The Formation of Vegetable Mould through the Action of Worms

“going on in the contents themselves. .. . “In Carnivora the contents of the coecum “are said to be alkaline, and naturally the “amount of fermentation will depend largely ‘“‘on the nature of the food.”* With worms not only the contents of the intestines, but their ejected matter or the castings, are generally acid. Thirty castings from different places were tested, and with three or four exceptions were found to be acid; and the exceptions may have been due to such castings not having been recently ejected; for some which were at first acid, were on the following morning, after being dried and again moistened, no longer acid ; and this probably resulted from the humus acids being, as 1s known to be the case, easily decomposed. Five fresh castings from worms which lived in mould close over the chalk, were of a whitish colour and abounded with caleareous matter; and these were not in the least acid. This shows how effectually carbonate of lime neutralises the intestinal acids. When worms were kept in pots filled

with fine ferruginous sand, it was manifest that the oxide of iron, with which the grains of silex were coated, had been dissolved and removed from them in the castings. The digestive fluid of worms resembles in its action, as already stated, the pancreatic secretion of the higher animals; and in these latter, “‘ pancreatic digestion is essentially “alkaline; the action will not take place “unless some alkali be present; and the “activity of an alkaline juice is arrested by “acidification, and hindered by neutraliza- “tion.”* Therefore it seems highly probable that the innumerable calciferous cells, which are poured from the four posterior glands into the alimentary canal of worms, serve to neutralise more or less completely the acids there generated by the half-decayed leaves. We have seen that these cells are instantly dissolved by a small quantity of acetic acid, and as they do not always suffice to neu- tralise the contents of even the upper part of the alimentary canal, the lime is perhaps avgregated into concretions in the anterior pair of glands, in order that some may be

carried down to the posterior parts of the intestine, where these concretions would be rolled about amongst the acid contents. The concretions found in the intestines and in the castings often have a worn appearance, but whether this is due to some amount of attrition or of chemical corrosion could not be told. Claparéde believes that they are formed for the sake of acting as mill-stones, and of thus aiding in the trituration of the food. They may give some aid in this way ; but I fully agree with Perrier that this must be of quite subordinate importance, seeing that the object is already attained by stones being generally present in the gizzards and intestines of worms. |

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 swal- lowing 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—Gigantie castings on the Nilgiri Mountains—Castings ejected in all countries.

Ix the pots in which worms were kept, leaves were pinned down to the soil, and at night the manner in which they were seized could be observed. The worms always endeavoured to drag the leaves towards their burrows; and they tore or sucked off small fragments, whenever the leaves were sufli- ciently tender. They generally seized the thin edge of a leaf with their mouths, between the projecting upper and lower lip; the thick and strong pharynx being at the same — time, as Perrier remarks, pushed forward within their bodies, so as to afford a point of resistance for the upper lip. In the case of broad flat objects they acted in a wholly different manner. The pointed anterior extremity of the body, after being brought into contact with an object of this kind, was drawn within tke adjoining rings, so that it appeared truncated and became as thick as the rest of the body. This part could then be seen to swell a little; and this, I believe, is due to the pharynx being pushed a little forwards. Then by a slight withdrawal of the pharynx or by its expansion, a vacuum was produced beneath the truncated slimy end of the body whilst in contact with the object; and by this means the two adhered firmly together.* That under these circum- stances a vacuum was produced was plainly

* Claparede remarks (‘ Zeitschrift fiir wissenschaft. Zoolog, B. 19, 1869, p. 602) that the pharynx appears from its structure to be adapted for suction. seen on one occasion, when a large worm lying beneath a flaccid cabbage leaf tried to drag it away; for the surface of the leaf directly over the end of the worm’s body became deeply pitted. On another occasion a worm suddenly lost its hold on a flat leaf; and the anterior end of the body was momer- tarily seen to be cup-formed. Worms can attach themselves to an object beneath water in the same manner; and I saw one thus dragging away a submerged slice of an onion-bulb.

The edges of fresh or nearly fresh leaves affixed to the ground were often nibbled by the worms; and sometimes the epidermis and all the parenchyma on one side was gnawed completely away over a considerable space; the epidermis alone on the opposite side being left quite clean. The veins were never touched, and leaves were thus some- times partly converted into skeletons. As worms have no teeth and as their mouths consist of very soft tissue, it may be pre- sumed that they consume by means of suction the edges and the parenchyma of fresh leaves, after they have been softened by the

digestive fluid. They cannot attack such strong leaves as those of sea-kale or large and thick leaves of ivy; though one of the latter after it had become rotten was reduced in parts to the state of a skeleton. Worms seize leaves and other objects, not only to serve as food, but for plugging up the mouths of their burrows; and this is one of their strongest instincts. Leaves and petioles of many kinds, some flower-pedun- cles, often decayed twigs of trees, bits of paper, feathers, tufts of wool and horse-hairs are dragged into their burrows for this pur- pose. I have seen as many as seventeen petioles of a Clematis projecting from the mouth of one burrow, and ten from the mouth of another. Some of these objects, such as the petioles just named, feathers, &., are never gnawed by worms. In a gravel walk in my garden I found many hundred leaves of a pine-tree (P. austriaca or nigri- cans) drawn by their bases into burrows. The surfaces by which these leaves are articu- lated to the branches are shaped in as pecu- liar a manner as is the joint between the leg- bones of a quadruped; and if these surfaces

had been in the least gnawed, the fact would have been immediately visible, but there was no trace of gnawing. Of ordinary dicotyle- donous leaves, all those which are dragged into burrows are not gnawed. I have seen as many as nine leaves of the lime-tree drawn into the same burrow, and not nearly all of them had been gnawed; but such leaves may serve as a store for future con- sumption. Where fallen leaves are abun- dant, many more are sometimes collected over the mouth of a burrow than can be used, so that a small pile of unused leaves is left like a roof over those which have been partly dragged in.

A leaf in being dragged a little way into a cylindrical burrow is necessarily much folded or crumpled. When another feaf is drawn in, this is done exteriorly to the first one, and so on with the succeeding leaves ; and finally all become closely folded and pressed ‘together. Sometimes the worm enlarges the mouth of its burrow, or makes a tresh one close by, so as to draw in a still larger number of leaves. They often or generally fill up the moist viscid earth ejected from their bodies, and thus the mouths of the burrows are securely plugged. Hundreds of such plugged burrows may be seen in many places, especially during the autumnal and early winter months. But, as will hereafter be shown, leaves are dragged into the burrows not only for plugging them up and for food, but for the sake of lining the upper part or mouth.

When worms cannot obtain leaves, petioles, sticks, &c., with which to plug up the mouths of their burrows, they often protect them by little heaps of stones; and such heaps of smooth rounded pebbles may frequently be seen on gravel-walks. Here there can be no question about food. A lady, who was in- terested in the habits of worms, removed the little heaps of stones from the mouths of several burrows and cleared the surface of the ground for some inches all round. She went out on the following night with a lantern, and saw the worms with their tails fixed in their burrows, dragging the stones inwards by the aid of their mouths, no doubt by suction. “After two nights some of the

“holes had 8 or 9 small stones over “them; after four nights one had about “30, and another 34 stones.”* One stone which had been dragged over the gravel-walk to the mouth of a burrow weighed two cinces; and this proves how strong worms are. But they show greater strength in some- times displacing stones in a well-trodden eravel-walk ; that they do so, may be inferred from the cavities left by the displaced stones being exactly filled by those lying over the mouths of adjoming burrows, as I have my- self observed,

Work of this kind is usually performed during the night; but I have occasionally known objects to be drawn into the burrows during the day. What advantage the worms derive from plugging up the mouths of their burrows with leaves, &ec., or from piling stones over them, is doubtful. They do not act in this manner at the times when they eject much earth from their burrows ; for their castings then serve to cover the mouth. When gardeners wish to kill worms on a

* An account of her observations is given in the ‘ Gardeners Chronicle,’ March 28th, 1868, p. 324. lawn, it is necessary first to brush or rake away the castings from the surface, in order that the lime-water may enter the burrows.* It might be inferred from this fact that the mouths are plugged up with leaves, &c., to prevent the entrance of water during heavy rain; but it may be urged against this view that a few, loose, well-rounded stones are ill- adapted to keep out water. I have moreover seen many burrows in the perpendicularly cut turfedgings to gravel-walks, into which water could hardly flow, as well plugged as burrows on a level surface. Can the plugs or piles of stones aid in concealing the bur- rows from scolopenders, which, according to Hoffmeister, t are the bitterest enemies of worms? Or may not worms when thus pro- tected be able to remain with safety with their heads close to the mouths of their bur- rows, which we know that they like to do, but which costs so many of them their lives? Or may not the plugs check the free ingress, of the lowest stratum of air, when chilled by

radiation at night, from the surrounding ground and herbage. J am inclined to be- lieve in this latter view; firstly, because when worms were kept in pots in a room with a fire, in which case cold air could not enter the burrows, they plugged them up in a slovenly manner; and secondarily, because they often coat the upper part of their burrows with leaves, apparently to prevent their bodies from coming into close contact with the cold damp earth. But the plugging-up process may perhaps serve for all the above purposes. Whatever the motive may be, it appears that worms much dislike leaving the mouths of their burrows open. Nevertheless they will reopen them at night, whether or not they can afterwards close them. Numerous open burrows may be seen on recently-dug eround, for in this case the worms eject their castings in cavities left in the ground, or in the old burrows, instead of piling them over the mouths of their burrows, and they cannot collect objects on the surface by which the mouths might be protected. So again on a recently disinterred pavement of a Roman villa at Abinger (hereafter to be described’

the worms pertinaciously opened their bur- rows almost every night, when these had been closed by being trampled on, although they were rarely able to find a few minute stones wherewith to protect them. Intelligence shown by worms in their manner of plugging up their burrows.—If a man had to plug up a small cylindrical hole, with such objects as leaves, petioles or twigs, he would drag or push them in by their pointed ends; but if these objects were very thin relatively to the size of the hole, he would probably insert some by their thicker or broader ends. The guide in his case would be intelligence. It seemed therefore worth while to observe carefully how worms dragged leaves into their burrows; whether by their tips or. bases or middle parts. It seemed more espe- cially desirable to do this in the case of plants not natives to our country; for although the habit of dragging leaves into their burrows is undoubtedly instinctive with worms, yet instinct could not tell them how to act in the case of leaves about which their pro- genitors knew nothing. If, moreover, worms acted solely through instinct or an unvary-

‘ing inherited impulse, they would draw all kinds of leaves into their burrows in the same manner. If they have no such definite instinct, we might expect that chance would determine whether the tip, base or middle was seized. If both these alternatives are ex- cluded, intelligence alone is left; unless the worm in each case first tries many different methods, and follows that alone which proves possible or the most easy ; but to act in this manner and to try different methods makes a near approach to intelligence.

In the first place 227 withered leaves of various kinds, mostly of English plants, were pulled out of worm-burrows in several places. Of these, 181 had been drawn into the -burrows by or near their tips, so that the foot-stalk projected nearly upright from the mouth of the burrow; 20 had been drawn in by their bases, and in this case the tips pro- jected from the burrows; and 26 had been seized near the middle, so that these had been drawn in transversely and were much erumpled. Therefore 80 per cent. (always using the nearest whole number) had been drawn in by the tip, 9 per cent. by the base

or footstalk, and 11 per cent. transversely or by the middle. This alone is almost suffi- cient to show that chance does not determine the manner in which leaves are dragged into the burrows. Of the above 227 leaves, 70 consisted of the fallen leaves of the common lime-tree, which is almost certainly not a native of England. These leaves are much acumin- ated towards the tip, and are very broad at the base with a well-developed foot-stalk. They are thin and quite flexible when half- withered. Of the 70, 79 per cent. had been drawn in by or near the tip; 4 per cent. by or near the base; and 17 per cent. trans- versely or by the middle. These proportions agree very closely, as far as the tip is con- cerned, with those before given. But the per- centage drawn in by the base is smaller, which may be attributed to the breadth of the basal part of the blade. We here, also, see that the presence of a foot-stalk, which it might have been expected would have tempted the worms as a convenient handle, has little or no in- fluence in determining the manner in which lime leaves are dragged into the burrows,

The considerable proportion, viz., 17 per cent., drawn in more or less transversely depends no doubt on the flexibility of these half-decayed leaves. The fact of so many having been drawn in by the middle, and of some few having been drawn in by the base, renders it improbable that the worms first tried to draw in most of the leaves by one or both of these methods, and that they after- wards drew in 79 per cent. by their tips; for it is clear that they would not have failed in drawing them in by the base or middle. The leaves of a foreign plant were next searched for, the blades of which were not more pointed towards the apex than towards the base. This proved to be the case with those of a laburnum (a hybrid between Cytisus alpinus and laburnum) for on doubling the terminal over the basal half, they gene- rally fitted exactly; and when there was any difference, the basal half was a little the narrower. It might, therefore, have been expected that an almost equal number of these leaves would have been drawn in by-the tip and base, or a slight excess in favour of the latter. But of 73 leaves (not included in

the first lot of 227) pulled out of worm- burrows, 63 per cent. had been drawn in by the tip; 27 per cent. by the base, and 10 per cent. transversely. We here see that a far larger proportion, viz., 27 per cent. were drawn in by the base than in the case of lime leaves, the blades of which are very broad at the base, and of which only 4 per cent. had thus been drawn in. We may perhaps account for the fact of a still larger proportion of the laburnum leaves not hav- ing been drawn in by the base, by worms having acquired the habit of generally draw ing im leaves by their tips and thus avoid- ing the foot-stalk. For the basa] margin of the blade in many kinds of leaves forms a large angle with the foot-stalk ; and if such a leaf were drawn in by the foot-stalk, the basal margin would come abruptly into contact with the ground on each side of the burrow, and would render the drawing in of the leaf very difficult,

Nevertheless worms break through their habit of avoiding the footstalk, if this part offers them the most convenient means for drawing leaves into their burrows. ‘The leaves of the endless hybridised varieties of the Rhododendron vary much in shape; some are narrowest towards the base and others to- wards the apex. After they have fallen off, the blade on each side of the midrib often becomes curled up while drying, sometimes along the whole length, sometimes chiefly at the base, sometimes towards the apex. Out of 28 fallen leaves on one bed of peat in my garden, no less than 23 were narrower in the basal quarter than in the terminal quarter of their length; and this narrowness was chiefly due to the curling in of the margins. Out of 36 fallen leaves on another bed, in which different varieties of the Rhododendron grew, only 17 were narrower towards the base than towards the apex. My son William, who first called my attention to this case, picked up 237 fallen leaves in his garden (where the Rhododendron grows in the natural soil) and of these 65 per cent. could have been drawn by worms into their bur- rows more easily by the base or foot-stalk than by the tip; and this was partly due to the shape of the leaf and in a less degree to the curling in of the margins: 27 per

cent. could have been drawn in more easily by the tip than by the base: and 8 per cent, with about equal ease by either end. The shape of a fallen leaf ought to be judged of before one end has been drawn into a burrow, for after this has happened, the free end, whether it be the base or apex, will dry more quickly than the end embedded in the damp ground; and the exposed margins of the free end will consequently tend to become more curled inwards than they were when the leaf was first seized by the worm. My son found 91 leaves which had been dragged by worms into their burrows, though not to a great depth; of these 66 per cent. had been -drawn in by the base or foot-stalk; and 34 per cent. by the tip. In this case, there- fore, the worms judged with a considerable degree of correctness how best to draw the withered leaves of this foreign plant into their burrows; notwithstanding that they had to depart from their usual habit of avoiding the foot-stalk.

On the gravel-walks in my garden a very large number of leaves of three species of Pinus (P. austriaca, nigricars and sylvestris) are regularly drawn into the mouths of worm- burrows. These leaves consist of two needles, which are of considerable length in the two first and short in the last named species, and are united to a common base; and it is by this part that they are almost invariably drawn into the burrows. I have seen only two or at most three exceptions to this rule with worms in a state of nature. As the sharply pointed needles diverge a little, and as several leaves are drawn into the same burrow, each tuft forms a perfect chevaux de frise. On two occasions many of these tufts were pulled up in the evening, but by the following morning fresh leaves had been pulled in, and the burrows were again well protected. These leaves could not be dragged into the burrows to any depth, except by their bases, as a worm cannot seize hold of the two needles at the same time, and if one alone were seized by the apex, the other would be pressed against the ground and would resist the entry of the seized one. This was manifest in the above mentioned two or three excep- tional cases. In order, therefore that worms should do their work well, they must drag

pine-leaves into their burrows by their bases, where the two needles are conjoined. But how they are guided in this work is a per- plexing question. This difficulty led my son Francis and my- self to observe worms in confinement during several nights by the aid of a dim light, while they dragged the leaves of the above named pines into their burrows. They moved the anterior extremities of their bodies about the leaves, and on several occasions when they touched the sharp end of a needle they with- drew suddenly as if pricked. But I doubt whether they were hurt, for they are indif- ferent to very sharp objects, and will swallow even rose-thorns and small splinters of glass. It may also be doubted, whether the sharp ends of the needles serve to tell them that this is the wrong end to seize; for the points were cut off many leaves for a length of about one inch, and fifty-seven of them thus treated were drawn into the burrows by their bases, and not one by the cut-off ends. The worms in confinement often seized the needles near the middle and drew them to- wards the mouths of their burrows; and one

worm tried in a senseless manner to drag them into the burrow by bending them. They sometimes collected many more leaves over the mouths of their burrows (as in the case formerly mentioned of lime-leaves) than could enter them. On other occasions, how- ever, they behaved very differently; for as soon as they touched the base of a pine-leaf, this was seized, being sometimes completely en- gulfed in their mouths, or a point very near the base was se‘zed, and the leaf was then quickly dragged or rather jerked into their burrows. It appeared both to my son and myself as if the worms instantly perceived as soon as they had seized a leaf in the proper manner. Nine such cases were observed, but in one of them the worm failed to drag the leaf into its burrow, as it was entangled by other leaves lying near. In another case « leaf stood nearly upright with the points of the needles partly inserted into a burrow, but how placed there was not seen; and then the worm reared itself up and seized the base. which was dragged into the mouth of the burrow by bowing the whole leaf. On the other hand, after a worm had seized the base

of a leaf, this was on two occasions relin- quished from some unknown motive. As already remarked, the habit of plugging ap the mouths of the burrows with various objects, is no doubt instinctive in worms ; and a very young one, born in one of my pots, dragged for some little distance a Scotzh- ir leaf, one needle of which was as long and almost as thick as its own body. No species of pine is endemic in this part of England, it is therefore incredible that the proper manner of dragging pine-leaves into the burrows can be instinctive with our worms. But as the worms on which the above obser- vations were made, were dug up beneath or near some pines, which had been planted there about forty years, it was desirable to prove that their actions were not instinctive. Accordingly, pine-leaves were scattered on the ground in places far removed from any pine-tree, and 90 of them were drawn into the burrows by their bases. Only two were drawn in by the tips of the needles, and these were not real exceptions, as one was drawn in for a very short distance, and the two needles of the other cohered. Other pine-

leaves were given to worms kept in pots in a warm room, and here the result was different ; for out of 42 leaves drawn into the burrows, no less than 16 were drawn in by the tips of the needles. These worms, however, worked in a careless or slovenly manner ; for the leaves were often drawn in to only a small depth; sometimes they were merely beaped over the mouths of the burrows, and sometimes none were drawn in. I believe that this carelessness may be accounted for by the air of the room being warm, and the worms consequently not being anxious to plug up their hcles effectually. Pots tenanted by worms and covered with a net which allowed the entrance of cold air, were left out of doors for several nights, and now 72 leaves were all properly drawn in by their bases.

It might perhaps be inferred from the facts as yet given, that worms somehow gain a general notion of the shape or structure of pine leaves, and perceive that it is necessary for them to seize the base where the two needles are conjoined. But the following cases make this more than doubtful. The tips of a large number of needles of P. austriaca were cemented together with shell-lac dis- solved in alcohol, and were kept for some days, until, as I believe, all odour or taste had been lost; and they were then scattered on the ground where no pine-trees grew, near burrows from which the plugging had been removed. Such leaves could have been drawn into the burrows with equal ease by either end; and judging from analogy and more especially from the case presently to be given of the petioles of Clematis montana, I expected that the apex would have been preferred. But the result was that out of 121 leaves with the tips cemented, which were drawn into bur- rows, 108 were drawn in by their bases, and only 13 by their tips. Thinking that the worms might possibly perceive and dislike the smell or taste of the shell-lac, though this was very improbable, especially after the leaves had been left out during several nights, the tips of the needles of many leaves were tied together with fine thread. Of leaves thus treated 150 were drawn into burrows— 123 by the base and 27 by the tied tips; so that between four and five times as many were

drawn in by the base as by the tip. It is possible that the short cut-off ends of the thread with which they were tied, may have tempted the worms to drag in a larger propor- tional number by the tips than when cement was used. Of the leaves with tied and cemented tips taken together (271 in number) 85 per cent. were drawn in by the base and 15 per cent. by the tips. We may therefore infer that it is not the divergence of the two needles which leads worms in a state of nature almost invariably to drag pine-leaves into their burrows by the base. Nor can it be the sharpness of the points of the needles which determines them; for, as we have seen, many leaves with the points cut off were drawn in by their bases. We are thus led to conclude, that with pine-leaves there must be something attractive to worms in the base, notwithstand- ing that few ordinary leaves are drawn in by the base or footstalk.