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the formation of vegetable mould-第13部分

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forwards into this part; which consequently swells and pushes away 
the earth on all sides。  The anterior extremity thus serves as a 
wedge。  It also serves; as we have before seen; for prehension and 
suction; and as a tactile organ。  A worm was placed on loose mould; 
and it buried itself in between two and three minutes。  On another 
occasion four worms disappeared in 15 minutes between the sides of 
the pot and the earth; which had been moderately pressed down。  On 
a third occasion three large worms and a small one were placed on 
loose mould well mixed with fine sand and firmly pressed down; and 
they all disappeared; except the tail of one; in 35 minutes。  On a 
fourth occasion six large worms were placed on argillaceous mud 
mixed with sand firmly pressed down; and they disappeared; except 
the extreme tips of the tails of two of them; in 40 minutes。  In 
none of these cases; did the worms swallow; as far as could be 
seen; any earth。  They generally entered the ground close to the 
sides of the pot。

A pot was next filled with very fine ferruginous sand; which was 
pressed down; well watered; and thus rendered extremely compact。  A 
large worm left on the surface did not succeed in penetrating it 
for some hours; and did not bury itself completely until 25 hrs。 40 
min。 had elapsed。  This was effected by the sand being swallowed; 
as was evident by the large quantity ejected from the vent; long 
before the whole body had disappeared。  Castings of a similar 
nature continued to be ejected from the burrow during the whole of 
the following day。

As doubts have been expressed by some writers whether worms ever 
swallow earth solely for the sake of making their burrows; some 
additional cases may be given。  A mass of fine reddish sand; 23 
inches in thickness; left on the ground for nearly two years; had 
been penetrated in many places by worms; and their castings 
consisted partly of the reddish sand and partly of black earth 
brought up from beneath the mass。  This sand had been dug up from a 
considerable depth; and was of so poor a nature that weeds could 
not grow on it。  It is therefore highly improbable that it should 
have been swallowed by the worms as food。  Again in a field near my 
house the castings frequently consist of almost pure chalk; which 
lies at only a little depth beneath the surface; and here again it 
is very improbable that the chalk should have been swallowed for 
the sake of the very little organic matter which could have 
percolated into it from the poor overlying pasture。  Lastly; a 
casting thrown up through the concrete and decayed mortar between 
the tiles; with which the now ruined aisle of Beaulieu Abbey had 
formerly been paved; was washed; so that the coarser matter alone 
was left。  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。 {37}

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 earth was examined under a high power; and the number of 
spores of various shapes and sizes which it contained was 
astonishingly great; and these crushed in the gizzards of worms may 
largely aid in supporting them。  Whenever castings are thrown up in 
the greatest number; few or no leaves are drawn into the burrows; 
for instance the turf along a hedgerow; about 200 yards in length; 
was daily observed in the autumn during several weeks; and every 
morning many fresh castings were seen; but not a single leaf was 
drawn into these burrows。  These castings from their blackness and 
from the nature of the subsoil could not have been brought up from 
a greater depth than 6 or 8 inches。  On what could these worms have 
subsisted during this whole time; if not on matter contained in the 
black earth?  On the other hand; whenever a large number of leaves 
are drawn into the burrows; the worms seem to subsist chiefly on 
them; for few earth…castings are then ejected on the surface。  This 
difference in the behaviour of worms at different times; perhaps 
explains a statement by Claparede; namely; that triturated leaves 
and earth are always found in distinct parts of their intestines。

Worms sometimes abound in places where they can rarely or never 
obtain dead or living leaves; for instance; beneath the pavement in 
well…swept courtyards; into which leaves are only occasionally 
blown。  My son Horace examined a house; one corner of which had 
subsided; and he found here in the cellar; which was extremely 
damp; many small worm…castings thrown up between the stones with 
which the cellar was paved; and in this case it is improbable that 
the worms could ever have obtained leaves。  Mr。 A。 C。 Horner 
confirms this account; as he has seen castings in the cellars of 
his house; which is an old one at Tonbridge。

But the best evidence; known to me; of worms subsisting for at 
least considerable periods of time solely on the organic matter 
contained in earth; is afforded by some facts communicated to me by 
Dr。 King。  Near Nice large castings abound in extraordinary 
numbers; so that 5 or 6 were often found within the space of a 
square foot。  They consist of fine; pale…coloured earth; containing 
calcareous matter; which after having passed through the bodies of 
worms and being dried; coheres with considerable force。  I have 
reason to believe that these castings had been formed by species of 
Perichaeta; which have been naturalized here from the East。 {38}  
They rise like towers; with their summits often a little broader 
than their bases; sometimes to a height of above 3 and often to a 
height of 2。5 inches。  The tallest of those which were measured was 
3。3 inches in height and 1 inch in diameter。  A small cylindrical 
passage runs up the centre of each tower; through which the worm 
ascends to eject the earth which it has swallowed; and thus to add 
to its height。  A structure of this kind would not allow leaves 
being easily dragged from the surrounding ground into the burrows; 
and Dr。 King; who looked carefully; never saw even a fragment of a 
leaf thus drawn in。  N
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