The Formation of Vegetable Mould through the action of worms with observations of their habits
by
Charles Darwin
Part 1 out of 4
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THE FORMATION OF VEGETABLE MOULD
THROUGH THE ACTION OF WORMS
WITH OBSERVATIONS ON THEIR HABITS.
by Charles Darwin
INTRODUCTION.
The share which worms have taken in the formation of the layer of
vegetable mould, which covers the whole surface of the land in
every moderately humid country, is the subject of the present
volume. This mould is generally of a blackish colour and a few
inches in thickness. In different districts it differs but little
in appearance, although it may rest on various subsoils. The
uniform fineness of the particles of which it is composed is one of
its chief characteristic features; and this may be well observed in
any gravelly country, where a recently-ploughed field immediately
adjoins one which has long remained undisturbed for pasture, and
where the vegetable mould is exposed on the sides of a ditch or
hole. The subject may appear an insignificant one, but we shall
see that it possesses some interest; and the maxim "de minimis non
curat lex," does not apply to science. Even Elie de Beaumont, who
generally undervalues small agencies and their accumulated effects,
remarks: {1} "La couche tres-mince de la terre vegetale est un
monument d'une haute antiquite, et, par le fait de sa permanence,
un objet digne d'occuper le geologue, et capable de lui fournir des
remarques interessantes." Although the superficial layer of
vegetable mould as a whole no doubt is of the highest antiquity,
yet in regard to its permanence, we shall hereafter see reason to
believe that its component particles are in most cases removed at
not a very slow rate, and are replaced by others due to the
disintegration of the underlying materials.
As I was led to keep in my study during many months worms in pots
filled with earth, I became interested in them, and wished to learn
how far they acted consciously, and how much mental power they
displayed. I was the more desirous to learn something on this
head, as few observations of this kind have been made, as far as I
know, on animals so low in the scale of organization and so poorly
provided with sense-organs, as are earth-worms.
In the year 1837, a short paper was read by me before the
Geological Society of London, {2} "On the Formation of Mould," in
which it was shown that small fragments of burnt marl, cinders,
&c., which had been thickly strewed over the surface of several
meadows, were found after a few years lying at the depth of some
inches beneath the turf, but still forming a layer. This apparent
sinking of superficial bodies is due, as was first suggested to me
by Mr. Wedgwood of Maer Hall in Staffordshire, to the large
quantity of fine earth continually brought up to the surface by
worms in the form of castings. These castings are sooner or later
spread out and cover up any object left on the surface. I was thus
led to conclude that all the vegetable mould over the whole country
has passed many times through, and will again pass many times
through, the intestinal canals of worms. Hence the term "animal
mould" would be in some respects more appropriate than that
commonly used of "vegetable mould."
Ten years after the publication of my paper, M. D'Archiac,
evidently influenced by the doctrines of Elie de Beaumont, wrote
about my "singuliere theorie," and objected that it could apply
only to "les prairies basses et humides;" and that "les terres
labourees, les bois, les prairies elevees, n'apportent aucune
preuve a l'appui de cette maniere de voir." {3} But M. D'Archiac
must have thus argued from inner consciousness and not from
observation, for worms abound to an extraordinary degree in kitchen
gardens where the soil is continually worked, though in such loose
soil they generally deposit their castings in any open cavities or
within their old burrows instead of on the surface. Hensen
estimates that there are about twice as many worms in gardens as in
corn-fields. {4} With respect to "prairies elevees," I do not know
how it may be in France, but nowhere in England have I seen the
ground so thickly covered with castings as on commons, at a height
of several hundred feet above the sea. In woods again, if the
loose leaves in autumn are removed, the whole surface will be found
strewed with castings. Dr. King, the superintendent of the Botanic
Garden in Calcutta, to whose kindness I am indebted for many
observations on earth-worms, informs me that he found, near Nancy
in France, the bottom of the State forests covered over many acres
with a spongy layer, composed of dead leaves and innumerable worm-
castings. He there heard the Professor of "Amenagement des Forets"
lecturing to his pupils, and pointing out this case as a "beautiful
example of the natural cultivation of the soil; for year after year
the thrown-up castings cover the dead leaves; the result being a
rich humus of great thickness."
In the year 1869, Mr. Fish {5} rejected my conclusions with respect
to the part which worms have played in the formation of vegetable
mould, merely on account of their assumed incapacity to do so much
work. He remarks that "considering their weakness and their size,
the work they are represented to have accomplished is stupendous."
Here we have an instance of that inability to sum up the effects of
a continually recurrent cause, which has often retarded the
progress of science, as formerly in the case of geology, and more
recently in that of the principle of evolution.
Although these several objections seemed to me to have no weight,
yet I resolved to make more observations of the same kind as those
published, and to attack the problem on another side; namely, to
weigh all the castings thrown up within a given time in a measured
space, instead of ascertaining the rate at which objects left on
the surface were buried by worms. But some of my observations have
been rendered almost superfluous by an admirable paper by Hensen,
already alluded to, which appeared in 1877. {6} Before entering on
details with respect to the castings, it will be advisable to give
some account of the habits of worms from my own observations and
from those of other naturalists.
[FIRST EDITION, October 10th, 1881.]
CHAPTER I--HABITS OF WORMS.
Nature of the sites inhabited--Can live long under water--
Nocturnal--Wander about at night--Often lie close to the mouths of
their burrows, and are thus destroyed in large numbers by birds--
Structure--Do not possess eyes, but can distinguish between light
and darkness--Retreat rapidly when brightly illuminated, not by a
reflex action--Power of attention--Sensitive to heat and cold--
Completely deaf--Sensitive to vibrations and to touch--Feeble power
of smell--Taste--Mental qualities--Nature of food--Omnivorous--
Digestion--Leaves before being swallowed, moistened with a fluid of
the nature of the pancreatic secretion--Extra-stomachal digestion--
Calciferous glands, structure of--Calcareous concretions formed in
the anterior pair of glands--The calcareous matter primarily an
excretion, but secondarily serves to neutralise the acids generated
during the digestive process.
Earth-worms are distributed throughout the world under the form of
a few genera, which externally are closely similar to one another.
The British species of Lumbricus have never been carefully
monographed; but we may judge of their probable number from those
inhabiting neighbouring countries. In Scandinavia there are eight
species, according to Eisen; {7} but two of these rarely burrow in
the ground, and one inhabits very wet places or even lives under
the water. We are here concerned only with the kinds which bring
up earth to the surface in the form of castings. Hoffmeister says
that the species in Germany are not well known, but gives the same
number as Eisen, together with some strongly marked varieties. {8}
Earth-worms abound in England in many different stations. Their
castings may be seen in extraordinary numbers on commons and chalk-
downs, so as almost to cover the whole surface, where the soil is
poor and the grass short and thin. But they are almost or quite as
numerous in some of the London parks, where the grass grows well
and the soil appears rich. Even on the same field worms are much
more frequent in some places than in others, without any visible
difference in the nature of the soil. They abound in paved court-
yards close to houses; and an instance will be given in which they
had burrowed through the floor of a very damp cellar. I have seen
worms in black peat in a boggy field; but they are extremely rare,
or quite absent in the drier, brown, fibrous peat, which is so much
valued by gardeners. On dry, sandy or gravelly tracks, where heath
with some gorse, ferns, coarse grass, moss and lichens alone grow,
hardly any worms can be found. But in many parts of England,
wherever a path crosses a heath, its surface becomes covered with a
fine short sward. Whether this change of vegetation is due to the
taller plants being killed by the occasional trampling of man and
animals, or to the soil being occasionally manured by the droppings
from animals, I do not know. {9} On such grassy paths worm-
castings may often be seen. On a heath in Surrey, which was
carefully examined, there were only a few castings on these paths,
where they were much inclined; but on the more level parts, where a
bed of fine earth had been washed down from the steeper parts and
had accumulated to a thickness of a few inches, worm-castings
abounded. These spots seemed to be overstocked with worms, so that
they had been compelled to spread to a distance of a few feet from
the grassy paths, and here their castings had been thrown up among
the heath; but beyond this limit, not a single casting could be
found. A layer, though a thin one, of fine earth, which probably
long retains some moisture, is in all cases, as I believe,
necessary for their existence; and the mere compression of the soil
appears to be in some degree favourable to them, for they often
abound in old gravel walks, and in foot-paths across fields.
Beneath large trees few castings can be found during certain
seasons of the year, and this is apparently due to the moisture
having been sucked out of the ground by the innumerable roots of
the trees; for such places may be seen covered with castings after
the heavy autumnal rains. Although most coppices and woods support
many worms, yet in a forest of tall and ancient beech-trees in
Knole Park, where the ground beneath was bare of all vegetation,
not a single casting could be found over wide spaces, even during
the autumn. Nevertheless, castings were abundant on some grass-
covered glades and indentations which penetrated this forest. On
the mountains of North Wales and on the Alps, worms, as I have been
informed, are in most places rare; and this may perhaps be due to
the close proximity of the subjacent rocks, into which worms cannot
burrow during the winter so as to escape being frozen. Dr.
McIntosh, however, found worm-castings at a height of 1500 feet on
Schiehallion in Scotland. They are numerous on some hills near
Turin at from 2000 to 3000 feet above the sea, and at a great
altitude on the Nilgiri Mountains in South India and on the
Himalaya.
Earth-worms must be considered as terrestrial animals, though they
are still in one sense semi-aquatic, like the other members of the
great class of annelids to which they belong. M. Perrier found
that their exposure to the dry air of a room for only a single
night was fatal to them. On the other hand he kept several large
worms alive for nearly four months, completely submerged in water.
{10} During the summer when the ground is dry, they penetrate to a
considerable depth and cease to work, as they do during the winter
when the ground is frozen. Worms are nocturnal in their habits,
and at night may be seen crawling about in large numbers, but
usually with their tails still inserted in their burrows. By the
expansion of this part of their bodies, and with the help of the
short, slightly reflexed bristles, with which their bodies are
armed, they hold so fast that they can seldom be dragged out of the
ground without being torn into pieces. {11} During the day they
remain in their burrows, except at the pairing season, when those
which inhabit adjoining burrows expose the greater part of their
bodies for an hour or two in the early morning. Sick individuals,
which are generally affected by the parasitic larvae of a fly, must
also be excepted, as they wander about during the day and die on
the surface. After heavy rain succeeding dry weather, an
astonishing number of dead worms may sometimes be seen lying on the
ground. Mr. Galton informs me that on one such occasion (March,
1881), the dead worms averaged one for every two and a half paces
in length on a walk in Hyde Park, four paces in width. He counted
no less than 45 dead worms in one place in a length of sixteen
paces. From the facts above given, it is not probable that these
worms could have been drowned, and if they had been drowned they
would have perished in their burrows. I believe that they were
already sick, and that their deaths were merely hastened by the
ground being flooded.
It has often been said that under ordinary circumstances healthy
worms never, or very rarely, completely leave their burrows at
night; but this is an error, as White of Selborne long ago knew.
In the morning, after there has been heavy rain, the film of mud or
of very fine sand over gravel-walks is often plainly marked with
their tracks. I have noticed this from August to May, both months
included, and it probably occurs during the two remaining months of
the year when they are wet. On these occasions, very few dead
worms could anywhere be seen. On January 31, 1881, after a long-
continued and unusually severe frost with much snow, as soon as a
thaw set in, the walks were marked with innumerable tracks. On one
occasion, five tracks were counted crossing a space of only an inch
square. They could sometimes be traced either to or from the
mouths of the burrows in the gravel-walks, for distances between 2
or 3 up to 15 yards. I have never seen two tracks leading to the
same burrow; nor is it likely, from what we shall presently see of
their sense-organs, that a worm could find its way back to its
burrow after having once left it. They apparently leave their
burrows on a voyage of discovery, and thus they find new sites to
inhabit.
Morren states {12} that worms often lie for hours almost motionless
close beneath the mouths of their burrows. I have occasionally
noticed the same fact with worms kept in pots in the house; so that
by looking down into their burrows, their heads could just be seen.
If the ejected earth or rubbish over the burrows be suddenly
removed, the end of the worm's body may very often be seen rapidly
retreating. This habit of lying near the surface leads to their
destruction to an immense extent. Every morning during certain
seasons of the year, the thrushes and blackbirds on all the lawns
throughout the country draw out of their holes an astonishing
number of worms, and this they could not do, unless they lay close
to the surface. It is not probable that worms behave in this
manner for the sake of breathing fresh air, for we have seen that
they can live for a long time under water. I believe that they lie
near the surface for the sake of warmth, especially in the morning;
and we shall hereafter find that they often coat the mouths of
their burrows with leaves, apparently to prevent their bodies from
coming into close contact with the cold damp earth. It is said
that they completely close their burrows during the winter.
Structure.--A few remarks must be made on this subject. The body
of a large worm consists of from 100 to 200 almost cylindrical
rings or segments, each furnished with minute bristles. The
muscular system is well developed. Worms can crawl backwards as
well as forwards, and by the aid of their affixed tails can retreat
with extraordinary rapidity into their burrows. The mouth is
situated at the anterior end of the body, and is provided with a
little projection (lobe or lip, as it has been variously called)
which is used for prehension. Internally, behind the mouth, there
is a strong pharynx, shown in the accompanying diagram (Fig. 1)
which is pushed forwards when the animal eats, and this part
corresponds, according to Perrier, with the protrudable trunk or
proboscis of other annelids. The pharynx leads into the
oesophagus, on each side of which in the lower part there are three
pairs of large glands, which secrete a surprising amount of
carbonate of lime. These calciferous glands are highly remarkable,
for nothing like them is known in any other animal. Their use will
be discussed when we treat of the digestive process. In most of
the species, the oesophagus is enlarged into a crop in front of the
gizzard. This latter organ is lined with a smooth thick chitinous
membrane, and is surrounded by weak longitudinal, but powerful
transverse muscles. Perrier saw these muscles in energetic action;
and, as he remarks, the trituration of the food must be chiefly
effected by this organ, for worms possess no jaws or teeth of any
kind. Grains of sand and small stones, from the 1/20 to a little
more than the 1/10 inch in diameter, may generally be found in
their gizzards and intestines. As it is certain that worms swallow
many little stones, independently of those swallowed while
excavating their burrows, it is probable that they serve, like
mill-stones, to triturate their food. The gizzard opens into the
intestine, which runs in a straight course to the vent at the
posterior end of the body. The intestine presents a remarkable
structure, the typhlosolis, or, as the old anatomists called it, an
intestine within an intestine; and Claparede {13} has shown that
this consists of a deep longitudinal involution of the walls of the
intestine, by which means an extensive absorbent surface is gained.
The circulatory system is well developed. Worms breathe by their
skin, as they do not possess any special respiratory organs. The
two sexes are united in the same individual, but two individuals
pair together. The nervous system is fairly well developed; and
the two almost confluent cerebral ganglia are situated very near to
the anterior end of the body.
Senses.--Worms are destitute of eyes, and at first I thought that
they were quite insensible to light; for those kept in confinement
were repeatedly observed by the aid of a candle, and others out of
doors by the aid of a lantern, yet they were rarely alarmed,
although extremely timid animals. Other persons have found no
difficulty in observing worms at night by the same means. {14}
Hoffmeister, however, states {15} that worms, with the exception of
a few individuals, are extremely sensitive to light; but he admits
that in most cases a certain time is requisite for its action.
These statements led me to watch on many successive nights worms
kept in pots, which were protected from currents of air by means of
glass plates. The pots were approached very gently, in order that
no vibration of the floor should be caused. When under these
circumstances worms were illuminated by a bull's-eye lantern having
slides of dark red and blue glass, which intercepted so much light
that they could be seen only with some difficulty, they were not at
all affected by this amount of light, however long they were
exposed to it. The light, as far as I could judge, was brighter
than that from the full moon. Its colour apparently made no
difference in the result. When they were illuminated by a candle,
or even by a bright paraffin lamp, they were not usually affected
at first. Nor were they when the light was alternately admitted
and shut off. Sometimes, however, they behaved very differently,
for as soon as the light fell on them, they withdrew into their
burrows with almost instantaneous rapidity. This occurred perhaps
once out of a dozen times. When they did not withdraw instantly,
they often raised the anterior tapering ends of their bodies from
the ground, as if their attention was aroused or as if surprise was
felt; or they moved their bodies from side to side as if feeling
for some object. They appeared distressed by the light; but I
doubt whether this was really the case, for on two occasions after
withdrawing slowly, they remained for a long time with their
anterior extremities protruding a little from the mouths of their
burrows, in which position they were ready for instant and complete
withdrawal.
When the light from a candle was concentrated by means of a large
lens on the anterior extremity, they generally withdrew instantly;
but this concentrated light failed to act perhaps once out of half
a dozen trials. The light was on one occasion concentrated on a
worm lying beneath water in a saucer, and it instantly withdrew
into its burrow. In all cases the duration of the light, unless
extremely feeble, made a great difference in the result; for worms
left exposed before a paraffin lamp or a candle invariably
retreated into their burrows within from five to fifteen minutes;
and if in the evening the pots were illuminated before the worms
had come out of their burrows, they failed to appear.
From the foregoing facts it is evident that light affects worms by
its intensity and by its duration. It is only the anterior
extremity of the body, where the cerebral ganglia lie, which is
affected by light, as Hoffmeister asserts, and as I observed on
many occasions. If this part is shaded, other parts of the body
may be fully illuminated, and no effect will be produced. As these
animals have no eyes, we must suppose that the light passes through
their skins, and in some manner excites their cerebral ganglia. It
appeared at first probable that the different manner in which they
were affected on different occasions might be explained, either by
the degree of extension of their skin and its consequent
transparency, or by some particular incident of the light; but I
could discover no such relation. One thing was manifest, namely,
that when worms were employed in dragging leaves into their burrows
or in eating them, and even during the short intervals whilst they
rested from their work, they either did not perceive the light or
were regardless of it; and this occurred even when the light was
concentrated on them through a large lens. So, again, whilst they
are paired, they will remain for an hour or two out of their
burrows, fully exposed to the morning light; but it appears from
what Hoffmeister says that a light will occasionally cause paired
individuals to separate.
When a worm is suddenly illuminated and dashes like a rabbit into
its burrow--to use the expression employed by a friend--we are at
first led to look at the action as a reflex one. The irritation of
the cerebral ganglia appears to cause certain muscles to contract
in an inevitable manner, independently of the will or consciousness
of the animal, as if it were an automaton. But the different
effect which a light produced on different occasions, and
especially the fact that a worm when in any way employed and in the
intervals of such employment, whatever set of muscles and ganglia
may then have been brought into play, is often regardless of light,
are opposed to the view of the sudden withdrawal being a simple
reflex action. With the higher animals, when close attention to
some object leads to the disregard of the impressions which other
objects must be producing on them, we attribute this to their
attention being then absorbed; and attention implies the presence
of a mind. Every sportsman knows that he can approach animals
whilst they are grazing, fighting or courting, much more easily
than at other times. The state, also, of the nervous system of the
higher animals differs much at different times, for instance, a
horse is much more readily startled at one time than at another.
The comparison here implied between the actions of one of the
higher animals and of one so low in the scale as an earth-worm, may
appear far-fetched; for we thus attribute to the worm attention and
some mental power, nevertheless I can see no reason to doubt the
justice of the comparison.
Although worms cannot be said to possess the power of vision, their
sensitiveness to light enables them to distinguish between day and
night; and they thus escape extreme danger from the many diurnal
animals which prey on them. Their withdrawal into their burrows
during the day appears, however, to have become an habitual action;
for worms kept in pots covered by glass plates, over which sheets
of black paper were spread, and placed before a north-east window,
remained during the day-time in their burrows and came out every
night; and they continued thus to act for a week. No doubt a
little light may have entered between the sheets of glass and the
blackened paper; but we know from the trials with coloured glass,
that worms are indifferent to a small amount of light.
Worms appear to be less sensitive to moderate radiant heat than to
a bright light. I judge of this from having held at different
times a poker heated to dull redness near some worms, at a distance
which caused a very sensible degree of warmth in my hand. One of
them took no notice; a second withdrew into its burrow, but not
quickly; the third and fourth much more quickly, and the fifth as
quickly as possible. The light from a candle, concentrated by a
lens and passing through a sheet of glass which would intercept
most of the heat-rays, generally caused a much more rapid retreat
than did the heated poker. Worms are sensitive to a low
temperature, as may be inferred from their not coming out of their
burrows during a frost.
Worms do not possess any sense of hearing. They took not the least
notice of the shrill notes from a metal whistle, which was
repeatedly sounded near them; nor did they of the deepest and
loudest tones of a bassoon. They were indifferent to shouts, if
care was taken that the breath did not strike them. When placed on
a table close to the keys of a piano, which was played as loudly as
possible, they remained perfectly quiet.
Although they are indifferent to undulations in the air audible by
us, they are extremely sensitive to vibrations in any solid object.
When the pots containing two worms which had remained quite
indifferent to the sound of the piano, were placed on this
instrument, and the note C in the bass clef was struck, both
instantly retreated into their burrows. After a time they emerged,
and when G above the line in the treble clef was struck they again
retreated. Under similar circumstances on another night one worm
dashed into its burrow on a very high note being struck only once,
and the other worm when C in the treble clef was struck. On these
occasions the worms were not touching the sides of the pots, which
stood in saucers; so that the vibrations, before reaching their
bodies, had to pass from the sounding board of the piano, through
the saucer, the bottom of the pot and the damp, not very compact
earth on which they lay with their tails in their burrows. They
often showed their sensitiveness when the pot in which they lived,
or the table on which the pot stood, was accidentally and lightly
struck; but they appeared less sensitive to such jars than to the
vibrations of the piano; and their sensitiveness to jars varied
much at different times.
It has often been said that if the ground is beaten or otherwise
made to tremble, worms believe that they are pursued by a mole and
leave their burrows. From one account that I have received, I have
no doubt that this is often the case; but a gentleman informs me
that he lately saw eight or ten worms leave their burrows and crawl
about the grass on some boggy land on which two men had just
trampled while setting a trap; and this occurred in a part of
Ireland where there were no moles. I have been assured by a
Volunteer that he has often seen many large earth-worms crawling
quickly about the grass, a few minutes after his company had fired
a volley with blank cartridges. The Peewit (Tringa vanellus,
Linn.) seems to know instinctively that worms will emerge if the
ground is made to tremble; for Bishop Stanley states (as I hear
from Mr. Moorhouse) that a young peewit kept in confinement used to
stand on one leg and beat the turf with the other leg until the
worms crawled out of their burrows, when they were instantly
devoured. Nevertheless, worms do not invariably leave their
burrows when the ground is made to tremble, as I know by having
beaten it with a spade, but perhaps it was beaten too violently.
The whole body of a worm is sensitive to contact. A slight puff of
air from the mouth causes an instant retreat. The glass plates
placed over the pots did not fit closely, and blowing through the
very narrow chinks thus left, often sufficed to cause a rapid
retreat. They sometimes perceived the eddies in the air caused by
quickly removing the glass plates. When a worm first comes out of
its burrow, it generally moves the much extended anterior extremity
of its body from side to side in all directions, apparently as an
organ of touch; and there is some reason to believe, as we shall
see in the next chapter, that they are thus enabled to gain a
general notion of the form of an object. Of all their senses that
of touch, including in this term the perception of a vibration,
seems much the most highly developed.
In worms the sense of smell apparently is confined to the
perception of certain odours, and is feeble. They were quite
indifferent to my breath, as long as I breathed on them very
gently. This was tried, because it appeared possible that they
might thus be warned of the approach of an enemy. They exhibited
the same indifference to my breath whilst I chewed some tobacco,
and while a pellet of cotton-wool with a few drops of millefleurs
perfume or of acetic acid was kept in my mouth. Pellets of cotton-
wool soaked in tobacco juice, in millefleurs perfume, and in
paraffin, were held with pincers and were waved about within two or
three inches of several worms, but they took no notice. On one or
two occasions, however, when acetic acid had been placed on the
pellets, the worms appeared a little uneasy, and this was probably
due to the irritation of their skins. The perception of such
unnatural odours would be of no service to worms; and as such timid
creatures would almost certainly exhibit some signs of any new
impression, we may conclude that they did not perceive these
odours.
The result was different when cabbage-leaves and pieces of onion
were employed, both of which are devoured with much relish by
worms. Small square pieces of fresh and half-decayed cabbage-
leaves and of onion bulbs were on nine occasions buried in my pots,
beneath about 0.25 of an inch of common garden soil; and they were
always discovered by the worms. One bit of cabbage was discovered
and removed in the course of two hours; three were removed by the
next morning, that is, after a single night; two others after two
nights; and the seventh bit after three nights. Two pieces of
onion were discovered and removed after three nights. Bits of
fresh raw meat, of which worms are very fond, were buried, and were
not discovered within forty-eight hours, during which time they had
not become putrid. The earth above the various buried objects was
generally pressed down only slightly, so as not to prevent the
emission of any odour. On two occasions, however, the surface was
well watered, and was thus rendered somewhat compact. After the
bits of cabbage and onion had been removed, I looked beneath them
to see whether the worms had accidentally come up from below, but
there was no sign of a burrow; and twice the buried objects were
laid on pieces of tin-foil which were not in the least displaced.
It is of course possible that the worms whilst moving about on the
surface of the ground, with their tails affixed within their
burrows, may have poked their heads into the places where the above
objects were buried; but I have never seen worms acting in this
manner. Some pieces of cabbage-leaf and of onion were twice buried
beneath very fine ferruginous sand, which was slightly pressed down
and well watered, so as to be rendered very compact, and these
pieces were never discovered. On a third occasion the same kind of
sand was neither pressed down nor watered, and the pieces of
cabbage were discovered and removed after the second night. These
several facts indicate that worms possess some power of smell; and
that they discover by this means odoriferous and much-coveted kinds
of food.
It may be presumed that all animals which feed on various
substances possess the sense of taste, and this is certainly the
case with worms. Cabbage-leaves are much liked by worms; and it
appears that they can distinguish between different varieties; but
this may perhaps be owing to differences in their texture. On
eleven occasions pieces of the fresh leaves of a common green
variety and of the red variety used for pickling were given them,
and they preferred the green, the red being either wholly neglected
or much less gnawed. On two other occasions, however, they seemed
to prefer the red. Half-decayed leaves of the red variety and
fresh leaves of the green were attacked about equally. When leaves
of the cabbage, horse-radish (a favourite food) and of the onion
were given together, the latter were always, and manifestly
preferred. Leaves of the cabbage, lime-tree, Ampelopsis, parsnip
(Pastinaca), and celery (Apium) were likewise given together; and
those of the celery were first eaten. But when leaves of cabbage,
turnip, beet, celery, wild cherry and carrots were given together,
the two latter kinds, especially those of the carrot, were
preferred to all the others, including those of celery. It was
also manifest after many trials that wild cherry leaves were
greatly preferred to those of the lime-tree and hazel (Corylus).
According to Mr. Bridgman the half-decayed leaves of Phlox verna
are particularly liked by worms. {16}
Pieces of the leaves of cabbage, turnip, horse-radish and onion
were left on the pots during 22 days, and were all attacked and had
to be renewed; but during the whole of this time leaves of an
Artemisia and of the culinary sage, thyme and mint, mingled with
the above leaves, were quite neglected excepting those of the mint,
which were occasionally and very slightly nibbled. These latter
four kinds of leaves do not differ in texture in a manner which
could make them disagreeable to worms; they all have a strong
taste, but so have the four first mentioned kinds of leaves; and
the wide difference in the result must be attributed to a
preference by the worms for one taste over another.
Mental Qualities.--There is little to be said on this head. We
have seen that worms are timid. It may be doubted whether they
suffer as much pain when injured, as they seem to express by their
contortions. Judging by their eagerness for certain kinds of food,
they must enjoy the pleasure of eating. Their sexual passion is
strong enough to overcome for a time their dread of light. They
perhaps have a trace of social feeling, for they are not disturbed
by crawling over each other's bodies, and they sometimes lie in
contact. According to Hoffmeister they pass the winter either
singly or rolled up with others into a ball at the bottom of their
burrows. {17} Although worms are so remarkably deficient in the
several sense-organs, this does not necessarily preclude
intelligence, as we know from such cases as those of Laura
Bridgman; and we have seen that when their attention is engaged,
they neglect impressions to which they would otherwise have
attended; and attention indicates the presence of a mind of some
kind. They are also much more easily excited at certain times than
at others. They perform a few actions instinctively, that is, all
the individuals, including the young, perform such actions in
nearly the same fashion. This is shown by the manner in which the
species of Perichaeta eject their castings, so as to construct
towers; also by the manner in which the burrows of the common
earth-worm are smoothly lined with fine earth and often with little
stones, and the mouths of their burrows with leaves. One of their
strongest instincts is the plugging up the mouths of their burrows
with various objects; and very young worms act in this manner. But
some degree of intelligence appears, as we shall see in the next
chapter, to be exhibited in this work,--a result which has
surprised me more than anything else in regard to worms.
Food and Digestion.--Worms are omnivorous. They swallow an
enormous quantity of earth, out of which they extract any
digestible matter which it may contain; but to this subject I must
recur. They also consume a large number of half-decayed leaves of
all kinds, excepting a few which have an unpleasant taste or are
too tough for them; likewise petioles, peduncles, and decayed
flowers. But they will also consume fresh leaves, as I have found
by repeated trials. According to Morren {18} they will eat
particles of sugar and liquorice; and the worms which I kept drew
many bits of dry starch into their burrows, and a large bit had its
angles well rounded by the fluid poured out of their mouths. But
as they often drag particles of soft stone, such as of chalk, into
their burrows, I feel some doubt whether the starch was used as
food. Pieces of raw and roasted meat were fixed several times by
long pins to the surface of the soil in my pots, and night after
night the worms could be seen tugging at them, with the edges of
the pieces engulfed in their mouths, so that much was consumed.
Raw fat seems to be preferred even to raw meat or to any other
substance which was given them, and much was consumed. They are
cannibals, for the two halves of a dead worm placed in two of the
pots were dragged into the burrows and gnawed; but as far as I
could judge, they prefer fresh to putrid meat, and in so far I
differ from Hoffmeister.
Leon Fredericq states {19} that the digestive fluid of worms is of
the same nature as the pancreatic secretion of the higher animals;
and this conclusion agrees perfectly with the kinds of food which
worms consume. Pancreatic juice emulsifies fat, and we have just
seen how greedily worms devour fat; it dissolves fibrin, and worms
eat raw meat; it converts starch into grape-sugar with wonderful
rapidity, and we shall presently show that the digestive fluid of
worms acts on starch. {20} But they live chiefly on half-decayed
leaves; and these would be useless to them unless they could digest
the cellulose forming the cell-walls; for it is well known that all
other nutritious substances are almost completely withdrawn from
leaves, shortly before they fall off. It has, however, now been
ascertained that some forms of cellulose, though very little or not
at all attacked by the gastric secretion of the higher animals, are
acted on by that from the pancreas. {21}
The half-decayed or fresh leaves which worms intend to devour, are
dragged into the mouths of their burrows to a depth of from one to
three inches, and are then moistened with a secreted fluid. It has
been assumed that this fluid serves to hasten their decay; but a
large number of leaves were twice pulled out of the burrows of
worms and kept for many weeks in a very moist atmosphere under a
bell-glass in my study; and the parts which had been moistened by
the worms did not decay more quickly in any plain manner than the
other parts. When fresh leaves were given in the evening to worms
kept in confinement and examined early on the next morning,
therefore not many hours after they had been dragged into the
burrows, the fluid with which they were moistened, when tested with
neutral litmus paper, showed an alkaline reaction. This was
repeatedly found to be the case with celery, cabbage and turnip
leaves. Parts of the same leaves which had not been moistened by
the worms, were pounded with a few drops of distilled water, and
the juice thus extracted was not alkaline. Some leaves, however,
which had been drawn into burrows out of doors, at an unknown
antecedent period, were tried, and though still moist, they rarely
exhibited even a trace of alkaline reaction.
The fluid, with which the leaves are bathed, acts on them whilst
they are fresh or nearly fresh, in a remarkable manner; for it
quickly kills and discolours them. Thus the ends of a fresh
carrot-leaf, which had been dragged into a burrow, were found after
twelve hours of a dark brown tint. Leaves of celery, turnip,
maple, elm, lime, thin leaves of ivy, and, occasionally those of
the cabbage were similarly acted on. The end of a leaf of Triticum
repens, still attached to a growing plant, had been drawn into a
burrow, and this part was dark brown and dead, whilst the rest of
the leaf was fresh and green. Several leaves of lime and elm
removed from burrows out of doors were found affected in different
degrees. The first change appears to be that the veins become of a
dull reddish-orange. The cells with chlorophyll next lose more or
less completely their green colour, and their contents finally
become brown. The parts thus affected often appeared almost black
by reflected light; but when viewed as a transparent object under
the microscope, minute specks of light were transmitted, and this
was not the case with the unaffected parts of the same leaves.
These effects, however, merely show that the secreted fluid is
highly injurious or poisonous to leaves; for nearly the same
effects were produced in from one to two days on various kinds of
young leaves, not only by artificial pancreatic fluid, prepared
with or without thymol, but quickly by a solution of thymol by
itself. On one occasion leaves of Corylus were much discoloured by
being kept for eighteen hours in pancreatic fluid, without any
thymol. With young and tender leaves immersion in human saliva
during rather warm weather, acted in the same manner as the
pancreatic fluid, but not so quickly. The leaves in all these
cases often became infiltrated with the fluid.
Large leaves from an ivy plant growing on a wall were so tough that
they could not be gnawed by worms, but after four days they were
affected in a peculiar manner by the secretion poured out of their
mouths. The upper surfaces of the leaves, over which the worms had
crawled, as was shown by the dirt left on them, were marked in
sinuous lines, by either a continuous or broken chain of whitish
and often star-shaped dots, about 2 mm. in diameter. The
appearance thus presented was curiously like that of a leaf, into
which the larva of some minute insect had burrowed. But my son
Francis, after making and examining sections, could nowhere find
that the cell-walls had been broken down or that the epidermis had
been penetrated. When the section passed through the whitish dots,
the grains of chlorophyll were seen to be more or less discoloured,
and some of the palisade and mesophyll cells contained nothing but
broken down granular matter. These effects must be attributed to
the transudation of the secretion through the epidermis into the
cells.
The secretion with which worms moisten leaves likewise acts on the
starch-granules within the cells. My son examined some leaves of
the ash and many of the lime, which had fallen off the trees and
had been partly dragged into worm-burrows. It is known that with
fallen leaves the starch-grains are preserved in the guard-cells of
the stomata. Now in several cases the starch had partially or
wholly disappeared from these cells, in the parts which had been
moistened by the secretion; while it was still well preserved in
the other parts of the same leaves. Sometimes the starch was
dissolved out of only one of the two guard-cells. The nucleus in
one case had disappeared, together with the starch-granules. The
mere burying of lime-leaves in damp earth for nine days did not
cause the destruction of the starch-granules. On the other hand,
the immersion of fresh lime and cherry leaves for eighteen hours in
artificial pancreatic fluid, led to the dissolution of the starch-
granules in the guard-cells as well as in the other cells.
From the secretion with which the leaves are moistened being
alkaline, and from its acting both on the starch-granules and on
the protoplasmic contents of the cells, we may infer that it
resembles in nature not saliva, {22} but pancreatic secretion; and
we know from Fredericq that a secretion of this kind is found in
the intestines of worms. As the leaves which are dragged into the
burrows are often dry and shrivelled, it is indispensable for their
disintegration by the unarmed mouths of worms that they should
first be moistened and softened; and fresh leaves, however soft and
tender they may be, are similarly treated, probably from habit.
The result is that they are partially digested before they are
taken into the alimentary canal. I am not aware of any other case
of extra-stomachal digestion having been recorded. The boa-
constrictor is said to bathe its prey with saliva, but this is
doubtful; and it is done solely for the sake of lubricating its
prey. Perhaps the nearest analogy may be found in such plants as
Drosera and Dionaea; for here animal matter is digested and
converted into peptone not within a stomach, but on the surfaces of
the leaves.
Calciferous Glands.--These glands (see Fig. 1), judging from their
size and from their rich supply of blood-vessels, must be of much
importance to the animal. But almost as many theories have been
advanced on their use as there have been observers. They consist
of three pairs, which in the common earth-worm debouch into the
alimentary canal in advance of the gizzard, but posteriorly to it
in Urochaeta and some other genera. {23} The two posterior pairs
are formed by lamellae, which, according to Claparede, are
diverticula from the oesophagus. {24} These lamellae are coated
with a pulpy cellular layer, with the outer cells lying free in
infinite numbers. If one of these glands is punctured and
squeezed, a quantity of white pulpy matter exudes, consisting of
these free cells. They are minute, and vary in diameter from 2 to
6 microns. They contain in their centres a little excessively fine
granular matter; but they look so like oil globules that Claparede
and others at first treated them with ether. This produces no
effect; but they are quickly dissolved with effervescence in acetic
acid, and when oxalate of ammonia is added to the solution a white
precipitate is thrown down. We may therefore conclude that they
contain carbonate of lime. If the cells are immersed in a very
little acid, they become more transparent, look like ghosts, and
are soon lost to view; but if much acid is added, they disappear
instantly. After a very large number have been dissolved, a
flocculent residue is left, which apparently consists of the
delicate ruptured cell-walls. In the two posterior pairs of glands
the carbonate of lime contained in the cells occasionally
aggregates into small rhombic crystals or into concretions, which
lie between the lamellae; but I have seen only one case, and
Claparede only a very few such cases.
The two anterior glands differ a little in shape from the four
posterior ones, by being more oval. They differ also conspicuously
in generally containing several small, or two or three larger, or a
single very large concretion of carbonate of lime, as much as 1.5
mm. in diameter. When a gland includes only a few very small
concretions, or, as sometimes happens, none at all, it is easily
overlooked. The large concretions are round or oval, and
exteriorly almost smooth. One was found which filled up not only
the whole gland, as is often the case, but its neck; so that it
resembled an olive-oil flask in shape. These concretions when
broken are seen to be more or less crystalline in structure. How
they escape from the gland is a marvel; but that they do escape is
certain, for they are often found in the gizzard, intestines, and
in the castings of worms, both with those kept in confinement and
those in a state of nature.
Claparede says very little about the structure of the two anterior
glands, and he supposes that the calcareous matter of which the
concretions are formed is derived from the four posterior glands.
But if an anterior gland which contains only small concretions is
placed in acetic acid and afterwards dissected, or if sections are
made of such a gland without being treated with acid, lamellae like
those in the posterior glands and coated with cellular matter could
be plainly seen, together with a multitude of free calciferous
cells readily soluble in acetic acid. When a gland is completely
filled with a single large concretion, there are no free cells, as
these have been all consumed in forming the concretion. But if
such a concretion, or one of only moderately large size, is
dissolved in acid, much membranous matter is left, which appears to
consist of the remains of the formerly active lamellae. After the
formation and expulsion of a large concretion, new lamellae must be
developed in some manner. In one section made by my son, the
process had apparently commenced, although the gland contained two
rather large concretions, for near the walls several cylindrical
and oval pipes were intersected, which were lined with cellular
matter and were quite filled with free calciferous cells. A great
enlargement in one direction of several oval pipes would give rise
to the lamellae.
Besides the free calciferous cells in which no nucleus was visible,
other and rather larger free cells were seen on three occasions;
and these contained a distinct nucleus and nucleolus. They were
only so far acted on by acetic acid that the nucleus was thus
rendered more distinct. A very small concretion was removed from
between two of the lamellae within an anterior gland. It was
imbedded in pulpy cellular matter, with many free calciferous
cells, together with a multitude of the larger, free, nucleated
cells, and these latter cells were not acted on by acetic acid,
while the former were dissolved. From this and other such cases I
am led to suspect that the calciferous cells are developed from the
larger nucleated ones; but how this was effected was not
ascertained.
When an anterior gland contains several minute concretions, some of
these are generally angular or crystalline in outline, while the
greater number are rounded with an irregular mulberry-like surface.
Calciferous cells adhered to many parts of these mulberry-like
masses, and their gradual disappearance could be traced while they
still remained attached. It was thus evident that the concretions
are formed from the lime contained within the free calciferous
cells. As the smaller concretions increase in size, they come into
contact and unite, thus enclosing the now functionless lamellae;
and by such steps the formation of the largest concretions could be
followed. Why the process regularly takes place in the two
anterior glands, and only rarely in the four posterior glands, is
quite unknown. Morren says that these glands disappear during the
winter; and I have seen some instances of this fact, and others in
which either the anterior or posterior glands were at this season
so shrunk and empty, that they could be distinguished only with
much difficulty.
With respect to the function of the calciferous glands, it is
probable that they primarily serve as organs of excretion, and
secondarily as an aid to digestion. Worms consume many fallen
leaves; and it is known that lime goes on accumulating in leaves
until they drop off the parent-plant, instead of being re-absorbed
into the stem or roots, like various other organic and inorganic
substances. {25} The ashes of a leaf of an acacia have been known
to contain as much as 72 per cent. of lime. Worms therefore would
be liable to become charged with this earth, unless there were some
special means for its excretion; and the calciferous glands are
well adapted for this purpose. The worms which live in mould close
over the chalk, often have their intestines filled with this
substance, and their castings are almost white. Here it is evident
that the supply of calcareous matter must be super-abundant.
Nevertheless with several worms collected on such a site, the
calciferous glands contained as many free calciferous cells, and
fully as many and large concretions, as did the glands of worms
which lived where there was little or no lime; and this indicates
that the lime is an excretion, and not a secretion poured into the
alimentary canal for some special purpose.
On the other hand, the following considerations render it highly
probable that the carbonate of lime, which is excreted by the
glands, aids the digestive process under ordinary circumstances.
Leaves during their decay generate an abundance of various kinds of
acids, which have been grouped together under the term of humus
acids. We shall have to recur to this subject in our fifth
chapter, and I need here only say that these acids act strongly on
carbonate of lime. The half-decayed leaves which are swallowed in
such large quantities by worms would, therefore, after they have
been moistened and triturated in the alimentary canal, be apt to
produce such acids. And in the case of several worms, the contents
of the alimentary canal were found to be plainly acid, as shown by
litmus paper. This acidity cannot be attributed to the nature of
the digestive fluid, for pancreatic fluid is alkaline; and we have
seen that the secretion which is poured out of the mouths of worms
for the sake of preparing the leaves for consumption, is likewise
alkaline. The acidity can hardly be due to uric acid, as the
contents of the upper part of the intestine were often acid. In
one case the contents of the gizzard were slightly acid, those of
the upper intestines being more plainly acid. In another case the
contents of the pharynx were not acid, those of the gizzard
doubtfully so, while those of the intestine were distinctly acid at
a distance of 5 cm. below the gizzard. Even with the higher
herbivorous and omnivorous animals, the contents of the large
intestine are acid. "This, however, is not caused by any acid
secretion from the mucous membrane; the reaction of the intestinal
walls in the larger as in the small intestine is alkaline. It must
therefore arise from acid fermentations going on in the contents
themselves . . . In Carnivora the contents of the coecum are said
to be alkaline, and naturally the amount of fermentation will
depend largely on the nature of the food." {26}
With worms not only the contents of the intestines, but their
ejected matter or the castings, are generally acid. Thirty
castings from different places were tested, and with three or four
exceptions were found to be acid; and the exceptions may have been
due to such castings not having been recently ejected; for some
which were at first acid, were on the following morning, after
being dried and again moistened, no longer acid; and this probably
resulted from the humus acids being, as is known to be the case,
easily decomposed. Five fresh castings from worms which lived in
mould close over the chalk, were of a whitish colour and abounded
with calcareous matter; and these were not in the least acid. This
shows how effectually carbonate of lime neutralises the intestinal
acids. When worms were kept in pots filled with fine ferruginous
sand, it was manifest that the oxide of iron, with which the grains
of silex were coated, had been dissolved and removed from them in
the castings.
The digestive fluid of worms resembles in its action, as already
stated, the pancreatic secretion of the higher animals; and in
these latter, "pancreatic digestion is essentially alkaline; the
action will not take place unless some alkali be present; and the
activity of an alkaline juice is arrested by acidification, and
hindered by neutralization." {27} Therefore it seems highly
probable that the innumerable calciferous cells, which are poured
from the four posterior glands into the alimentary canal of worms,
serve to neutralise more or less completely the acids there
generated by the half-decayed leaves. We have seen that these
cells are instantly dissolved by a small quantity of acetic acid,
and as they do not always suffice to neutralise the contents of
even the upper part of the alimentary canal, the lime is perhaps
aggregated into concretions in the anterior pair of glands, in
order that some may be carried down to the posterior parts of the
intestine, where these concretions would be rolled about amongst
the acid contents. The concretions found in the intestines and in
the castings often have a worn appearance, but whether this is due
to some amount of attrition or of chemical corrosion could not be
told. Claparede believes that they are formed for the sake of
acting as mill-stones, and of thus aiding in the trituration of the
food. They may give some aid in this way; but I fully agree with
Perrier that this must be of quite subordinate importance, seeing
that the object is already attained by stones being generally
present in the gizzards and intestines of worms.
CHAPTER II--HABITS OF WORMS--continued.
Manner in which worms seize objects--Their power of suction--The
instinct of plugging up the mouths of their burrows--Stones piled
over the burrows--The advantages thus gained--Intelligence shown by
worms in their manner of plugging up their burrows--Various kinds
of leaves and other objects thus used--Triangles of paper--Summary
of reasons for believing that worms exhibit some intelligence--
Means by which they excavate their burrows, by pushing away the
earth and swallowing it--Earth also swallowed for the nutritious
matter which it contains--Depth to which worms burrow, and the
construction of their burrows--Burrows lined with castings, and in
the upper part with leaves--The lowest part paved with little
stones or seeds--Manner in which the castings are ejected--The
collapse of old burrows--Distribution of worms--Tower-like castings
in Bengal--Gigantic castings on the Nilgiri Mountains--Castings
ejected in all countries.
In the pots in which worms were kept, leaves were pinned down to
the soil, and at night the manner in which they were seized could
be observed. The worms always endeavoured to drag the leaves
towards their burrows; and they tore or sucked off small fragments,
whenever the leaves were sufficiently tender. They generally
seized the thin edge of a leaf with their mouths, between the
projecting upper and lower lip; the thick and strong pharynx being
at the same time, as Perrier remarks, pushed forward within their
bodies, so as to afford a point of resistance for the upper lip.
In the case of broad flat objects they acted in a wholly different
manner. The pointed anterior extremity of the body, after being
brought into contact with an object of this kind, was drawn within
the adjoining rings, so that it appeared truncated and became as
thick as the rest of the body. This part could then be seen to
swell a little; and this, I believe, is due to the pharynx being
pushed a little forwards. Then by a slight withdrawal of the
pharynx or by its expansion, a vacuum was produced beneath the
truncated slimy end of the body whilst in contact with the object;
and by this means the two adhered firmly together. {28} That under
these circumstances a vacuum was produced was plainly seen on one
occasion, when a large worm lying beneath a flaccid cabbage leaf
tried to drag it away; for the surface of the leaf directly over
the end of the worm's body became deeply pitted. On another
occasion a worm suddenly lost its hold on a flat leaf; and the
anterior end of the body was momentarily seen to be cup-formed.
Worms can attach themselves to an object beneath water in the same
manner; and I saw one thus dragging away a submerged slice of an
onion-bulb.
The edges of fresh or nearly fresh leaves affixed to the ground
were often nibbled by the worms; and sometimes the epidermis and
all the parenchyma on one side was gnawed completely away over a
considerable space; the epidermis alone on the opposite side being
left quite clean. The veins were never touched, and leaves were
thus sometimes partly converted into skeletons. As worms have no
teeth and as their mouths consist of very soft tissue, it may be
presumed that they consume by means of suction the edges and the
parenchyma of fresh leaves, after they have been softened by the
digestive fluid. They cannot attack such strong leaves as those of
sea-kale or large and thick leaves of ivy; though one of the latter
after it had become rotten was reduced in parts to the state of a
skeleton.
Worms seize leaves and other objects, not only to serve as food,
but for plugging up the mouths of their burrows; and this is one of
their strongest instincts. They sometimes work so energetically
that Mr. D. F. Simpson, who has a small walled garden where worms
abound in Bayswater, informs me that on a calm damp evening he
there heard so extraordinary a rustling noise from under a tree
from which many leaves had fallen, that he went out with a light
and discovered that the noise was caused by many worms dragging the
dry leaves and squeezing them into the burrows. Not only leaves,
but petioles of many kinds, some flower-peduncles, often decayed
twigs of trees, bits of paper, feathers, tufts of wool and horse-
hairs are dragged into their burrows for this purpose. I have seen
as many as seventeen petioles of a Clematis projecting from the
mouth of one burrow, and ten from the mouth of another. Some of
these objects, such as the petioles just named, feathers, &c., are
never gnawed by worms. In a gravel-walk in my garden I found many
hundred leaves of a pine-tree (P. austriaca or nigricans) drawn by
their bases into burrows. The surfaces by which these leaves are
articulated to the branches are shaped in as peculiar a manner as
is the joint between the leg-bones of a quadruped; and if these
surfaces had been in the least gnawed, the fact would have been
immediately visible, but there was no trace of gnawing. Of
ordinary dicotyledonous leaves, all those which are dragged into
burrows are not gnawed. I have seen as many as nine leaves of the
lime-tree drawn into the same burrow, and not nearly all of them
had been gnawed; but such leaves may serve as a store for future
consumption. Where fallen leaves are abundant, many more are
sometimes collected over the mouth of a burrow than can be used, so
that a small pile of unused leaves is left like a roof over those
which have been partly dragged in.
A leaf in being dragged a little way into a cylindrical burrow is
necessarily much folded or crumpled. When another leaf is drawn
in, this is done exteriorly to the first one, and so on with the
succeeding leaves; and finally all become closely folded and
pressed together. Sometimes the worm enlarges the mouth of its
burrow, or makes a fresh one close by, so as to draw in a still
larger number of leaves. They often or generally fill up the
interstices between the drawn-in leaves with moist viscid earth
ejected from their bodies; and thus the mouths of the burrows are
securely plugged. Hundreds of such plugged burrows may be seen in
many places, especially during the autumnal and early winter
months. But, as will hereafter be shown, leaves are dragged into
the burrows not only for plugging them up and for food, but for the
sake of lining the upper part or mouth.
When worms cannot obtain leaves, petioles, sticks, &c., with which
to plug up the mouths of their burrows, they often protect them by
little heaps of stones; and such heaps of smooth rounded pebbles
may frequently be seen on gravel-walks. Here there can be no
question about food. A lady, who was interested in the habits of
worms, removed the little heaps of stones from the mouths of
several burrows and cleared the surface of the ground for some
inches all round. She went out on the following night with a
lantern, and saw the worms with their tails fixed in their burrows,
dragging the stones inwards by the aid of their mouths, no doubt by
suction. "After two nights some of the holes had 8 or 9 small
stones over them; after four nights one had about 30, and another
34 stones." {29} One stone--which had been dragged over the
gravel-walk to the mouth of a burrow weighed two ounces; and this
proves how strong worms are. But they show greater strength in
sometimes displacing stones in a well-trodden gravel-walk; that
they do so, may be inferred from the cavities left by the displaced
stones being exactly filled by those lying over the mouths of
adjoining burrows, as I have myself observed.
Work of this kind is usually performed during the night; but I have
occasionally known objects to be drawn into the burrows during the
day. What advantage the worms derive from plugging up the mouths
of their burrows with leaves, &c., or from piling stones over them,
is doubtful. They do not act in this manner at the times when they
eject much earth from their burrows; for their castings then serve
to cover the mouths. When gardeners wish to kill worms on a lawn,
it is necessary first to brush or rake away the castings from the
surface, in order that the lime-water may enter the burrows. {30}
It might be inferred from this fact that the mouths are plugged up
with leaves, &c., to prevent the entrance of water during heavy
rain; but it may be urged against this view that a few, loose,
well-rounded stones are ill-adapted to keep out water. I have
moreover seen many burrows in the perpendicularly cut turf-edgings
to gravel-walks, into which water could hardly flow, as well
plugged as burrows on a level surface. It is not probable that the
plugs or piles of stones serve to conceal the burrows from
scolopendras, which, according to Hoffmeister, {31} are the
bitterest enemies of worms, or from the larger species of Carabus
and Staphylinus which attack them ferociously, for these animals
are nocturnal, and the burrows are opened at night. May not worms
when the mouth of the burrow is protected be able to remain with
safety with their heads close to it, which we know that they like
to do, but which costs so many of them their lives? Or may not the
plugs check the free ingress of the lowest stratum of air, when
chilled by radiation at night, from the surrounding ground and
herbage? I am inclined to believe in this latter view: firstly,
because when worms were kept in pots in a room with a fire, in
which case cold air could not enter the burrows, they plugged them
up in a slovenly manner; and secondarily, because they often coat
the upper part of their burrows with leaves, apparently to prevent
their bodies from coming into close contact with the cold damp
earth. Mr. E. Parfitt has suggested to me that the mouths of the
burrows are closed in order that the air within them may be kept
thoroughly damp, and this seems the most probable explanation of
the habit. But the plugging-up process may serve for all the above
purposes.
Whatever the motive may be, it appears that worms much dislike
leaving the mouths of their burrows open. Nevertheless they will
reopen them at night, whether or not they can afterwards close
them. Numerous open burrows may be seen on recently-dug ground,
for in this case the worms eject their castings in cavities left in
the ground, or in the old burrows instead of piling them over the
mouths of their burrows, and they cannot collect objects on the
surface by which the mouths might be protected. So again on a
recently disinterred pavement of a Roman villa at Abinger
(hereafter to be described) the worms pertinaciously opened their
burrows almost every night, when these had been closed by being
trampled on, although they were rarely able to find a few minute
stones wherewith to protect them.
Intelligence shown by worms in their manner of plugging up their
burrows.--If a man had to plug up a small cylindrical hole, with
such objects as leaves, petioles or twigs, he would drag or push
them in by their pointed ends; but if these objects were very thin
relatively to the size of the hole, he would probably insert some
by their thicker or broader ends. The guide in his case would be
intelligence. It seemed therefore worth while to observe carefully
how worms dragged leaves into their burrows; whether by their tips
or bases or middle parts. It seemed more especially desirable to
do this in the case of plants not natives to our country; for
although the habit of dragging leaves into their burrows is
undoubtedly instinctive with worms, yet instinct could not tell
them how to act in the case of leaves about which their progenitors
knew nothing. If, moreover, worms acted solely through instinct or
an unvarying inherited impulse, they would draw all kinds of leaves
into their burrows in the same manner. If they have no such
definite instinct, we might expect that chance would determine
whether the tip, base or middle was seized. If both these
alternatives are excluded, intelligence alone is left; unless the
worm in each case first tries many different methods, and follows
that alone which proves possible or the most easy; but to act in
this manner and to try different methods makes a near approach to
intelligence.
In the first place 227 withered leaves of various kinds, mostly of
English plants, were pulled out of worm-burrows in several places.
Of these, 181 had been drawn into the burrows by or near their
tips, so that the foot-stalk projected nearly upright from the
mouth of the burrow; 20 had been drawn in by their bases, and in
this case the tips projected from the burrows; and 26 had been
seized near the middle, so that these had been drawn in
transversely and were much crumpled. Therefore 80 per cent.
(always using the nearest whole number) had been drawn in by the
tip, 9 per cent. by the base or foot-stalk, and 11 per cent.
transversely or by the middle. This alone is almost sufficient to
show that chance does not determine the manner in which leaves are
dragged into the burrows.
Of the above 227 leaves, 70 consisted of the fallen leaves of the
common lime-tree, which is almost certainly not a native of
England. These leaves are much acuminated towards the tip, and are
very broad at the base with a well-developed foot-stalk. They are
thin and quite flexible when half-withered. Of the 70, 79 per
cent. had been drawn in by or near the tip; 4 per cent. by or near
the base; and 17 per cent. transversely or by the middle. These
proportions agree very closely, as far as the tip is concerned,
with those before given. But the percentage drawn in by the base
is smaller, which may be attributed to the breadth of the basal
part of the blade. We here, also, see that the presence of a foot-
stalk, which it might have been expected would have tempted the
worms as a convenient handle, has little or no influence in
determining the manner in which lime leaves are dragged into the
burrows. The considerable proportion, viz., 17 per cent., drawn in
more or less transversely depends no doubt on the flexibility of
these half-decayed leaves. The fact of so many having been drawn
in by the middle, and of some few having been drawn in by the base,
renders it improbable that the worms first tried to draw in most of
the leaves by one or both of these methods, and that they
afterwards drew in 79 per cent. by their tips; for it is clear that
they would not have failed in drawing them in by the base or
middle.
The leaves of a foreign plant were next searched for, the blades of
which were not more pointed towards the apex than towards the base.
This proved to be the case with those of a laburnum (a hybrid
between Cytisus alpinus and laburnum) for on doubling the terminal
over the basal half, they generally fitted exactly; and when there
was any difference, the basal half was a little the narrower. It
might, therefore, have been expected that an almost equal number of
these leaves would have been drawn in by the tip and base, or a
slight excess in favour of the latter. But of 73 leaves (not
included in the first lot of 227) pulled out of worm-burrows, 63
per cent. had been drawn in by the tip; 27 per cent. by the base,
and 10 per cent. transversely. We here see that a far larger
proportion, viz., 27 per cent. were drawn in by the base than in
the case of lime leaves, the blades of which are very broad at the
base, and of which only 4 per cent. had thus been drawn in. We may
perhaps account for the fact of a still larger proportion of the
laburnum leaves not having been drawn in by the base, by worms
having acquired the habit of generally drawing in leaves by their
tips and thus avoiding the foot-stalk. For the basal margin of the
blade in many kinds of leaves forms a large angle with the foot-
stalk; and if such a leaf were drawn in by the foot-stalk, the
basal margin would come abruptly into contact with the ground on
each side of the burrow, and would render the drawing in of the
leaf very difficult.
Nevertheless worms break through their habit of avoiding the foot-
stalk, if this part offers them the most convenient means for
drawing leaves into their burrows. The leaves of the endless
hybridised varieties of the Rhododendron vary much in shape; some
are narrowest towards the base and others towards the apex. After
they have fallen off, the blade on each side of the midrib often
becomes curled up while drying, sometimes along the whole length,
sometimes chiefly at the base, sometimes towards the apex. Out of
28 fallen leaves on one bed of peat in my garden, no less than 23
were narrower in the basal quarter than in the terminal quarter of
their length; and this narrowness was chiefly due to the curling in
of the margins. Out of 36 fallen leaves on another bed, in which
different varieties of the Rhododendron grew, only 17 were narrower
towards the base than towards the apex. My son William, who first
called my attention to this case, picked up 237 fallen leaves in
his garden (where the Rhododendron grows in the natural soil) and
of these 65 per cent. could have been drawn by worms into their
burrows more easily by the base or foot-stalk than by the tip; and
this was partly due to the shape of the leaf and in a less degree
to the curling in of the margins: 27 per cent. could have been
drawn in more easily by the tip than by the base: and 8 per cent.
with about equal ease by either end. The shape of a fallen leaf
ought to be judged of before one end has been drawn into a burrow,
for after this has happened, the free end, whether it be the base
or apex, will dry more quickly than the end imbedded in the damp
ground; and the exposed margins of the free end will consequently
tend to become more curled inwards than they were when the leaf was
first seized by the worm. My son found 91 leaves which had been
dragged by worms into their burrows, though not to a great depth;
of these 66 per cent. had been drawn in by the base or foot-stalk;
and 34 per cent, by the tip. In this case, therefore, the worms
judged with a considerable degree of correctness how best to draw
the withered leaves of this foreign plant into their burrows;
notwithstanding that they had to depart from their usual habit of
avoiding the foot-stalk.
On the gravel-walks in my garden a very large number of leaves of
three species of Pinus (P. austriaca, nigricans and sylvestris) are
regularly drawn into the mouths of worm burrows. These leaves
consist of two so-called needles, which are of considerable length
in the two first and short in the last named species, and are
united to a common base; and it is by this part that they are
almost invariably drawn into the burrows. I have seen only two or
at most three exceptions to this rule with worms in a state of
nature. As the sharply pointed needles diverge a little, and as
several leaves are drawn into the same burrow, each tuft forms a
perfect chevaux de frise. On two occasions many of these tufts
were pulled up in the evening, but by the following morning fresh
leaves had been pulled in, and the burrows were again well
protected. These leaves could not be dragged into the burrows to
any depth, except by their bases, as a worm cannot seize hold of
the two needles at the same time, and if one alone were seized by
the apex, the other would be pressed against the ground and would
resist the entry of the seized one. This was manifest in the above
mentioned two or three exceptional cases. In order, therefore,
that worms should do their work well, they must drag pine-leaves
into their burrows by their bases, where the two needles are
conjoined. But how they are guided in this work is a perplexing
question.
This difficulty led my son Francis and myself to observe worms in
confinement during several nights by the aid of a dim light, while
they dragged the leaves of the above named pines into their
burrows. They moved the anterior extremities of their bodies about
the leaves, and on several occasions when they touched the sharp
end of a needle they withdrew suddenly as if pricked. But I doubt
whether they were hurt, for they are indifferent to very sharp
objects, and will swallow even rose-thorns and small splinters of
glass. It may also be doubted, whether the sharp ends of the
needles serve to tell them that this is the wrong end to seize; for
the points were cut off many leaves for a length of about one inch,
and fifty-seven of them thus treated were drawn into the burrows by
their bases, and not one by the cut-off ends. The worms in
confinement often seized the needles near the middle and drew them
towards the mouths of their burrows; and one worm tried in a
senseless manner to drag them into the burrow by bending them.
They sometimes collected many more leaves over the mouths of their
burrows (as in the case formerly mentioned of lime-leaves) than
could enter them. On other occasions, however, they behaved very
differently; for as soon as they touched the base of a pine-leaf,
this was seized, being sometimes completely engulfed in their
mouths, or a point very near the base was seized, and the leaf was
then quickly dragged or rather jerked into their burrows. It
appeared both to my son and myself as if the worms instantly
perceived as soon as they had seized a leaf in the proper manner.
Nine such cases were observed, but in one of them the worm failed
to drag the leaf into its burrow, as it was entangled by other
leaves lying near. In another case a leaf stood nearly upright
with the points of the needles partly inserted into a burrow, but
how placed there was not seen; and then the worm reared itself up
and seized the base, which was dragged into the mouth of the burrow
by bowing the whole leaf. On the other hand, after a worm had
seized the base of a leaf, this was on two occasions relinquished
from some unknown motive.
As already remarked, the habit of plugging up the mouths of the
burrows with various objects, is no doubt instinctive in worms; and
a very young one, born in one of my pots, dragged for some little
distance a Scotch-fir leaf, one needle of which was as long and
almost as thick as its own body. No species of pine is endemic in
this part of England, it is therefore incredible that the proper
manner of dragging pine-leaves into the burrows can be instinctive
with our worms. But as the worms on which the above observations
were made, were dug up beneath or near some pines, which had been
planted there about forty years, it was desirable to prove that
their actions were not instinctive. Accordingly, pine-leaves were
scattered on the ground in places far removed from any pine-tree,
and 90 of them were drawn into the burrows by their bases. Only
two were drawn in by the tips of the needles, and these were not
real exceptions, as one was drawn in for a very short distance, and
the two needles of the other cohered. Other pine-leaves were given
to worms kept in pots in a warm room, and here the result was
different; for out of 42 leaves drawn into the burrows, no less
than i6 were drawn in by the tips of the needles. These worms,
however, worked in a careless or slovenly manner; for the leaves
were often drawn in to only a small depth; sometimes they were
merely heaped over the mouths of the burrows, and sometimes none
were drawn in. I believe that this carelessness may be accounted
for either by the warmth of the air, or by its dampness, as the
pots were covered by glass plates; the worms consequently did not
care about plugging up their holes effectually. Pots tenanted by
worms and covered with a net which allowed the free entrance of
air, were left out of doors for several nights, and now 72 leaves
were all properly drawn in by their bases.
It might perhaps be inferred from the facts as yet given, that
worms somehow gain a general notion of the shape or structure of
pine-leaves, and perceive that it is necessary for them to seize
the base where the two needles are conjoined. But the following
cases make this more than doubtful. The tips of a large number of
needles of P. austriaca were cemented together with shell-lac
dissolved in alcohol, and were kept for some days, until, as I
believe, all odour or taste had been lost; and they were then
scattered on the ground where no pine-trees grew, near burrows from
which the plugging had been removed. Such leaves could have been
drawn into the burrows with equal ease by either end; and judging
from analogy and more especially from the case presently to be
given of the petioles of Clematis montana, I expected that the apex
would have been preferred. But the result was that out of 121
leaves with the tips cemented, which were drawn into burrows, 108
were drawn in by their bases, and only 13 by their tips. Thinking
that the worms might possibly perceive and dislike the smell or
taste of the shell-lac, though this was very improbable, especially
after the leaves had been left out during several nights, the tips
of the needles of many leaves were tied together with fine thread.
Of leaves thus treated 150 were drawn into burrows--123 by the base
and 27 by the tied tips; so that between four land five times as
many were drawn in by the base as by the tip. It is possible that
the short cut-off ends of the thread with which they were tied, may
have tempted the worms to drag in a larger proportional number by
the tips than when cement was used. Of the leaves with tied and
cemented tips taken together (271 in number) 85 per cent. were
drawn in by the base and 15 per cent. by the tips. We may
therefore infer that it is not the divergence of the two needles
which leads worms in a state of nature almost invariably to drag
pine-leaves into their burrows by the base. Nor can it be the
sharpness of the points of the needles which determines them; for,
as we have seen, many leaves with the points cut off were drawn in
by their bases. We are thus led to conclude, that with pine-leaves
there must be something attractive to worms in the base,
notwithstanding that few ordinary leaves are drawn in by the base
or foot-stalk.
Petioles.--We will now turn to the petioles or foot-stalks of
compound leaves, after the leaflets have fallen off. Those from
Clematis montana, which grew over a verandah, were dragged early in
January in large numbers into the burrows on an adjoining gravel-
walk, lawn, and flower-bed. These petioles vary from 2.5 to 4.5
inches in length, are rigid and of nearly uniform thickness, except
close to the base where they thicken rather abruptly, being here
about twice as thick as in any other part. The apex is somewhat
pointed, but soon withers and is then easily broken off. Of these
petioles, 314 were pulled out of burrows in the above specified
sites; and it was found that 76 per cent. had been drawn in by
their tips, and 24 per cent by their bases; so that those drawn in
by the tip were a little more than thrice as many as those drawn in
by the base. Some of those extracted from the well-beaten gravel-
walk were kept separate from the others; and of these (59 in
number) nearly five times as many had been drawn in by the tip as
by the base; whereas of those extracted from the lawn and flower-
bed, where from the soil yielding more easily, less care would be
necessary in plugging up the burrows, the proportion of those drawn
in by the tip (130) to those drawn in by the base (48) was rather
less than three to one. That these petioles had been dragged into
the burrows for plugging them up, and not for food, was manifest,
as neither end, as far as I could see, had been gnawed. As several
petioles are used to plug up the same burrow, in one case as many
as 10, and in another case as many as 15, the worms may perhaps at
first draw in a few by the thicker end so as to save labour; but
afterwards a large majority are drawn in by the pointed end, in
order to plug up the hole securely.
The fallen petioles of our native ash-tree were next observed, and
the rule with most objects, viz., that a large majority are dragged
into the burrows by the more pointed end, had not here been
followed; and this fact much surprised me at first. These petioles
vary in length from 5 to 8.5 inches; they are thick and fleshy
towards the base, whence they taper gently towards the apex, which
is a little enlarged and truncated where the terminal leaflet had
been originally attached. Under some ash-trees growing in a grass-
field, 229 petioles were pulled out of worm burrows early in
January, and of these 51.5 per cent. had been drawn in by the base,
and 48.5 per cent. by the apex. This anomaly was however readily
explained as soon as the thick basal part was examined; for in 78
out of 103 petioles, this part had been gnawed by worms, just above
the horse-shoe shaped articulation. In most cases there could be
no mistake about the gnawing; for ungnawed petioles which were
examined after being exposed to the weather for eight additional
weeks had not become more disintegrated or decayed near the base
than elsewhere. It is thus evident that the thick basal end of the
petiole is drawn in not solely for the sake of plugging up the
mouths of the burrows, but as food. Even the narrow truncated tips
of some few petioles had been gnawed; and this was the case in 6
out of 37 which were examined for this purpose. Worms, after
having drawn in and gnawed the basal end, often push the petioles
out of their burrows; and then drag in fresh ones, either by the
base for food, or by the apex for plugging up the mouth more
effectually. Thus, out of 37 petioles inserted by their tips, 5
had been previously drawn in by the base, for this part had been
gnawed. Again, I collected a handful of petioles lying loose on
the ground close to some plugged-up burrows, where the surface was
thickly strewed with other petioles which apparently had never been
touched by worms; and 14 out of 47 (i.e. nearly one-third), after
having had their bases gnawed had been pushed out of the burrows
and were now lying on the ground. From these several facts we may
conclude that worms draw in some petioles of the ash by the base to
serve as food, and others by the tip to plug up the mouths of their
burrows in the most efficient manner.
The petioles of Robinia pseudo-acacia vary from 4 or 5 to nearly 12
inches in length; they are thick close to the base before the
softer parts have rotted off, and taper much towards the upper end.
They are so flexible that I have seen some few doubled up and thus
drawn into the burrows of worms. Unfortunately these petioles were
not examined until February, by which time the softer parts had
completely rotted off, so that it was impossible to ascertain
whether worms had gnawed the bases, though this is in itself
probable. Out of 121 petioles extracted from burrows early in
February, 68 were imbedded by the base, and 53 by the apex. On
February 5 all the petioles which had been drawn into the burrows
beneath a Robinia, were pulled up; and after an interval of eleven
days, 35 petioles had been again dragged in, 19 by the base, and 16
by the apex. Taking these two lots together, 56 per cent. were
drawn in by the base, and 44 per cent. by the apex. As all the
softer parts had long ago rotted off, we may feel sure, especially
in the latter case, that none had been drawn in as food. At this
season, therefore, worms drag these petioles into their burrows
indifferently by either end, a slight preference being given to the
base. This latter fact may be accounted for by the difficulty of
plugging up a burrow with objects so extremely thin as are the
upper ends. In support of this view, it may be stated that out of
the 16 petioles which had been drawn in by their upper ends, the
more attenuated terminal portion of 7 had been previously broken
off by some accident.
Triangles of paper.--Elongated triangles were cut out of moderately
stiff writing-paper, which was rubbed with raw fat on both sides,
so as to prevent their becoming excessively limp when exposed at
night to rain and dew. The sides of all the triangles were three
inches in length, with the bases of 120 one inch, and of the other
183 half an inch in length. These latter triangles were very
narrow or much acuminated. {32} As a check on the observations
presently to be given, similar triangles in a damp state were
seized by a very narrow pair of pincers at different points and at
all inclinations with reference to the margins, and were then drawn
into a short tube of the diameter of a worm-burrow. If seized by
the apex, the triangle was drawn straight into the tube, with its
margins infolded; if seized at some little distance from the apex,
for instance at half an inch, this much was doubled back within the
tube. So it was with the base and basal angles, though in this
case the triangles offered, as might have been expected, much more
resistance to being drawn in. If seized near the middle the
triangle was doubled up, with the apex and base left sticking out
of the tube. As the sides of the triangles were three inches in
length, the result of their being drawn into a tube or into a
burrow in different ways, may be conveniently divided into three
groups: those drawn in by the apex or within an inch of it; those
drawn in by the base or within an inch of it; and those drawn in by
any point in the middle inch.
In order to see how the triangles would be seized by worms, some in
a damp state were given to worms kept in confinement. They were
seized in three different manners in the case of both the narrow
and broad triangles: viz., by the margin; by one of the three
angles, which was often completely engulfed in their mouths; and
lastly, by suction applied to any part of the flat surface. If
lines parallel to the base and an inch apart, are drawn across a
triangle with the sides three inches in length, it will be divided
into three parts of equal length. Now if worms seized
indifferently by chance any part, they would assuredly seize on the
basal part or division far oftener than on either of the two other
divisions. For the area of the basal to the apical part is as 5 to
1, so that the chance of the former being drawn into a burrow by
suction, will be as 5 to 1, compared with the apical part. The
base offers two angles and the apex only one, so that the former
would have twice as good a chance (independently of the size of the
angles) of being engulfed in a worm's mouth, as would the apex. It
should, however, be stated that the apical angle is not often
seized by worms; the margin at a little distance on either side
being preferred. I judge of this from having found in 40 out of 46
cases in which triangles had been drawn into burrows by their
apical ends, that the tip had been doubled back within the burrow
for a length of between 1/20 of an inch and 1 inch. Lastly, the
proportion between the margins of the basal and apical parts is as
3 to 2 for the broad, and 2.5 to 2 for the narrow triangles. From
these several considerations it might certainly have been expected,
supposing that worms seized hold of the triangles by chance, that a
considerably larger proportion would have been dragged into the
burrows by the basal than by the apical part; but we shall
immediately see how different was the result.
Triangles of the above specified sizes were scattered on the ground
in many places and on many successive nights near worm-burrows,
from which the leaves, petioles, twigs, &c., with which they had
been plugged, were removed. Altogether 303 triangles were drawn by
worms into their burrows: 12 others were drawn in by both ends,
but as it was impossible to judge by which end they had been first
seized, these are excluded. Of the 303, 62 per cent. had been
drawn in by the apex (using this term for all drawn in by the
apical part, one inch in length); 15 per cent. by the middle; and
23 per cent. by the basal part. If they had been drawn
indifferently by any point, the proportion for the apical, middle
and basal parts would have been 33.3 per cent. for each; but, as we
have just seen, it might have been expected that a much larger
proportion would have been drawn in by the basal than by any other
part. As the case stands, nearly three times as many were drawn in
by the apex as by the base. If we consider the broad triangles by
themselves, 59 per cent. were drawn in by the apex, 25 per cent. by
the middle, and 16 per cent. by the base. Of the narrow triangles,
65 per cent. were drawn in by the apex, 14 per cent, by the middle,
and 21 per cent. by the base; so that here those drawn in by the
apex were more than 3 times as many as those drawn in by the base.
We may therefore conclude that the manner in which the triangles
are drawn into the burrows is not a matter of chance.
In eight cases, two triangles had been drawn into the same burrow,
and in seven of these cases, one had been drawn in by the apex and
the other by the base. This again indicates that the result is not
determined by chance. Worms appear sometimes to revolve in the act
of drawing in the triangles, for five out of the whole lot had been
wound into an irregular spire round the inside of the burrow.
Worms kept in a warm room drew 63 triangles into their burrows;
but, as in the case of the pine-leaves, they worked in a rather
careless manner, for only 44 per cent. were drawn in by the apex,
22 per cent. by the middle, and 33 per cent. by the base. In five
cases, two triangles were drawn into the same burrow.
It may be suggested with much apparent probability that so large a
proportion of the triangles were drawn in by the apex, not from the
worms having selected this end as the most convenient for the
purpose, but from having first tried in other ways and failed.
This notion was countenanced by the manner in which worms in
confinement were seen to drag about and drop the triangles; but
then they were working carelessly. I did not at first perceive the
importance of this subject, but merely noticed that the bases of
those triangles which had been drawn in by the apex, were generally
clean and not crumpled. The subject was afterwards attended to
carefully. In the first place several triangles which had been
drawn in by the basal angles, or by the base, or a little above the
base, and which were thus much crumpled and dirtied, were left for
some hours in water and were then well shaken while immersed; but
neither the dirt nor the creases were thus removed. Only slight
creases could be obliterated, even by pulling the wet triangles
several times through my fingers. Owing to the slime from the
worms' bodies, the dirt was not easily washed off. We may
therefore conclude that if a triangle, before being dragged in by
the apex, had been dragged into a burrow by its base with even a
slight degree of force, the basal part would long retain its
creases and remain dirty. The condition of 89 triangles (65 narrow
and 24 broad ones), which had been drawn in by the apex, was
observed; and the bases of only 7 of them were at all creased,
being at the same time generally dirty. Of the 82 uncreased
triangles, 14 were dirty at the base; but it does not follow from
this fact that these had first been dragged towards the burrows by
their bases; for the worms sometimes covered large portions of the
triangles with slime, and these when dragged by the apex over the
ground would be dirtied; and during rainy weather, the triangles
were often dirtied over one whole side or over both sides. If the
worms had dragged the triangles to the mouths of their burrows by
their bases, as often as by their apices, and had then perceived,
without actually trying to draw them into the burrow, that the
broader end was not well adapted for this purpose--even in this
case a large proportion would probably have had their basal ends
dirtied. We may therefore infer--improbable as is the inference--
that worms are able by some means to judge which is the best end by
which to draw triangles of paper into their burrows.
The percentage results of the foregoing observations on the manner
in which worms draw various kinds of objects into the mouths of
their burrows may be abridged as follows:-
Drawn
into the Drawn in, Drawn in,
Nature of Object. burrows, by or by or
by or near near
near the the the
apex. middle. base.
Leaves of various kinds 80 11 9
- of the Lime, basal margin
of blade broad, apex
acuminated 79 17 4
- of a Laburnum, basal part of
blade as narrow as, or some-
times little narrower than
the apical part 63 10 27
- of the Rhododendron, basal
part of blade often narrower
than the apical part 34 ... 66
- of Pine-trees, consisting of
two needles arising from a
common base ... ... 100
Petioles of a Clematis,
somewhat pointed at the apex,
and blunt at the base 76 ... 24
- of the Ash, the thick basal
end often drawn in to serve
as food 48.5 ... 51.5
- of Robinia, extremely thin,
especially towards the apex,
so as to be ill-fitted for
plugging up the burrows 44 ... 56
Triangles of paper, of the
two sizes 62 15 23
- of the broad ones alone 59 25 16
- of the narrow ones alone 65 14 21
If we consider these several cases, we can hardly escape from the
conclusion that worms show some degree of intelligence in their
manner of plugging up their burrows. Each particular object is
seized in too uniform a manner, and from causes which we can
generally understand, for the result to be attributed to mere
chance. That every object has not been drawn in by its pointed
end, may be accounted for by labour having been saved through some
being inserted by their broader or thicker ends. No doubt worms
are led by instinct to plug up their burrows; and it might have
been expected that they would have been led by instinct how best to
act in each particular case, independently of intelligence. We see
how difficult it is to judge whether intelligence comes into play,
for even plants might sometimes be thought to be thus directed; for
instance when displaced leaves re-direct their upper surfaces
towards the light by extremely complicated movements and by the
shortest course. With animals, actions appearing due to
intelligence may be performed through inherited habit without any
intelligence, although aboriginally thus acquired. Or the habit
may have been acquired through the preservation and inheritance of
beneficial variations of some other habit; and in this case the new
habit will have been acquired independently of intelligence
throughout the whole course of its development. There is no a
priori improbability in worms having acquired special instincts
through either of these two latter means. Nevertheless it is
incredible that instincts should have been developed in reference
to objects, such as the leaves of petioles of foreign plants,
wholly unknown to the progenitors of the worms which act in the
described manner. Nor are their actions so unvarying or inevitable
as are most true instincts.
As worms are not guided by special instincts in each particular
case, though possessing a general instinct to plug up their
burrows, and as chance is excluded, the next most probable
conclusion seems to be that they try in many different ways to draw
in objects, and at last succeed in some one way. But it is
surprising that an animal so low in the scale as a worm should have
the capacity for acting in this manner, as many higher animals have
no such capacity. For instance, ants may be seen vainly trying to
drag an object transversely to their course, which could be easily
drawn longitudinally; though after a time they generally act in a
wiser manner, M. Fabre states {33} that a Sphex--an insect
belonging to the same highly-endowed order with ants--stocks its
nest with paralysed grass-hoppers, which are invariably dragged
into the burrow by their antennae. When these were cut off close
to the head, the Sphex seized the palpi; but when these were
likewise cut off, the attempt to drag its prey into the burrow was
given up in despair. The Sphex had not intelligence enough to
seize one of the six legs or the ovipositor of the grasshopper,
which, as M. Fabre remarks, would have served equally well. So
again, if the paralysed prey with an egg attached to it be taken
out of the cell, the Sphex after entering and finding the cell
empty, nevertheless closes it up in the usual elaborate manner.
Bees will try to escape and go on buzzing for hours on a window,
one half of which has been left open. Even a pike continued during
three months to dash and bruise itself against the glass sides of
an aquarium, in the vain attempt to seize minnows on the opposite
side. {34} A cobra-snake was seen by Mr. Layard {35} to act much
more wisely than either the pike or the Sphex; it had swallowed a
toad lying within a hole, and could not withdraw its head; the toad
was disgorged, and began to crawl away; it was again swallowed and
again disgorged; and now the snake had learnt by experience, for it
seized the toad by one of its legs and drew it out of the hole.
The instincts of even the higher animals are often followed in a
senseless or purposeless manner: the weaver-bird will
perseveringly wind threads through the bars of its cage, as if
building a nest: a squirrel will pat nuts on a wooden floor, as if
he had just buried them in the ground: a beaver will cut up logs
of wood and drag them about, though there is no water to dam up;
and so in many other cases.
Mr. Romanes, who has specially studied the minds of animals,
believes that we can safely infer intelligence, only when we see an
individual profiting by its own experience. By this test the cobra
showed some intelligence; but this would have been much plainer if
on a second occasion he had drawn a toad out of a hole by its leg.
The Sphex failed signally in this respect. Now if worms try to
drag objects into their burrows first in one way and then in
another, until they at last succeed, they profit, at least in each
particular instance, by experience.
But evidence has been advanced showing that worms do not habitually
try to draw objects into their burrows in many different ways.
Thus half-decayed lime-leaves from their flexibility could have
been drawn in by their middle or basal parts, and were thus drawn
into the burrows in considerable numbers; yet a large majority were
drawn in by or near the apex. The petioles of the Clematis could
certainly have been drawn in with equal ease by the base and apex;
yet three times and in certain cases five times as many were drawn
in by the apex as by the base. It might have been thought that the
foot-stalks of leaves would have tempted the worms as a convenient
handle; yet they are not largely used, except when the base of the
blade is narrower than the apex. A large number of the petioles of
the ash are drawn in by the base; but this part serves the worms as
food. In the case of pine-leaves worms plainly show that they at
least do not seize the leaf by chance; but their choice does not
appear to be determined by the divergence of the two needles, and
the consequent advantage or necessity of drawing them into their
burrows by the base. With respect to the triangles of paper, those
which had been drawn in by the apex rarely had their bases creased
or dirty; and this shows that the worms had not often first tried
to drag them in by this end.
If worms are able to judge, either before drawing or after having
drawn an object close to the mouths of their burrows, how best to
drag it in, they must acquire some notion of its general shape.
This they probably acquire by touching it in many places with the
anterior extremity of their bodies, which serves as a tactile
organ. It may be well to remember how perfect the sense of touch
becomes in a man when born blind and deaf, as are worms. If worms
have the power of acquiring some notion, however rude, of the shape
of an object and of their burrows, as seems to be the case, they
deserve to be called intelligent; for they then act in nearly the
same manner as would a man under similar circumstances.
To sum up, as chance does not determine the manner in which objects
are drawn into the burrows, and as the existence of specialized
instincts for each particular case cannot be admitted, the first
and most natural supposition is that worms try all methods until
they at last succeed; but many appearances are opposed to such a
supposition. One alternative alone is left, namely, that worms,
although standing low in the scale of organization, possess some
degree of intelligence. This will strike every one as very
improbable; but it may be doubted whether we know enough about the
nervous system of the lower animals to justify our natural distrust
of such a conclusion. With respect to the small size of the
cerebral ganglia, we should remember what a mass of inherited
knowledge, with some power of adapting means to an end, is crowded
into the minute brain of a worker-ant.
Means by which worms excavate their burrows.--This is effected in
two ways; by pushing away the earth on all sides, and by swallowing
it. In the former case, the worm inserts the stretched out and
attenuated anterior extremity of its body into any little crevice,
or hole; and then, as Perrier remarks, {36} the pharynx is pushed
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. Nor could any trace be discovered of the worms
having crawled down the exterior surfaces of the towers in search
of leaves; and had they done so, tracks would almost certainly have
been left on the upper part whilst it remained soft. 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.--Although worms usually live near the surface, yet they
burrow to a considerable depth during long-continued dry weather
and severe cold. In Scandinavia, according to Eisen, and in
Scotland, according to Mr. Lindsay Carnagie, the burrows run down
to a depth of from 7 to 8 feet; in North Germany, according to
Hoffmeister, from 6 to 8 feet, but Hensen says, from 3 to 6 feet.
This latter observer has seen worms frozen at a depth of 1.5 feet
beneath the surface. I have not myself had many opportunities for
observation, but I have often met with worms at depths of 3 to 4
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