This consisted of grains of quartz, micaceous slate,
other rocks, and bricks or tiles, many of them from 1/20 to 1/10
inch in diameter. No one will suppose that these grains were
swallowed as food, yet they formed more than half of the casting,
for they weighed 19 grains, the whole casting having weighed 33
grains. Whenever a worm burrows to a depth of some feet in
undisturbed compact ground, it must form its passage by swallowing
the earth; for it is incredible that the ground could yield on all
sides to the pressure of the pharynx when pushed forwards within
the worm's body.
That worms swallow a larger quantity of earth for the sake of
extracting any nutritious matter which it may contain than for
making their burrows, appears to me certain. But as this old
belief has been doubted by so high an authority as Claparede,
evidence in its favour must be given in some detail. There is no a
priori improbability in such a belief, for besides other annelids,
especially the Arenicola marina, which throws up such a profusion
of castings on our tidal sands, and which it is believed thus
subsists, there are animals belonging to the most distinct classes,
which do not burrow, but habitually swallow large quantities of
sand; namely, the molluscan Onchidium and many Echinoderms.
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