The formation of vegetable mould through the action of worms : with observations on their habits
Darwin, Charles, 1809-1882
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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 atten- tion— 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 — Cal- careous concretions formed in the anterior pair of glands — The calcareous matter primarily an excre- tion, but secondarily serves to neutralise the acids generated during the digestive process . 8-56 30667 iv CONTENTS. CHAPTER II. habits of worms — continued. Manner in which worms seize objects — Their power of suction — The instinct of plugging up the mouths of their burrows — Stones piled over the burrows — The advantages thus gained — Intelligence shown by worms in their manner of plugging up their burrows — Various kinds of leaves and other objects thus used — Triangles of paper — Summary of reasons for believing that worms exhibit some intelligence — Means by which, they excavate their burrows, by pushing away the earth and swallowing it — Earth also swallowed for the nutritious matter which it contains — Depth to which worms burrow, and the construction of their burrows — Burrows lined with castings, and in the upper part with leaves — The lowest part paved with little stones or seeds — Manner in which the castings are ejected — The collapse of old burrows — Distribution of worms — Tower-like castings in Bengal — Gigantic castings on the Kilgiri Mountains — Castings ejected in all countries ..... Pages 57-130 CHAPTER III. THE AMOUNT OF FINE EARTH BROUGHT UP BY WORMS v 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. v 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. Pages 131-177 CHAPTEE IV. THE PART 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 — 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 — Eoman villas at Chedworth and Brading — The remains of the Eoman town at Silchester — The nature of the debris 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 Eoman city of Wroxeter — Thickness of the mould — Depth of the foundations of some of the buildings — Conclusion. 178-231 CHAPTEE Y. THE ACTION OF WORMS IN THE DENUDATION OF THE LAND. Evidence of the amount of denudation which the land has undergone — Sub-aerial denudation — The deposi- tion of dust — Vegetable mould, its dark colour and vi CONTENTS. 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 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 . Pages 232-261 CHAPTER VI. THE DENUDATION OF THE LAND — Continued. Denudation aided by recently ejected castings flowing 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 . . 262-307 CONTENTS. vii CHAPTER VII. CONCLUSION. Summary of the part which worms have played in the history of the world — Their aid in the disintegra- tion of rocks — In the denudation of the land — In the preservation of ancient remains — In the pre- paration of the soil for the growth of plants — Mental powers of worms — Conclusion. Pages 308-316 Index 317-328 THE FORMATION OF VEGETABLE MOULD. THROUGH THE ACTION OF WORMS, WITH OBSERVATIONS ON THEIR HABITS. 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 com- posed is one of its chief characteristic features ; and this may be well observed in any gravelly country, where a recently-ploughed field B 2 INTRODUCTION. immediately adjoins one which has long re- mained 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 ge'ologue, 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 here- after 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, * ' Lecons de Geologie Pratique,' torn. i. 1845, p. 140. INTRODUCTION. 3 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,* " 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 * ' Transactions Geolog. Soc' vol. v. p. 505. Read Novem- ber 1, 1837. B 2 4 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 coun- try 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 doc- trines of Elie de Beaumont, wrote about my "singuliere theorie," and objected that it could apply only to "les prairies basses et humides;" and that "les terres labourees, les bois, les prairies elevees, n'apportent aucune preuve a l'appui de cette maniere de voir." * 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 * ' Histoire des progrfes de la Geologie,' torn. i. 1847, p. 224. INTRODUCTION. 5 about twice as many worms in gardens as in corn-fields.* With respect to " prairies elevees," I do not know how it may be in France, but nowhere in England have I seen the ground so thickly covered with castings as on commons, at a height of several hundred feet above the sea. In woods again, if the loose leaves in autumn are removed, the whole surface will be found strewed with castings. Dr. King, the superintendent of the Botanic Garden in Calcutta, to whose kindness I am indebted for many observations on earth- worms, informs me that he found, near Nancy in France, the bottom of the State forests covered over many acres with a spongy layer, composed of dead leaves and innumerable worm-castings. He there heard the Professor of " Amenagement des Forets " lecturing to his pupils, and pointing out this case as a u 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 fur wissenschaft. Zoologie,' B. xxviii. 1877, p. 361. 6 INTRODUCTION. In the year 1869, Mr. Fish* rejected my conclusions with respect to the part which worms have played in the formation of veget- able 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 cast- ings 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 ob- servations have been rendered almost super- fluous by an admirable paper by Hensen, * ' Gardeners' Chronicle,' April 17, 1869, p. 418. INTRODUCTION. 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. 9 8 HABITS OF WORMS, Chap. I. 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 Scan- dinavia there are eight species, according to Chap. I. SITES INHABITED. 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 cast- ings. Hoffmeister says that the species in Germany are not well known, but gives the same number as Eisen, together with some strongly marked varieties.*)* 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 * ' Bidrag till Skandinaviens Oligochajtfauna,' 1871. t ' Die bis jetzt bekannten Arten aus der Familie der Eegen- wtirmer ' 1845. 10 HABITS OF WOKMS. Chap. 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 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 Koyal Agricultural College, remarks (' Gardeners' Chronicle,' 1854, p. 619) : " Another circumstance in the cultiva- tion of grasses in the separate form or small patches, is the impossibility of rolling or treading them firmly, without which no pasture can continue good." Chap. I. SITES INHABITED. 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 thick- ness 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 innu- merable roots of the trees ; for such places may be seen covered with castings after the heavy autumnal rains. Although most cop- pices and woods support many worms, yet in a forest of tall and ancient beech-trees in Knole 12 HABITS OF WORMS. Chap. I. 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 abun- dant on some grass-covered glades and in- dentations 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 sub- jacent 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 terres- trial 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 ex- posure to the dry air of a room for only a single night was fatal to them. On the Chap. I. NOCTURNAL. 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 consider- able 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, j 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 * I shall have occasion often to refer to M. Perrier's admirable memoir, ' Organisation des Lombriciens terrestres ' in ' Archives de Zoolog. exper.' torn. iii. 1874, p. 372. C. F. Morren (' De Lumbrici terrestris Hist. Nat.' 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 terrestris Hist. Nat.' &c, 1829, p. 67. 14 HABITS OF WOEMS. Chap. I. individuals, which are generally affected by the parasitic larvae of a fly, must also be ex- cepted, as they wander about during* the day and die on the surface. After heavy rain succeeding dry weather, an astonishing num- ber 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 Sel- borne 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 Chap. I. 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. 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 * 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 * ■ De Lumbrici terrestris Hist. Nat.' &c, p. 14. 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 barrows 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 cer- tain 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, es- pecially in the morning ; and we shall here- after 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 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 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 ac- companying diagram (Fig. 1) which is pushed forwards when the animal eats, and this part corresponds, according to Perrier, with the pro- trudable 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 remark- able, 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 18 HABITS OF WORMS. Chap. I. latter organ is fiji Mouth. Pharynx. «« H (Esophagus. Calciferous glands. (Esophagus. Crop. Gizzard. Upper part of in- testine. Fig. 1. Diagram of the alimen- tary canal of an earth- worm (Lumbricus), copied from Ray Lan- kester in ' Quart. Journ. of Microscop. Sc* vol. xv. N.S. pi. vii. lined with a smooth thick chitinous membrane, and is surrounded by weak longitudinal, but power- ful 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 ^ to a little more than the ^ inch in diameter, may generally be found in their gizzards and intestines. As it is certain that worms swal- low many little stones, in- dependently of those swal- lowed while excavating their burrows, it is prob- able that thev serve, like mill-stones, to triturate their food. The gizzard opens into the intestine, Chap. I. THEIR SENSES. 19 which runs in a straight course to the vent at the posterior end of the body. The intes- tine presents a remarkable structure, the typhi osolis, or, as the old anatomists called it, an intestine within an intestine ; and Cla- parede* 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 in- sensible to light; for those kept in confine- ment 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. 'Zeit- schrift fur wissenschaft. Zoologie,' B. xk., 1869, p. 611. c 2 20 HABITS OF WORMS. Chap. I. persons have found no difficulty 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 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 in- tercepted 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 * For instance, Mr. Bridgrnan and Mr. Newman ('The Zoologist/ vol. vii. 1849, p. 2576), and some friends who observed worms for me. | ' Familie der Regenwumier,' 1845, p. 18. Chap. I. THEIK 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. Some- times, however, they behaved very differ- ently, 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 con- centrated by means of a large lens on the anterior extremity, they generally withdrew instantly ; but this concentrated light failed 22 HABITS OF WORMS. Chap. I. 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. it 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 dif- ferent manner in which they were affected on Chap. I. THEIR SENSES. 23 different occasions might be explained, either by the degree of extension of their skin and its consequent transparency, or by some particular incidence 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 24 HABITS OF WOEMS. Chap. I. 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 com ting, 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- Chap. I. THEIR SENSES. 25 fetched ; for we thus attribute to the worm attention and some mental power, neverthe- less 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 win- dow, 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 26 HABITS OF WOEMS. Chap. I. 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 with- drew 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 temper- ature, 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 re- peatedly 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 undula- tions in the air audible by us, they are Chap. I. THEIR SENSES. 27 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 Gr 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 sensi- tive to such jars than to the vibrations of the 28 HABITS OF WORMS. Chap. I. 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 Chap. I. THEIR SENSES. 29 devoured. Nevertheless, worms do not in- variably 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 ex- tended anterior extremity of its body from side to side in all directions, apparently as an organ of touch ; and there is some reasou 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, 30 HABITS OF WOKMS. Chap. I. 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. 