The Formation of Vegetable Mould, Through the Action of Worms

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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 burrow^ 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 dtaf — 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 . 8-54 ir CONTENTa

CHAPTER n.

HABITS OF WORMS — Continued.

llanner in which worms seize objects — ^Their power of suction — The instinct of plugging up the mouths of their burrows — Stones piled over the burrows— The advantages thus gained — Intelligence shown by worms in their manner of plugging up their burrows — Various kinds of leaves and other objects thus used — Triangles of paper — Summary of reasons for believing that worms exhibit some intelligence — Means by which they excavate their buiTows, by pushing away the earth and swallowing it — Earth also swallowed fur 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 see<1s — Manner in which the castings are ejected — The collapse of old burrows — ^Distribution of worms — Tower-like castings in Bengal — Gigantic castings on the Kilgiri Mountains — Castings ejected in all oountries ..... Pag(^ 65-128

CHAPTEE ni.

I'HE AMOUNT OF FINE EAETH BROaOHT UP BY WORMS

TO THE SURFACE.

Rate at which various objects strewed on the surface of 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

CONTENTS. f

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 un 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 PAUT WHICH WORMS HAVE PLAYED IN THE BURIAL OF ANCIENT BUILDINGS.

The accumulation of rubbish on the sites of great cities independent of the action of worms — Tho burial of a Boman villa at Abinger — The floors and walls penetrated by worms — Subsidence of a modem pavement — The buried pavement at Beaulieu Abbey — Boman villas at Chedworth and Brading — ^The remains of the Boman town at Silchester — The nature of the debris by which the remains are covered— The penetrati(.>n of the tesselatoii floors and walls by worms — Subsidence uf the floors — Thickness of the mould — The old Boman city of W'roxeter — Thickness of the mould — ^Depth of the foundatioDS of some of the buildings — Conclusion

176-229

CHAPTEB V.

THE ACTION OF WORMS IN THE DENUDATION OP

THE LAND.

Flviience of the amount of denudation which the land has undergone — Subaerial denudation — The deposi* tion of dust — ^Vegetable mould, its dark colour and

Contents

fine texture largely due to the action of worms — • The disintegration of rocks by the humus-acids — Similar acids apparently geneiated within the bodies of worms — The action of thebe acids facilitated by the continued movement of the panicles of earth — ^A thick bed of mould checks the disintegration of tbo underlying soil and rocks — Particles of stone worn or triturated in the gizzards of worms — Swallowed stones serve as millstones — 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 . Page 230-268

CHAPTEK VI.

THE DENUDATION OF THE LAND — Continued,

Conclusion

Summary of the part which worms havo played in th* history of the world — Their aid in the dibintegration of rocks — In the denudation of the land — Ic the preservation of ancient remains — In the preparation, of the soil for the growth of plants — Mental pow<*rs of worms — Conclusion . 305-313

XHB

FORMATION OF VEGETABLE MOULD,

THROUGH THE ACTION OP WORMS, WITH OBSERVATIONS ON THEIR HABITa

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 aiM a few inches in thickness. In different dratricts 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

2 INTRODUCTION.

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 lex non curat," does not apply to science. Even Elie de Beaumont, who generally undervalues small agencies and their accumulated effects, remarks* " la " couche tres-mince de la terre vegetale est un " monument d'une haute antiquite, et, par le " fait de sa permanence, un objet digne d'oc- " cuper le geologue, et capable de lui fournir " des remarques interessantes." Although the superficial layer of vegetable mould as a whole no doubt is of the highest antiquity, yet in regard to its permanence, we shall hereafter &em reason to believe that its component partlclePare in most cases rq^Qved at not a very slow rate, and are replaced by others due to the disintegration of ^e underlying materials.

As I was led to keep in my study during many months worms in po^ filled with earth,

• * Legons de G^logie Pratique,' torn. i. 1845, p. 140.

INTRODUCTION. 8

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 G-eological Society of London,* " On the Formation of Mould," in which it was shown that small fragments of burnt marl, cinders, &c.j 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 €ue, as was first suggested to me by Mr. W^gwood of Maer Hall in Stafibrdshire, 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

• * Transactions Geolog. Soc' vol. v. p. 605. Read November 1, 1837.

I INTRODUCTION.

spread out and cover up any object left on the surface. I was thus led to conclude that all the vegetable mould over the whole country has passed many times through, and will again pass many times through, the intestinal canals of worms. Hence the term "animal mould*' would be in some respects more appropriate than that commonly used of " vegetable mould."

Ten years after the publication of my paper, M. D' Archiac, evidently influenced by the doctrines of Elie de Beaumont, wrote about my "singuliere theorie," and objected that it could apply only to **les prairies basses et humides;'* and that "les terres labourees, les bois, les prairies elevees, n'apportent aucune preuve a Tappui de cette maniere de voir.'* * But M. D'Archiac must have thus argued from inner conscicMisness and not from observation, for wormsvftbound 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. Yon Hensen estimates that there are

* * Histoire des progrb de la G^logie/ torn. L 1847, p. 22i»

INTRODUCTION. 6

about twice as many worms in gardens as in corn-fields.* With respect to ** prairies felevees," I do not know how it may be in Prance, but nowhere in England have I seen the ground so thickly covered with castings as on commons, at a height of several hundred feet above the sea. In woods again, if the loose leaves in autumn are removed, the whole surface will be found strewed with castings. Dr. King, the superintendent of the Botanic Garden in Calcutta, to whose kindness I am indebted for many observations on earth-worms, informs me that he found, near Nancy in France, the bottom of the State forests covered over many acres with a spongy layer, composed of dead leaves and innumerable worm-castings. He there heard the Professor of " Amenagement des Forets '* lecturing to his pupils, and pointing out this case as a ** beautiful example of the natural cultiva- "tion of the soil; for year after year the " thrown-up castings cover the dead leaves ; "the result being a rich humus of great ** thickness.**

* 'Zeitschrift filr wissenschaft. Zoologi),' B. zxviii. 187T p. 861.

6 INTRODUCTION.

In the year 1869, Mr, Fish* 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 anothier 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 Von Hensen,

• « Gardeners' Chronicle,' April 17, 1869, p. 418.

IKTRODUTIOK. 7

already alluded to, which appeared in 1877. 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.

8 HABITS OP WORMS, Ciiap. L

CHAPTER L

I^aiure 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 — Oalciferous glands, structure of — Calcareous concretions formed in the anterior pair of glands — ^The calcareous matter primarily an excretion, but sesondarily 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

Chap, L SITES EmABITED, 9

Eisen ;* 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.f

Earth-worms abound in England in many different static^^^Their castings may be seen in extraoll(|pt^^ numbers on commons and chalk-downs, |0 as almost to cover the whole surface, wliere 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

I 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.* 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 ji

these paths, where they were much inclined ;

* There is even some reason to believe that pressure is actually favourable to the growth of grasses, for Professor Buckman, who made many observations on their growth in the experimental gardens of the Royal Agricultural College, remarks (* Gardeners' Chronicle,' 1854, p. 619) ; " Another circumstance in the cultivation of grasses in the separate form or small patches, is the impossibility of rolling or treading them firmly, withonl which no pasture can continue good.'' i

Chap. I. SITES INHABTTEU 11

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 cfafew 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 gi'assy 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 2

Park, where the ground beneath was bare of all vegetation, not a single casting could be f jund 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-c:astings 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 altitade on the Nilgiii 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

Ohap. 1. NOCTUBNAL. 13

other hand he kept several large worms alive for nearly four months, completely submerged in water.* 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.f During the day they remain in their burrows, except at tlie 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

* I shall have occasion often to refer to M. Perricr's admirable memoir, 'Organisation des Lombriciens terrestres' in * Archives de Zoolcg. exp^r.' torn. iii. 1874, p. 372. C. F. Morren (*De Lumbrici terrestris,' 1829, p. 14) found that worms endured immersion for fifteen to twenty days in summer, but that in winter they died when thus treated.

t Morren, 'De Lumbrici tevestris,' &c., 1829, p. 67.

14 HABITS OF WOllMS. Chap. L

individuals, which are generally affected by the parasitic Iftrvas 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 thev 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, aftei there has been heavy rain, the film of mud or of very fine sand over gravel-walks is often

Chap. L WANDER FROM THEIR BURROWS. 15

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. 1 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 * that worms often lie for hours almost motionless close beneath the mouths of their burrows. I have occasionally aoticed the same fact with worms kept in

* * De Liimbrici terrestris/ &c., p. 14.

'i

16 HABITS OF WORMS. Chap. I

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.

Sti^ucture. — A few remarks must be made on this subject. The body of a hirge worm iv

Chap. I. THEIR STRUCTURE. 17

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 aflSxed 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 carbonat/e 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 cf the species, the oesophagus is enlarged into a crop in front of the gizzard. This

HABITS OP WORMS.

Chap. L i

Montb.

Pharynx.

(Esophagus.

latter organ is lined with a smooth thick

chitinous membrane, and is surrounded by weak longitudinal, but by 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 -^^ ^o a little more than the ^V ^^^^ ^^ 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 thev serve, like mill-stones, to triturate their food. The gizzard opens into the intestine,

Calclferous glands.

(Esophagus.

Oop.

Gizzard.

dpper part of intestine.

Fig . 1.

Diagram of the alimentary canal of an earth-worm (Lumhricus), copied fromi Ray Lankester in * Quart.

Joum. of Microscop.

Soc.* vol. XV. N.S. pi. vii.

I

Chap. L THEIR SENSES. 19

which runs in a straight course to the vent at the posterior end of the body. The intestine presents a remarkable structure, the typhosolis, or, as the old anatomists called it, an intestine within an intestine ; and Claparede* 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 circulator]^ 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

• Histolog. Untersuchungen iiber die Regenwurmer. *Zei1r ichrift fiir wissenschaft. Zoologie/ B. xix., 1869, p. 611.

20 HABITS OP WORMS. Cuap. L

persons have found no difiSculty in observing worms at night by the same means.*

Hoffmeister, however, states f 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 veiy 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 Qo difference in the result. When they were

* For instance, Mr. Bridgman and Mr. Newman ('The Zoologist/ vol. vii. 1849, p. 2576), and some friends who observed worms for me.

t ' Familie der Begenwiirmer,' 1845, p. 18.

Chap. I. THEIR SENSES. 21

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, olher 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