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. {97}
If earth were swallowed only when worms deepened their burrows or made
new ones, castings would be thrown up only occasionally; but in many
places fresh castings may be seen every morning, and the amount of earth
ejected from the same burrow on successive days is large. Yet worms do
not burrow to a great depth, except when the weather is very dry or
intensely cold. On my lawn the black vegetable mould or humus is only
about 5 inches in thickness, and overlies light-coloured or reddish
clayey soil: now when castings are thrown up in the greatest profusion,
only a small proportion are light coloured, and it is incredible that the
worms should daily make fresh burrows in every direction in the thin
superficial layer of dark-coloured mould, unless they obtained nutriment
of some kind from it. I have observed a strictly analogous case in a
field near my house where bright red clay lay close beneath the surface.
Again on one part of the Downs near Winchester the vegetable mould
overlying the chalk was found to be only from 3 to 4 inches in thickness;
and the many castings here ejected were as black as ink and did not
effervesce with acids; so that the worms must have confined themselves to
this thin superficial layer of mould, of which large quantities were
daily swallowed. In another place at no great distance the castings were
white; and why the worms should have burrowed into the chalk in some
places and not in others, I am unable to conjecture.
Two great piles of leaves had been left to decay in my grounds, and
months after their removal, the bare surface, several yards in diameter,
was so thickly covered during several months with castings that they
formed an almost continuous layer; and the large number of worms which
lived here must have subsisted during these months on nutritious matter
contained in the black earth.
The lowest layer from another pile of decayed leaves mixed with some
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