When worms make their burrows through earth abounding with little stones,
no doubt many will be unavoidably swallowed; but it must not be supposed
that this fact accounts for the frequency with which stones and sand are
found in their gizzards. For beads of glass and fragments of brick and
of hard tiles were scattered over the surface of the earth, in pots in
which worms were kept and had already made their burrows; and very many
of these beads and fragments were picked up and swallowed by the worms,
for they were found in their castings, intestines, and gizzards. They
even swallowed the coarse red dust, formed by the pounding of the tiles.
Nor can it be supposed that they mistook the beads and fragments for
food; for we have seen that their taste is delicate enough to distinguish
between different kinds of leaves. It is therefore manifest that they
swallow hard objects, such as bits of stone, beads of glass and angular
fragments of bricks or tiles for some special purpose; and it can hardly
be doubted that this is to aid their gizzards in crushing and grinding
the earth, which they so largely consume. That such hard objects are not
necessary for crushing leaves, may be inferred from the fact that certain
species, which live in mud or water and feed on dead or living vegetable
matter, but which do not swallow earth, are not provided with gizzards,
{227} and therefore cannot have the power of utilising stones.
During the grinding process, the particles of earth must be rubbed
against one another, and between the stones and the tough lining membrane
of the gizzard. The softer particles will thus suffer some attrition,
and will perhaps even be crushed. This conclusion is supported by the
appearance of freshly ejected castings, for these often reminded me of
the appearance of paint which has just been ground by a workman between
two flat stones. Morren remarks that the intestinal canal is impleta
tenuissima terra, veluti in pulverem redacta. {228a} Perrier also
speaks of l etat de pate excessivement fine a laquelle est reduite la
terre qu ils rejettent, &c. {228b}
As the amount of trituration which the particles of earth undergo in the
gizzards of worms possesses some interest (as we shall hereafter see), I
endeavoured to obtain evidence on this head by carefully examining many
of the fragments which had passed through their alimentary canals. With
worms living in a state of nature, it is of course impossible to know how
much the fragments may have been worn before they were swallowed. It is,
however, clear that worms do not habitually select already rounded
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