The Formation of Vegetable Mould through the Action of Worms

depth, and was of so poor a nature that weeds could not grow onit. It is therefore highly improbable that it should have been swallowed by the worms as food. Again in a field near my house the castings frequently consist of almost pure chalk, which lies at only a little depth beneath the surface ; and here again it is very improbable that the chalk should have been swallowed for the sake of the very little organic matter which could have percolated into it from the poor over- lying pasture. Lastly, a casting thrown up through the concrete and decayed mortar between the tiles, with which the now ruined aisle of Beaulieu Abbey had formerly been paved, was washed, so that the coarser matter alone was left. This consisted of grains of quartz, micaceous slate, other rocks, and bricks or tiles, many of them from , to yy inch in diameter. No one will suppose that these grains were swallowed as food, yet they formed more than half of the casting, for they weighed 19 grains, the whole cast- ing having weighed 33 grains. Whenever a worm burrows to a depth of some feet in undisturbed compact ground, it must form its

passage by swallowing the earth; for it 1s incredible that the ground could yield on all sides to the pressure of the pharynx when pushed forwards within the worm’s body. That worms swallow a larger quantity of earth for the sake of extracting any nutritious matter which it may contain than for making their burrows, appears to me certain. But as this old belief has been doubted by so high an authority as Claparede, evidence in its favour must be given in some detail. There is no @ priori improbability in such a belief, for besides other annelids, especially the Arenicola marina, which throws up such a profusion of castings on our tidal sands, and which it is believed thus subsists, there are animals belonging to the most distinct classes, which do not burrow, but habitually swallow large quantities of sand; namely the mollus- ean Onchidium and many Hchinoderms.*

If earth were swallowed only when worms deepened their burrows or made new ones, castings would be thrown up only occasion- ally ; but in many places fresh castings may * I state this on the authority of Semper, ‘Reisen im Archipel der Philippinen,” Th. ii. 1877, p. 30. be seen every morning, and the amount of earth ejected from the same burrow on successive days is large. Yet worms do not burrow to a great depth, except when the weather is very dry or intensely cold. On my lawn the black vegetable mould is only about 5 inches in thickness, and overlies light- coloured or reddish clayey soil: now when castings are thrown up in the greatest profusion, only a small proportion are light coloured, and it is incredible that the worms should daily make fresh burrows in every direction in the thin superficial layer of dark-coloured humus, unless they obtained nutriment of some kind from it. I have observed a strictly analogous case in a field near my house where bright red clay lay close beneath the surface. Again on one part of the Downs near Winchester the vegetable mould overlying the chalk was found to be only from 3 to 4 inches in thickness; and the many castings here ejected were as black as ink and did not effervesce with acids; so that the worms must have confined themselves to this thin superficial layer of mould, of which large quantities were daily swallowed. In

another place at no great distance the castings were white; and why the worms should have burrowed into the chalk in some places and not in others, I am unable to conjecture. Two great piles of leaves had been left to decay in my grounds, and months after their removal, the bare surface, several yards in diameter, was so thickly covered during several months with castings that they formed an almost continuous layer; and the large number of worms which lived here must have subsisted during these months on nutritious matter contained in the black earth.

The lowest layer from another pile of de- cayed leaves mixed with some earth was ex- amined under a high power, and the number of spores of various shapes and sizes which it contained was astonishingly great; and these crushed in the gizzards of worms may largely aid in supporting them. When- ever castings are thrown up in the greatest number, few or no leaves are drawn into the burrows; for instance the turf along a hedge- row, about 200 yards in length, was daily observed in the autumn during several weeks,

and every morning many fresh castings were seen ; but nota single leaf was drawn into these burrows. These castings from their blackness and from the nature of the subsoil could not have been brought up from a greater depth than 6 or 8 inches. On what could these worms have subsisted during this whole time, if not on matter contained in the black earth? On the other hand, whenever a large number of leaves are drawn into the burrows, the: worms seem to subsist chiefly on them, for few earth-castings are then. ejected on the surface. This difference in the behaviour of worms at different times, perhaps explains a statement by Claparede, namely, that triturated leaves and earth are always found in distinct parts of their intestines,

Worms sometimes abound in places where they can rarely or never obtain dead or living leaves; for instance, beneath the pave- ment in well-swept courtyards, into which leaves are only occasionally blown. My son Horace examined a house, one corner of which had subsided; and he found here in the cellar, which was extremely damp, many small worm-castings thrown up between the stones with which the cellar was paved ; and in this case it is improbable that the worms could ever have obtained leaves.

But the best evidence, known to me, of worms subsisting for at least considerable periods of time solely on the organic matter contained in earth, is afforded by some facts communicated to me by Dr. King. Near Nice large castings abound in extraordinary “numbers, so that 5 or 6 were often found within the space of a square foot. They consist of fine, pale-coloured earth, containing calcareous matter, which after having passed through the bodies of worms and being dried, coheres with considerable force. I have reason to believe that these castings had been formed by species of Perichzta, which have been naturalised here from the East.* Thev

* Dr. King gave me some worms collected near Nice, which, as he believes, had constructed these castings. They were sent to M. Perrier, who with great kindness examined and named them for me: they consisted of Pericheta affinis, a native of Cochin China and of the Philippines; P. Luzonica, a native of Luzon in the Philippines ; and P. Houlleti, which lives near Calcutta, M. Perrier informs me that species of Pericheta have been natural- ized in the gardens near Montpellier and in Algiers. Before I had any reason to suspect that the tower-like castings from Nice had been formed by worms not endemic in the country, I wag

rise like towers (see Fig. 2), with their sum- mits often a little broader than their bases, Tower-like casting from near Nice, constructed of earth, voided probably by a species of Perichata : of natural size, copied from a photograph. sometimes to a height of above 3 and often toa height of 24 inches. The tallest. of those greatly surprised to see how closely they resembled castings sent to me from near Calcutta, where it is known that species of Pericheta abound.

which were measured was 3°53 inch in height and 1 in diameter. A small cylindrical pas- sage runs up the centre of each tower, through which the worm ascends to eject the earth which it has swallowed, and thus to add to its height. A structure of this kind would not allow leaves being easily dragged from the surrounding ground into the bur- rows; and Dr. King, who looked carefully, never saw even a fragment of a leaf thus drawn in. Nor could any trace be discovered of the worms having crawled down the ex- terior surfaces of the towers in search of leaves; and had they done so, tracks would almost certainly have been left on the upper part whilst it remained soft. It does not, however, follow that these worms do not draw leaves into their burrows during some other season of the year, at which time they would not build up their towers.

From the several foregoing cases, it can hardly be doubted that worms swallow earth, not only for the sake of making their bur- rows, but for obtaining food. Hensen, how- ever, concludes from his analyses of humus that worms probably could not live on ordinary vegetable mould, though he admits that they might be nourished to some extent by leaf-mould.* But we have seen that worms eagerly devour raw meat, fat, and dead worms ; and ordinary mould can hardly fail to contain many ova, larve, and small living or dead creatures, spores of crypto- gamic plants, and micrococci, such as those which give rise to saltpetre. These various organisms, together with some cellulose from any leaves and roots not utterly decayed, might well account for such large quantities of mould being swallowed by worms. It may be worth while here to recall the fact that certain species of Utricularia, which grow in damp places in the tropics, possess bladders beautifully constructed for catching minute subterranean animals; and these traps would not have been developed unless many small animals inhabited such soil.

The depth to which worms penetrate, and the construction of their burrows. — Although worms usually live near the surface, yet they burrow to a considerable depth during long: continued dry weather and severe cold. In Scandinavia, according to Eisen, and in Scot- land, according to Mr. Lindsay Carnagie, the burrows run down to a depth of from 7 to 8 feet ; in North Germany, according to Hoff- meister, from 6 to 8 feet, but Hensen says, from 8 to 6 feet. This latter observer has seen worms frozen at a depth of 14 feet beneath the surface. I have not myself had many opportunities for observation, but I have often met with worms at depths of 3 to 4 feet. In a bed of fine sand overlying the chalk, which had never been disturbed, a worm was cut into two at 55 inches, and another was found here in December at the bottom of its burrow, at 61 inches beneath the surface. Lastly, in earth near an old Roman Villa, which had not been disturbed for many centuries, a worm was met with at a depth of 66 inches; and this was in the middle of August.

The burrows run down perpendicularly, or more commonly a little obliquely. They are said sometimes to branch, but as far as I have seen this does not occur, except in recently dug ground and near the surface. They are generally, or as I believe invariably, lined with a thin layer of fine, dark-coloured earth voided by the worms; so that they must at first be made a little wider than their ultimate diameter. I have seen several burrows in undisturbed sand thus lined at a depth of 4 ft. 6 in.3; and others close to the surface thus lined in recently dug ground. The walls of fresh burrows are often dotted with little globular pellets of voided earth, still soft and viscid; and these, as it appears, are spread out on all sides by the worm as it travels up or down its burrow. The lining thus formed becomes very com pact and smooth when nearly dry, and closely fits the worm’s body. The minute reflexed bristles which project in rows on all sides {rom the body, thus have excellent points of support ; and the burrow is rendered well adapted for the rapid movement of the animal. The lining appears also to strengthen the walls, and perhaps saves the worm’s body from being scratched. I think so because several burrows which passed through a layer of sifted coal-cinders, spread over turf to a

unusual thickness. In this case the worms, judging from the castings, had pushed the cinders away on all sides and had_ not swallowed any of them. In another place, burrows similarly lined, passed through a layer of coarse coal-cinders, 33 inches in thickness. We thus see that the burrows are not mere excavations, but may rather be compared with tunnels lined with cement. The mouths of the burrow are in addition often lined with leaves; and this is an instinct distinct from that of plugging them up, and does not appear to have been hitherto noticed. Many leaves of the Scotch-fir or pine (Pinus sylvestris) were given to worms kept in con- finement in two pots; and when after several weeks the earth was carefully broken up, the upper parts of three oblique burrows were found surrouuded for lengths of 7, 4, and é4 inches with pine-leaves, together with fragments of other leaves which had been given the worms as food. Glass beads and bits of tile, which had been strewed on the surface of the soil, were stuck into the inter- stices between the pine-leaves; and _ these interstices were likewise plastered with the

viscid castings voided by the worms. The structures thus formed cohered so well, that I succeeded in removing one with only a little earth adhering to it. It consisted of a sli ghtly curved cylindrical case, the interior of which could be seen through holes in the sides and at either end. The pine-leaves had all been drawn in by their bases; and the sharp points of the needles had been pressed into the lining of voided earth. Had this not been effectually done, the sharp points would have prevented the retreat of the worms into their burrows; and these structures would have resembled traps armed with converging points of wire, rendering the ingress of an animal easy and its egress difficult or ime possible. The skill shown by these worms is noteworthy and is the more remarkable, as the Scotch pine is not a native of this district

After having examined these burrows made by worms in confinement, I looked at those in a flower-bed near some Scotch pines. These had all been plugged up in the ordinary manner with the leaves of this tree, drawn in for a length of from 1 to 1§ inch; but the mouths of many of them were likew?se lined with them, mingled with fragments of other kinds of leaves, drawn in to a depth of 4 or 5 inches. Worms often remain, as formerly stated, for a long time close to the mouths of their burrows, apparently for warmth; and the basket-like structures formed of leaves would keep their bodies from coming into close contact with the cold damp earth. That they habitually rested on the pine-leaves, was rendered probable by their clean and almost polished surfaces.

The burrows which run far down into the ground, generally, or at least often, terminate in a little enlargement or chamber. Here, ac- cording to Hoffmeister, one or several worms pass the winter rolled up into a ball. Mr. Lindsay Carnagie informed me (1838) that he had examined many burrows over a stone- quarry in Scotland, where the overlying boulder-clay and mould had recently been cleared away, and a little vertical cliff thus left. In several cases the same burrow was a little enlarged at two or three .points one beneath the other; and all the burrows ter- minated in a rather large chamber, at a depth of 7 or 8 feet from the surface. These cham.

bers contained many small sharp bits of stone and husks of flax-seeds. They must also have contained living seeds, for on the follow- ing spring Mr. Carnagie saw grass-plants sprouting out of some of the intersected chambers. I found at Abinger in Surrey two burrows terminating in similar chambers at a depth of 36 and 41 inches, and these were lined or paved with little pebbles, about as large as mustard seeds; and in one of the chambers there was a decayed _oat-grain, with its husk. Hensen likewise states that the bottoms of the burrows are lined with little stones; and where these could not be procured, seeds, apparently of the pear, had been used, as many as fifteen having been carried down into a single burrow, one of which had germinated.* We thus see how easily a botanist might be deceived who wished to learn how long deeply buried seeds remained alive, if he were to collect earth from a considerable depth, on the supposition that it could contain only seeds which had long lain buried. It is probable that the little stones,

as well as the seeds, are carried down from the surface by being swallowed; for a sur- prising number of glass beads, bits of tile and of glass were certainly thus carried down by worms kept in pots; but some may have been carried down within their mouths. The sole conjecture which I can form why worms line their winter-quarters with little stones and seeds, is to prevent their closely coiled-up bodies from coming into close centact with the surrounding cold soil; and such contact would perhaps interfere with their respiration which is effected by the skin alone.

A worm after swallowing earth, whether for making its burrow or for food, soon comes to the surface to empty its body. The ejected earth is thoroughly mingled with the intestinal secretions, and is thus rendered viscid. After being dried it sets hard. I have watched worms during the act of ejection, and when the earth was in a very liquid state it was ejected in little spurts, and when not so liquid by a slow peristaltic movement. It is not cast indifferently on any side, but with some care, first on one and then on another side; the tail being used almost like a trowel.

As soon as a little heap is formed, the worm apparently avoids, for the sake of safety, protruding its tail; and the earthy matter is forced up through the previously deposited soft mass. The mouth of the same burrow is used for this purpose for a consider- able time. In the case of the tower-like castings (see Fig. 2) near Nice, and of the similar but still taller towers from Bengal (hereafter to be described and figured) a considerable degree of skill is exhibited in their construction. Dr. King also observed that the passage up these towers hardly ever ran in the same exact line with the under- lying burrow, so that a thin cylindrical object such as a haulm of grass, could not be passed down the tower into the burrow; and this change of direction probably serves in some manner as a protection. When a worm comes to the surface to eject earth, the tail protrudes, but when it collects leaves its head must protrude. Worms therefore must have the power of turning round in their closely- fitting burrows; and this, as it appears to us, would be a difficult feat.

the surface of the ground. When they can find any cavity, as when burrowing in newly turned-up earth, or between the stems of banked-up plants, they deposit their castings in such places. So again any hollow beneath a large stone lying on the surface of the ground, is soon filled up with their castings. According to Hensen, old burrows are habitu- ally used for this purpose; but as far as my experience serves, this is not the case, except- ing with those near the surface in recently dug ground. I think that Hensen may have been deceived by the walls of old burrows, lined with black earth, having sunk in or collapsed ; for black streaks are thus left, and these are conspicuous when passing through light- coloured soil, and might be mistaken for completely filled-up burrows.

It is certain that old burrows collapse in the course of time; for as we shall see in the next chapter, the fine earth voided by worms, if spread out uniformly, would form in many places in the course of a year a layer + of an inch in thickness; so that at any rate this large amount is not deposited within the old unused burrows. If the burrows did not collapse, the whole ground would be first thickly riddled with holes to a depth of about ten inches, and in fifty years a hollow unsup- ported space, ten inches in depth, would be left. The holes left by the decay of succes- sively formed roots of trees and plants must likewise collapse in the course of time.

The burrows of worms run down perpen- ‘dicularly or a little obliquely, and where the ‘soil is at all argillaceous, there is no difficulty in believing that the walls would slowly flow or slide inwards during very wet weather. When, however, the soil is sandy or mingled with many small stones, it can hardly be viscous enough to flow inwards during even the wettest weather; but another -agency may here come into play. After much rain the ground swells, and as it cannot expand laterally, the surface rises; during dry weather it sinks again. For instance, a large flat stone laid on the surface of a field sank 3°33 mm. whilst the weather was dry between May 9th and June 13th, and rose 1°91 mm. between September 7th and 19th, much rain having fallen during the latter part of this time. During frosts and thaws -

the movements were twice as great. These observations were made by my son Horace, who will hereafter publish an account of the movements of this stone during successive wet and dry seasons, and of the effects of its being undermined by worms. Now when the ground swells, if it be penetrated by cylindrical holes, such as worm-burrows, their walls will tend to yield and be pressed inwards; and the yielding will be greater in the deeper parts (supposing the whole to be equally moistened) from the greater weight of the superincumbent soil which has to be raised, than in the parts near the sur- face. When the ground dries, the walls will shrink a little and the burrows will be a litile enlarged. Their enlargement, however, through the lateral contraction of the ground, will not be favoured, but rather op- posed, by the weight of the superincumbent soil.

Mstribution of Worms.—Earth-worms are found in all parts of the world, and some of the genera have an enormous range.* They inhabit the most isolated islands; they abound in Iceland, and are known to exist in the West Indies, St. Helena, Madagascar, New Caledonia and Tahiti. In the Antarctic regions, worms from Kerguelen Land have been described by Ray Lankester; and I found them in the Falkland Islands. How they reach such isolated islands is at present quite unknown. They are easily killed by salt-water, and it does not appear probable that young worms or their egg-capsules could be carried in earth adhering to the feet or beaks of land-birds. Moreover Kerguelen Land is not now inhabited by any land-bird. In this volume we are chiefly concerned with the earth cast up by worms, and I have gleaned a few facts on this subject with respect to distant lands. Worms throw up plenty of castings in the United States. In Venezuela, castings, probably ejected by species of Urocheta, are common in the gardens and fields, but not in the forests, as I hear from Dr. Ernst of Caracas. He collected 156 eastings from the court-yard of his house, having an area of 200 square yards. They varied in bulk from half a cubic centimeter to five cubic centimeters, and were on an average

three cubic centimeters. They were, therefore of small size in comparison with those often found in England; for six large castings from . a field near my house averaged 16 cubic centi- meters. Several species of earth-worms are common in St. Catharina in South Brazil, and Fritz Miller informs me “ that in most parts of “ the forests and pasture-lands, the whole soil, “toa depth ofa quarter ofa metre, looks as if it “had passed repeatedly through the intestines “ of earth-worms, even where hardly any cast- “ings are to be seen on the surface.” A gigantic but very rare species is found there, the burrows of which are sometimes even two centimeters or nearly $ of an inch in diameter, and which apparently penetrate the ground to a great depth.

In the dry climate of New South Wales, I hardly expected that worms would be com- mon; but Dr. G. Krefft of Sydney, to whom I applied, after making enquiries from gardeners and others, and from his own observations, informs me that their castings abound. He sent me some collected after heavy rain, and they consisted of little pellets, about °15 inch in diameter; and the blackened sandy earth of which they were formed still cohered with considerable tenacity.

The late Mr. John Scott of the Botanic Gardens near Calcutta made many observa- tions for me on worms living under the hot and humid climate of Bengal. The castings abound almost everywhere, in jungles and in the open ground, to a greater degree, as he thinks, than in England. After the water has subsided from the flooded rice-fields, the whole surface very soon becomes studded with castings—a fact which much surprised Mr. Scott, as he did not know how long worms could survive beneath water. They cause much trouble in the Botanic garden, “ for “some of the finest of our lawns can be kept “in anything like order only by being almost “ daily rolled ; if left undisturbed for a few days “they become studded with large castings.” These closely resemble those described as abounding near Nice ; and they are probably the work of a species of Pericheta. They stand up like towers, with an open passage in the centre.