the shells of land-molluscs, leaves, twigs, &c., are before long all
buried beneath the accumulated castings of worms, and are thus brought in
a more or less decayed state within reach of the roots of plants. Worms
likewise drag an infinite number of dead leaves and other parts of plants
into their burrows, partly for the sake of plugging them up and partly as
food.
The leaves which are dragged into the burrows as food, after being torn
into the finest shreds, partially digested, and saturated with the
intestinal and urinary secretions, are commingled with much earth. This
earth forms the dark coloured, rich humus which almost everywhere covers
the surface of the land with a fairly well-defined layer or mantle.
Hensen {285} placed two worms in a vessel 18 inches in diameter, which
was filled with sand, on which fallen leaves were strewed; and these were
soon dragged into their burrows to a depth of 3 inches. After about 6
weeks an almost uniform layer of sand, a centimeter (0.4 inch) in
thickness, was converted into humus by having passed through the
alimentary canals of these two worms. It is believed by some persons
that worm-burrows, which often penetrate the ground almost
perpendicularly to a depth of 5 or 6 feet, materially aid in its
drainage; notwithstanding that the viscid castings piled over the mouths
of the burrows prevent or check the rain-water directly entering them.
They allow the air to penetrate deeply into the ground. They also
greatly facilitate the downward passage of roots of moderate size; and
these will be nourished by the humus with which the burrows are lined.
Many seeds owe their germination to having been covered by castings; and
others buried to a considerable depth beneath accumulated castings lie
dormant, until at some future time they are accidentally uncovered and
germinate.
Worms are poorly provided with sense-organs, for they cannot be said to
see, although they can just distinguish between light and darkness; they
are completely deaf, and have only a feeble power of smell; the sense of
touch alone is well developed. They can therefore learn but little about
the outside world, and it is surprising that they should exhibit some
skill in lining their burrows with their castings and with leaves, and in
the case of some species in piling up their castings into tower-like
constructions. But it is far more surprising that they should apparently
exhibit some degrees of intelligence instead of a mere blind instinctive
impulse, in their manner of plugging up the mouths of their burrows.
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