Before the anatomy of this destructive creature had been carefully
examined, there was as much difficulty in guessing at the means it
employed in penetrating the solid timbers in which it is found, as
there is with regard to the operations of the Pholas; but subsequent
observation shows that the hard parts, of which the mouth is formed,
are fully equal to the task.
[Illustration: _Teredo navalis._]
Perhaps we may say, with propriety, that this is the only species of
the shell-bearing tribes that is decidedly injurious to mankind. The
animal of the Teredo is a long worm-shaped creature, dwelling in a
tube of a shelly substance, which it forms for itself in its progress
through the wood; the small pointed shell-like pieces, to the right
in the engraving, form the jaws of the animal. With the assistance of
these it cuts its way into the timber, and, at the same time, lines the
excavation it is making with a shelly substance, which is gradually
formed into a tube, the animal occupying that part which is most
deeply sunken in the timber; in directing its course it generally
excavates in the direction of the grain of the wood, but in some
instances it crosses this grain.
In Holland a great part of the country is below the level of high
water, and, to prevent the irruption of the sea, immense dykes have
been formed along the coast; these are framed, on the sea-side, of
large masses of sand, while to the landward they are strengthened by
means of strong piles driven into the ground and wattled together.
These piles were once discovered pierced in all directions by this
destructive worm, to such an extent as to endanger their safety, and
had it not been for a timely discovery of the mischief, immense tracts
of country would have been laid under water, and irretrievably lost.