[Picture: Book cover]
THE FORMATION OF
VEGETABLE MOULD
THROUGH THE ACTION OF WORMS
WITH OBSERVATIONS ON THEIR HABITS.
BY CHARLES DARWIN, LL.D., F.R.S.
THIRTEENTH THOUSAND
WITH ILLUSTRATIONS
* * * * *
LONDON
JOHN MURRAY, ALBEMARLE STREET
1904
* * * * *
PRINTED BY
WILLIAM CLOWES AND SONS, LIMITED,
LONDON AND BECCLES.
* * * * *
CONTENTS.
INTRODUCTION Page 1–6
CHAPTER I.
HABITS OF WORMS.
Nature of the sites inhabited—Can live long under 7–15
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.
CHAPTER II.
HABITS OF WORMS—_continued_.
Manner in which worms seize objects—Their power of 52–120
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.
CHAPTER III.
THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS TO THE SURFACE.
Rate at which various objects strewed on the surface of 121–163
grass-fields are covered up by the castings of
worms—The burial of a paved path—The slow subsidence of
great stones left on the surface—The number of worms
which live within a given space—The weight of earth
ejected from a burrow, and from all the burrows within
a given space—The thickness of the layer of mould which
the castings on a given space would form within a given
time if uniformly spread out—The slow rate at which
mould can increase to a great thickness—Conclusion.
CHAPTER IV.
THE PART WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.
The accumulation of rubbish on the sites of great 164–208
cities independent of the action of worms—The burial of
a Roman villa at Abinger—The floors and walls
penetrated by worms—Subsidence of a modern pavement—The
buried pavement at Beaulieu Abbey—Roman villas at
Chedworth and Brading—The remains of the Roman town at
Silchester—The nature of the débris by which the
remains are covered—The penetration of the tesselated
floors and walls by worms—Subsidence of the
floors—Thickness of the mould—The old Roman city of
Wroxeter—Thickness of the mould—Depth of the
foundations of some of the Buildings—Conclusion.
CHAPTER V.
THE ACTION OF WORMS IN THE DENUDATION OF THE LAND.
Evidence of the amount of denudation which the land has 209–236
undergone—Sub-aerial denudation—The deposition of
dust—Vegetable mould, its dark colour and fine texture
largely due to the action of worms—The disintegration
of rocks by the humus-acids—Similar acids apparently
generated within the bodies of worms—The action of
these acids facilitated by the continued movement of
the particles of earth—A thick bed of mould checks the
disintegration of the underlying soil and rocks.
Particles of stone worn or triturated in the gizzards
of worms—Swallowed stones serve as mill-stones—The
levigated state of the castings—Fragments of brick in
the castings over ancient buildings well rounded. The
triturating power of worms not quite insignificant
under a geological point of view.
CHAPTER VI.
THE DENUDATION OF THE LAND—_continued_.
Denudation aided by recently ejected castings flowing 237–279
down inclined grass-covered surfaces—The amount of
earth which annually flows downwards—The effect of
tropical rain on worm castings—The finest particles of
earth washed completely away from castings—The
disintegration of dried castings into pellets, and
their rolling down inclined surfaces—The formation of
little ledges on hill-sides, in part due to the
accumulation of disintegrated castings—Castings blown
to leeward over level land—An attempt to estimate the
amount thus blown—The degradation of ancient
encampments and tumuli—The preservation of the crowns
and furrows on land anciently ploughed—The formation
and amount of mould over the Chalk formation.
CHAPTER VII.
CONCLUSION.
Summary of the part which worms have played in the 280–288
history of the world—Their aid in the disintegration of
rocks—In the denudation of the land—In the preservation
of ancient remains—In the preparation of the soil for
the growth of plants—Mental powers of worms—Conclusion.
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 Élie de
Beaumont, who generally undervalues small agencies and their accumulated
effects, remarks: {2} “La couche très-mince de la terre végétale est un
monument d’une haute antiquité, et, par le fait de sa permanence, un
objet digne d’occuper le géologue, et capable de lui fournir des
remarques intéressantes.” 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, {3} “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 Élie de Beaumont, wrote about my
“singulière théorie,” and objected that it could apply only to “les
prairies basses et humides;” and that “les terres labourées, les bois,
les prairies élevées, n’apportent aucune preuve à l’appui de cette
manière de voir.” {4a} 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. {4b} With respect to “prairies élevées,” 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 “Aménagement des Forêts” 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_ 10_th_, 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;
{8a} 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. {8b}
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.
{9b} 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. {11} 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. {12} 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 larvæ 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 {14} 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 œsophagus, 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 œsophagus 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 Claparède {17} 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.
[Picture: Fig. 1: Diagram of the alimentary canal of an earth-worm. Fig.
2: Tower-like casting from near Nice]
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. {18a}
Hoffmeister, however, states {18b} 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 ¼ 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. {31}
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. {32} 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 Perichæta 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 {33} 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.
Léon Fredericq states {34} 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. {35a} 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. {35b}
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, {40} 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 Dionæa; 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 Urochæta and some other genera. {41a}
The two posterior pairs are formed by lamellæ, which, according to
Claparède, are diverticula from the œsophagus. {41b} These lamellæ 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 _μ_. They contain in
their centres a little excessively fine granular matter; but they look so
like oil globules that Claparède 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 lamellæ; but I have seen only one
case, and Claparède 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 1½ 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.
Claparède 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, lamellæ 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
lamellæ. After the formation and expulsion of a large concretion, new
lamellæ 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 lamellæ.
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 lamellæ 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 lamellæ; 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. {46} 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.” {49}
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.” {50}
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. Claparède 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.