For those without javascript use this menu
THE FORMATION OF VEGETABLE MOULD
THROUGH THE ACTION OF WORMS
WITH OBSERVATIONS ON THEIR HABITS.
by Charles Darwin
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.
|
