1. THE PIDDOCK.2. AND 3. THE LITTLE PIDDOCK.
PLATE XVIII
THE SHIP-WORM (1 and 2)
This creature certainly does not look in the very least like a mollusc; and I do not think that anybody who had never seen it before would ever guess that it is really quite a near relation of the piddocks. It looks much more like a kind of worm, for it has a soft round body no larger than an ordinary drawing pencil, though it is often as much as ten or even twelve inches in length. But if you were to look at the head end of its body you would see its bivalve shells, though they are so very small that they might easily be mistaken for jaws. And these would show you that the animal is really a shell-bearing mollusc.
The shipworm is a most mischievous creature, for instead of burrowing into chalk or limestone rocks, like the piddocks, it bores into timber, such as the hulls of ships, and the posts which support jetties and piers. Very often it cuts away more than half the wood in a great beam, leaving only the thinnest walls between its tunnels. And as it works along it lines these tunnels with a curious shelly substance, which strengthens them and prevents them from breaking down.
By burrowing into timber in this way the shipworm often does most terrible damage. But it seems to dislike the taste of iron rust very much indeed. So when a beam of timber has to be protected from its attacks, a number of iron nails with very broad, flat heads are driven into the surface, with only the space of an inch or two between them. The salt-water acts upon these very quickly, and the result is that the whole of the beam is very soon covered over with a thin coating of rust, so that no shipworm will attempt to touch it.