It does not,
however, follow that these worms do not draw leaves into their
burrows during some other season of the year, at which time they
would not build up their towers.
From the several foregoing cases, it can hardly be doubted that
worms swallow earth, not only for the sake of making their burrows,
but for obtaining food. Hensen, however, concludes from his
analyses of mould that worms probably could not live on ordinary
vegetable mould, though he admits that they might be nourished to
some extent by leaf-mould. {39} But we have seen that worms
eagerly devour raw meat, fat, and dead worms; and ordinary mould
can hardly fail to contain many ova, larvae, and small living or
dead creatures, spores of cryptogamic plants, and micrococci, such
as those which give rise to saltpetre. These various organisms,
together with some cellulose from any leaves and roots not utterly
decayed, might well account for such large quantities of mould
being swallowed by worms. It may be worth while here to recall the
fact that certain species of Utricularia, which grow in damp places
in the tropics, possess bladders beautifully constructed for
catching minute subterranean animals; and these traps would not
have been developed unless many small animals inhabited such soil.
The depth to which worms penetrate, and the construction of their
burrows.
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