopod.)

Drawing from bruceruiz.net

I was looking teredos up because Laurie took an interesting photo of shipworm-eaten wood today. I wanted to check my spelling, and ended up wandering around shipworm pages. Fascinating!

Like other clams, the teredo is a filter feeder, straining plankton from the water. But it also eats some of the wood it burrows in, with the aid of the bacteria it houses in special structures called "bacteriocytes". These bacteria produce enzymes that digest the cellulose of the wood. (Nobanis) They also "fix" nitrogen, and produce necessary amino acids.

(In this way, they are similar to termites, which also rely on protozoans and bacteria to digest the cellulose in wood. And similar, also, to us humans, whose intestines are home to many bacteria that work with us to digest our food. Not wood, though. Our bacteria can't handle cellulose.)

Laurie's photo. A rain- and wave-worn section of shipwormy log. Our local teredo is Bankia setacea.

Or there is Answer #2: When it's a crustacean.

The gribble, or gribble worm, as it is often called, also drills through wood found underwater,  like pilings, float logs, and boats. It is not a worm, but a crustacean, a marine isopod.

Limnoriidae are second only to theTeredinidae in the amount of destruction caused to marine timber structures such as jetties and piers. ... Gribbles bore the surface layers of wood, unlike the Teredinidae which attack more deeply. Their burrows are 1–2 mm diameter, may be several centimetres long, and have the burrow’s roof punctured with a series of smaller ventilation holes. Attacked wood can become spongy and friable. (From Wikipedia)

These critters seem to make their own enzymes to digest cellulose, but at least one species does find a use for symbionts:

L. tripunctata is unusually tolerant ofcreosote, a preservative often used to protect timber piles, due to symbiosis with creosote-degrading bacteria. 

Log showing gribble damage, and a teredo tube. Oyster Bay.

Come to think of it; I've never seen a live gribble or teredo; just their holes. I must remedy that.