The plan adopted by the Ship-worm is at the same time simple and effective. It feeds upon wood, and gradually eats its way through almost any timber that may be submerged. It does not, however, merely bore its way through the timber, but lines its burrow with a coating of hard, shelly material. Taking this hint, Brunel proceeded in the same fashion to drive his tunnel through the very ungrateful soils which form the bed of the Thames. He built a “shield,” as he called it, of iron, exactly fitting the tunnel, and divided into a number of compartments, each of which could be pushed forwards independently of the others. In each compartment was a single workman, and, as he excavated the earth in front of him, he pushed forward his portion of the shield, while the interior was cased with brickwork, just as a Teredo tunnel is cased with shell. * * * * * ABOVE the Teredo is represented another marine tunnel-maker, as it appears in its burrow. This is the mollusc popularly known as the Piddock, and scientifically as _Pholas dactylus_. It may be found abundantly in all our chalk cliffs, boring its tunnels deeply into the stone, and aiding the sea in its slow, but never-ending task of breaking down the cliffs on one side, while it gradually rears them up on another. As the material into which the Piddock burrows is so hard, there is no need for lining the tunnel, as is done by the Teredo. In this point, too, our engineers follow its example. When their tunnels pass through comparatively soft ground, they line it with masonry, proportioning the thickness of the lining to the looseness of the soil. But, when they come to solid rock, they are content with its strength, and do not trouble themselves about the lining. The mode of action adopted by the Pholas has long been a disputed point, and even now appears to be not quite settled. I think, however, that William Robertson has proved by his experiments that the shell and the siphon are both brought into requisition. The shell perpetually rotates in one direction, and then back again, just like the action of a bradawl, and, by the file-like projections on its surface, rasps away the chalk, converting it into a fine powder. This powder, being of course mixed with water, passes into the interior of the animal, and is ejected through the siphon. There are many species of Pholas which burrow into various substances, even in floating cakes of wax and resin. The same species, too, will burrow into different substances, and it is worthy of notice that those specimens which burrow into soft ground attain a much larger size, and their shells are in better preservation, than those which force their way through hard rock. * * * * *