As worms have no teeth, and their mouths consist of very soft tissue, it may be presumed that they consume by means of suction of the edges and 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. They not only seize leaves and other objects for purposes of food, but for plugging up the mouths of their burrows. Flower-peduncles, decayed twigs of trees, bits of paper, feathers, tufts of wool and horse-hair are some of the many things other than leaves that are dragged into their burrows for this purpose. Many hundred leaves of the pine-tree have been found drawn by their bases into burrows. Where fallen leaves are abundant, especially ordinary dicotyledonous leaves, many more than can be used are collected over the mouth of a burrow, 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 necessarily becomes much folded or crumpled, and when another is drawn in, this is done exteriorly to the first, and so on with succeeding leaves, till finally they all become closely folded and pressed together. Sometimes the mouth of a burrow is enlarged, or a fresh one is made close by, so that a larger number of leaves may be drawn in. Generally the interstices between the drawn-in leaves are filled with moist, viscid earth ejected from their bodies, thus rendering them doubly secure. Hundreds of such plugged burrows may be seen during the autumnal and early winter months.
When leaves, petioles, sticks, etc., cannot be obtained for the mouths of their burrows, heaps of stones, smooth, rounded pebbles, are utilized for protection. When the stones are removed and the surface of the ground is cleared for some inches round the burrow, the worms may be seen with their tails fixed in their burrows dragging the stones inward by the aid of their mouths, stones weighing as much as two ounces often being found in the little heaps, which goes to show how strong these apparently weak creatures are. Work of this kind is usually performed during the night, although objects have been occasionally known to be drawn into the burrows during the day. What advantage worms derive from plugging up the mouths of their burrows, or from piling stones over them, cannot be satisfactorily answered. They do not act in this manner when they eject much earth from their burrows, for then their castings serve to cover the mouth. Perhaps the plugs serve to protect them from the attacks of scolopenders, their most inveterate enemies, or to enable them to remain with safety with their heads close to the mouths of their burrows, which they like so well to do, but which, unless protected, costs many a fellow its life. Besides, 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? The last view of the matter seems especially well taken, because worms kept in pots where there is fire, having no cold air with which to contend, plug up their burrows in a slovenly manner, and because they often coat the upper part of their burrows with leaves, apparently to prevent their bodies from coming into contact with the cold, damp earth. But the plugging-up process may undoubtedly serve for all these purposes. Whatever the motive may be, it seems that worms much dislike leaving the mouths of their burrows open, yet, nevertheless, they will reopen them at night, whether or not they are able afterwards to close them.
Considerable intelligence is shown by worms in their manner of plugging up their burrows. If man had to plug up a cylindrical hole with such objects as leaves, petioles or twigs, he would push them in by their pointed ends, but if these were thin relatively to the size of the hole, he would probably insert some by their broader ends. Intelligence would certainly be his guide in such a case. But how worms would drag leaves into their burrows, whether by their tips, bases, or middle parts, has been a matter of interest to many. Darwin, who experimented upon the subject, found it especially desirable to experiment with plants not native to his country, for he conceived that although the habit of dragging leaves into their burrows is undoubtedly instinctive with worms, yet instinct could not teach them how to act in the case of leaves about which their progenitors knew nothing. Did they act solely through instinct, or an unvarying inherited impulse, they would draw all kinds of leaves into their burrows in the same manner. Having no such definite instinct, chance might be expected to determine whether the tip, base, or middle might be seized. If the worm in each case first tries many different methods, and follows that alone which proves possible or the most easy, then both instinct and chance are ruled out of the solution of the question. But to act in this manner, and to try different methods, makes what in man would be called intelligent action.
Three species of pine-leaves are mentioned by Darwin as being regularly drawn into the mouths of worm-burrows on the gravel-walk in his garden. These leaves consist of two needles, which are united to a common base, and it is by this point that they are almost invariably drawn into the burrows. As the sharply-pointed needles diverge somewhat, and as several are drawn into the same burrow, each tuft forms a perfect chevaux-de-frise. Many tufts were pulled up in the evening, but by the ensuing morning fresh leaves had taken their places, and the burrows again well protected. Impossible it would be to drag these leaves to any depth into the burrows, except by their bases, as a worm cannot seize hold of the two leaves at the same time, and if one alone were seized by the apex, the other would be pressed against the ground and resist the entry of the one that was seized. That the worms should do their work well, it was very essential that they drag the pine-leaves into their burrows by their bases, that is, where the two needles are conjoined. But how they are guided in this work was at first perplexing. The difficulty, however, was soon settled. With the assistance of his son Francis, the elder Darwin set to work to observe worms in confinement during several nights by the aid of a dim light, while they dragged the leaves of the aforementioned kinds into their burrows. They were seen to move the anterior extremities of their bodies about the leaves, and on several occasions when they touched the sharp end of the needle they suddenly withdrew as though they had been pricked, but it is doubtful that they were hurt, for they are indifferent to sharp objects, being known to swallow rose-thorns and small splinters of glass. It may be doubted whether the sharp end of the needle serves to tell them that is the wrong end to seize, for the points of many were cut off for the length of an inch, and these leaves were always drawn in by their bases and not by the cut-off ends. The worms, it seemed, almost instantly perceived as soon as they had seized a leaf in the proper manner. Many leaves were cemented together at the top, or tied together by fine thread, and these in the majority of instances were dragged in by their bases, which leads to the conclusion that there must be something attractive to worms in the base of pine-leaves, notwithstanding that few ordinary leaves are drawn in by their base or footstalk. Leaves of other plants, and also the petioles of some compound plants, as well as triangular bits of paper, dry and damp, were experimented with, and the manner of seizing the objects and bearing them into their burrows were as amusing as they were novel and interesting. The leaves and stems used were such as the worms had not been accustomed to in their respective haunts.
When the several cases experimented on are considered, one can hardly escape from the conclusion that some degree of intelligence is shown by worms in plugging up their burrows. Each particular object is seized in too uniform a manner, and from causes which we can generally understand, for the result to be attributed to mere chance. That every object has not been drawn in by its pointed end may be accounted for by labor having been saved by some being carried in by their broader ends. There is no doubt that worms are governed by instinct in plugging up their burrows, and it might be expected that they would have been taught in every particular instance how to act independently of intelligence. It is very difficult to judge when intelligence comes into play. The actions of animals, appearing due to intelligence, may be performed through inherited habit without any intelligence, although aboriginally acquired, or the habit may be acquired through the preservation and inheritance of some other action, and in the latter case the new habit will have been acquired independently of intelligence throughout the entire course of its development. There is no à priori improbability in worms having acquired special instincts through either of these two latter means. Nevertheless it is incredible that instincts should have been developed in reference to objects, such as the leaves and petioles of foreign plants, wholly unknown to the progenitors of the worms which have acted in the manner just described. Nor are their actions so unvarying or inevitable as are most true instincts.
As worms are not controlled by special instincts in each particular case, though possessing a general instinct to plug up their burrows, and as chance is excluded, the next most probable conclusion is that they try in many ways to draw in objects and finally succeed in some one way. It is surprising, however, that an animal so low in the scale as a worm should have the capacity to act in this way, as many higher animals have no such capacity, the instincts of the latter often being followed in a senseless or purposeless manner.
We can safely infer intelligence, as Mr. Romanes, who has specially studied animals, says, only when we see an individual profiting by his own experiences. That worms are able to judge either before or after having drawn an object close to the mouths of their burrows how best to drag it in, shows that they must have acquired some notion of its general shape. This they probably acquire by touching it in many places with the anterior extremity of their bodies, which serves them as a tactile organ. Man, even when born blind and deaf, shows how perfect the sense of touch may become, and if worms, which also come into being in the same condition, have the power of acquiring some notion, however rude, of the shape of an object and their burrows, they deserve, it must seem to every sensible mind, to be called intelligent creatures, for they act in such a case in nearly the same manner as a man would under similar circumstances. That worms, which stand so low in the scale of organization, should possess some degree of intelligence, will doubtless strike everyone as very improbable. It may be doubted, however, whether we know enough about the nervous system of the lower animals to justify our natural distrust of such a conclusion. With regard to the small size of the cerebral ganglia, we would do well to remember what a mass of inherited knowledge, with some power of adapting means to an end, is crowded into the minute brain of a worker ant.
Two ways are adopted by worms in excavating their burrows. Either the earth is pushed away on all sides or it is swallowed by the animal. In the former case the worm inserts the stretched-out and attenuated anterior extremity of its body into any little crevice or hole, and the pharynx is pushed forward into this part, which consequently swells and pushes away the earth on all sides, the anterior extremity thus acting as a wedge. When placed in loose mould a worm will bury itself in between two and three minutes, but in earth that is moderately pressed down it often requires as many as fifteen minutes for its disappearance. But whenever a worm burrows to a depth of several feet in undisturbed compact ground, it must form its passage by swallowing the earth, for it is impossible that the ground could yield on all sides to the pressure of the pharynx when pushed forward within the worm’s body. Great depths are reached only during continued dry weather and severe cold, the burrows sometimes attaining to a depth of from seven to eight feet. The burrows run down perpendicularly, or, more commonly, obliquely, and are sometimes said to branch. Generally, or invariably as I think, they are lined with fine, dark-colored earth voided by the worm, so that at first they must be made a little wider than their ultimate diameter. Little globular pellets of voided earth, still soft and viscid, often dot the walls of fresh burrows, and these are spread out on all sides by the worm as it travels up or down its burrow, the lining thus formed becoming very compact and smooth when nearly dry and closely fitting the worm’s body. Excellent points of support are thus afforded for the minute reflexed bristles which project in rows on all sides from the body, thus rendering the burrow 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, which would assuredly be the case when the burrows, as is occasionally observed, pass through a layer of sifted coal cinders. The burrows are thus seen to be not mere excavations, but may be compared with tunnels lined with cement. Those which run far down into the ground generally, or at least frequently, terminate in little chambers, where one or several worms pass the winter rolled up into a ball. Small pebbles and seeds as large as grains of mustard are carried down from the surface by being swallowed or within the mouths of worms, as well as bits of glass and tile, whose only use in their winter-quarters seems to be the prevention of their closely coiled-up bodies from coming into contiguity with the surrounding cold soil, for such contact would perhaps interfere with their respiration, which is effected by the skin alone.
After swallowing earth, whether for making its burrow or for food, the earth-worm soon comes to the surface to empty its body. The rejected matter is thoroughly mixed with the intestinal secretions, and is thus rendered viscid. After becoming dried, it sets hard. When in a very liquid state the earth is thrown out in little spurts, and when not so liquid by a slow peristaltic movement of the intestine. It is not cast indifferently on any side, but first on one and then on another, the tail being used almost like a trowel. The little heap being formed the worm seemingly avoids, for the sake of safety, the use of its tail, the earthy matter being forced up through the previously deposited soft mass. The mouth of the same burrow is used for this purpose for a considerable time. When a worm comes to the surface to eject earth, the tail protrudes, but when it collects leaves its head must protrude, and thus worms must have the power of performing the difficult feat, as it seems to us, of turning round in their closely-fitting burrows. Worms do not always eject their castings upon the surface of the ground, for when burrowing in newly turned-up earth, or between the stems of banked-up plants, they deposit their castings in such places, and even hollows beneath large stems lying on the surface of the ground are filled up with their ejections. Old burrows collapse in time. The fine earth voided by worms, if spread out uniformly, would form in many places a layer of one-fifth of an inch in thickness. But 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 the depth of ten inches or more, which in fifty years would grow into a hollow, unsupported place ten inches deep.
Hardly any animal is more universally distributed than worms. The earth-worm is found in all parts of the world, and some of the genera have an enormous range. They inhabit the most isolated islands, abounding in Iceland, and also being known to exist in the West Indies, St. Helena, Madagascar, New Caledonia and Tahiti. Worms from Kergulen Land in the Antarctic regions have been described by Ray Lankester, and Darwin has reported them as being found in the Falkland Islands. How they reach such isolated islands is quite unknown. They are easily killed by salt water, and it does not seem likely that young worms or their egg-capsules could be carried in earth adhering to the feet or beaks of land-birds, especially to Kergulen Land, for it is not now inhabited by any terrestrial bird.