Part 5

Some remarks about the structure of the earth-worm now appear apropos. Its body consists of from one hundred to two hundred almost cylindrical rings, each provided with minute bristles. The muscular system is well developed, thus enabling these animals to crawl backwards as well as forwards, and to retreat by the help of their affixed tails into their burrows with extraordinary rapidity. Situated at the anterior end of the body is the mouth. It is furnished with a little projection, variously called the lobe or lip, which is used for prehension. Behind the mouth, internally located, is a strong pharynx, which is pushed forwards when the animal eats, corresponding, it is said, with the protrudable trunk of other Annelids. The pharynx conducts to the œsophagus, on each side of the lower part of which are placed three pairs of large glands, called calciferous glands, whose function is the secretion of carbonate of lime. These glands are very remarkable organs, and their like is not to be found in any other animal. Their use is connected in some way with the process of digestion. The œsophagus, in most of the species, is enlarged into a crop in front of the gizzard. This latter organ is lined with a smooth, thick chitinous membrane, and is surrounded by weak, longitudinal, but powerful transverse muscles, whose energetic action is most effectual in the trituration of the food, for these worms possess no jaws, or teeth of any kind. Grains of sand and small stones, from the one-twentieth to the one-tenth of an inch in size, are found in their gizzards and intestines, and these little stones, independently of those swallowed while excavating their burrows, most probably serve, like millstones, to triturate their food. The gizzard opens into the intestine--a most remarkable structure, an intestine within an intestine--which runs in a straight line to the vent at the posterior end of the body. But this curious structure, as shown by Claparède, merely consists of a deep longitudinal involution of the walls of the intestine, by which means an extensive absorbent surface is secured.

Worms have a well-developed circulating system. Their breathing is effected by the skin, and so they do not possess any special respiratory apparatus. Each individual unites the two sexes in its own body, but two individuals pair together. The nervous system is fairly well developed, the two nearly confluent cerebral ganglia being situated very close to the anterior extremity of the body.

Being destitute of eyes, we would naturally conclude that worms were quite insensible to light; but from many experiments that have been made by Darwin, Hofmeister and others, it is evident that light affects them, but only by its intensity and duration. It is the anterior extremity of the body, where the cerebral ganglia lie, that is affected, for if this part is shaded and other parts of the body are illuminated no effect will be produced. As these animals have no eyes, it is probable that the light passes through their skins and excites in some manner their cerebral ganglia. When worms are employed in dragging leaves into their burrows or in eating them, and even during the brief intervals of rest from their labors, they either do not perceive the light or are regardless of it, and this is even the case when the light is concentrated upon them through a large lens. Paired individuals will remain for an hour or two together out of their burrows, fully exposed to the morning light, but it appears, from what some writers have said, that a light will occasionally cause paired individuals to separate. When a worm is suddenly illuminated and dashes into its burrow, one is led to look at the action as a reflex one, the irritation of the cerebral ganglia apparently causing certain muscles to contract in an inevitable manner, without the exercise of the will or consciousness of the animal, as though it was an automaton. But the different effect which a light produces on different occasions, and especially the fact that a worm when in any way occupied, no matter what set of muscles and ganglia may be brought into play, is often regardless of light, are antagonistic 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 upon them, we ascribe this to their attention being then absorbed, and attention necessarily implies the presence of mind. Although worms cannot be said to possess the power of vision, yet their sensitiveness to light enables them to discriminate between day and night, and thus they escape the attacks of the many diurnal animals that would prey upon them. They are less sensitive to a moderate radiant heat than to a bright light, as repeated experiments have conclusively shown; and their disinclination to leave their burrows during a frost proves that they are sensitive to a low temperature.

Investigation fails to locate in worms any organ of hearing, from which must be concluded that they are insensible to sounds. The shrill notes of a metallic whistle sounded near them, and the deepest and loudest tones of a bassoon, failed to awaken the least notice. Although indifferent to modulations in the air, audible to human ears, yet they are extremely sensitive to vibrations in any solid object. Even the light and delicate tread of a robin affrights and sends them deep into their burrows. It has been said that if the ground is beaten, or otherwise made to tremble, that worms believe they are pursued by a mole and leave their burrows, but this does not stand the test of experiment, for the writer has frequently beaten the ground in many places where these creatures abounded, but not one emerged. A worm’s entire body is sensitive to contact, the slightest puff of air from the mouth causing an instant retreat. When a worm first comes out of its burrow it generally moves the much-extended anterior extremity of its body from side to side in all directions, apparently as an object of touch, and there is good reason to believe that they are thus enabled to gain a general knowledge of the form of an object. Touch, including in this term the perception of a vibration, seems much the most highly developed of all their senses. The sense of smell is quite feeble, and is apparently confined to the perception of certain odors. They are quite indifferent to the human breath, even when tainted by tobacco, or to a pellet of cotton-wool with a few drops of Millefleur’s perfume when held by pincers and moved about within a few inches of them. The perception of such an unnatural odor would be of no service to them. Now, as such timid creatures would almost certainly exhibit some signs of any new impression, we may reasonably conclude that they did not perceive these odors. But when cabbage leaves and pieces of onion were employed, both of which are devoured with much relish by worms, the result was different. These, with bits of fresh raw meat, have been buried in pots beneath one-fourth of an inch of common garden soil, or sometimes laid on pieces of tin foil in the earth, the ground being pressed down slightly, so as not to prevent the emission of any odor, and yet they were always discovered by the worms that were placed in the pots, and removed after varying periods of time. These facts indicate that worms possess some power of smell, and that they discover by this means odoriferous and much-coveted kinds of food.

That all animals which feed on various substances possess the sense of taste, is a wise presumption. This is certainly the case with worms. Cabbage leaves are much liked by worms, and it would seem that they are able to distinguish between the different varieties, but this may perhaps be owing to differences in their texture. When leaves of the cabbage, horse-radish and onion were given together, they manifestly preferred the last to the others. Celery is preferred to the leaves of the cabbage, lime-tree, ampelopsis and parsnip, and the leaves of the wild cherry and carrots, especially the latter, to all the others. That the worms have a preference for one taste over another, is still further shown from what follows. Pieces of the leaves of cabbage, turnip, horse-radish and onion have been fed to the worms, mingled with the leaves of an Artemisia and of the culinary sage, thyme and mint, differing in no material degree in texture from the foregoing four, yet quite as strong in taste, but the latter were quite neglected excepting those of the mint, which were slightly nibbled, but the others were all attacked and had to be renewed.

There is little to be noted about the mental qualities of worms. They have been seen to be timid creatures. Their eagerness for certain kinds of food manifestly shows that they must enjoy the pleasure of eating. So strong is their sexual passion that they overcome for a time their dread of light. They seem to have a trace of social feeling, for they are not disturbed by crawling over each other’s bodies, and they sometimes lie in contact. Although remarkably deficient in the several sense-organs, yet this does not necessarily preclude intelligence, for it has been shown that when their attention is engaged they neglect impressions to which they would otherwise have attended, and attention, as is well known, indicates the presence of a mind of some kind. A few actions are performed instinctively, that is, all the individuals, including the young, perform each action in nearly the same manner. The various species of Perichæta eject their castings so as to construct towers, and the burrows of the Common Earth-worm--_Lumbricus terrestris_--are smoothly lined with fine earth and often with little stones, and the mouth with leaves. One of their strongest instincts is the plugging up of the mouths of their burrows with various objects, the very young worms acting in a similar manner. But some degree of intelligence is manifested, as will subsequently appear.

Almost everything is eaten by worms. They swallow enormous quantities of earth, from which they extract any digestible matter it may contain. Large numbers of half-decayed leaves of all kinds, excepting a few that are too tough and unpleasant to the taste, and likewise petioles, peduncles, and decayed flowers. Fresh leaves are consumed as well. Particles of sugar, licorice and starch, and bits of raw and roasted meat, and preferably raw fat, are eaten when they come into their possession, but the last article with a better relish than any other substance given to them. They are cannibals to a certain extent, and have been known to eat the dead bodies of their own companions.

The digestive fluid of worms, according to León Frédéricq, is analogous in nature to the pancreatic secretion of the higher animals, and this conclusion agrees perfectly with the kinds of food which they consume. Pancreatic juice emulsifies fat, dissolves fibrin, and worms greedily devour fat and eat raw meat. It converts starch into grape-sugar with wonderful rapidity, and the digestive fluid of worms acts upon the starch of leaves. But worms live chiefly on half-decayed leaves, and these would be useless to them unless they could digest the cellulose forming the cell-walls, for all other nutritious substances, as is well known, are almost completely withdrawn from leaves shortly before they fall off. It has been ascertained that cellulose, though very little or not at all attacked by the gastric juice of the higher animals, is acted on by that from the pancreas, and so worms eat the leaves as much for the cellulose as for the starch they contain. The half-decayed or fresh leaves which are intended for food are dragged into the mouths of their burrows to a depth of from one to three inches, and are then moistened with a secreted fluid, which has been assumed to hasten their decay, but which, from its alkaline nature, and from its acting both on the starch-granules and on the protoplasmic contents of the cells, is not of the nature of saliva, but a pancreatic secretion, and of the same kind as is found in the intestines of worms. As the leaves which are dragged into the burrows are often dry and shrivelled, it is indispensable for the unarmed mouths of worms that they should first be moistened and softened, their disintegration being thereby the more readily effected. Fresh leaves, however soft and tender they may be, are similarly treated, probably from habit. Thus the leaves are partially digested before they are taken into the alimentary canal, an instance of extra-stomachal digestion, whose nearest analogy is to be found in such plants as Dionæa and Drosera, for in them animal matter is digested and converted into peptone, not within a stomach, but on the surfaces of the leaves.

But no portion of the economy of worms has been more the subject of speculation than the calciferous glands. About as many theories have been advanced on their utility as there have been observers. Judging from their size and from their rich supply of blood-vessels, they must be of vast importance to these animals. They consist of three pairs, which in the Common Earth-worm debouch into the alimentary canal in front of the gizzard, but posteriorly to it, in some genera. The two posterior pairs are formed by lamellæ, diverticula from the œsophagus, which are coated with a pulpy cellular layer, with the outer cells lying free in infinite numbers. If one of these glands is punctured and squeezed, a quantity of white, pulpy matter exudes, consisting of these free cells, which are minute bodies, varying in diameter from two to six millimetres. They contain in their centres a small quantity of excessively fine granular matter, that looks so like oil globules that many scientists are deceived by its appearance. When treated with acetic acid they quickly dissolve with effervescence. An addition of oxalate of ammonia to the solution throws down a white precipitate, showing that the cells contain carbonate of lime. The two anterior glands differ a little in shape from the four posterior ones by being more oval, and also conspicuously in generally containing several small, or two or three larger, or a single very large concretion of carbonate of lime, as much as one and one-half millimetres in diameter. With respect to the function of the calciferous glands, it is likely that they primarily serve as organs of excretion, and secondarily as an aid to digestion. Worms consume many fallen leaves. It is known that lime goes on accumulating in leaves until they drop off the parent-plant, instead of being re-absorbed into the stem or roots, like various other organic and inorganic substances, and worms would therefore be liable to become charged with this earth, unless there was some special apparatus for its excretion, and for this purpose the calciferous glands are ably adapted. On the other hand, the carbonate of lime, which is excreted by the glands, aids the digestive process under ordinary circumstances. Leaves during their decay generate an abundance of various kinds of acids, which have been grouped together under the term of humus acids. These half-decayed leaves, which are swallowed by worms in large quantities, would, therefore, after having been moistened and triturated in the alimentary canal, be apt to produce such acids, and in the case of several worms, whose alimentary canals were examined, their contents were plainly shown by litmus paper to be decidedly acid. This acidity cannot be attributed to the nature of the digestive fluid, for pancreatic juice is alkaline, and so also is the secretion which is poured out of the mouths of worms for the preparation of the leaves for consumption. With worms not only the contents of the intestines, but their ejected matter or the castings are generally acid. 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 necessarily alkaline, and 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 neutralization. Therefore is seems probable that innumerable calciferous cells, which are emptied from the four posterior glands in the alimentary canal, serve to neutralize more or less completely the acids generated there by the half-decayed leaves. These cells, as has been seen, are instantly dissolved by a small quantity of acetic acid, and as they do not always suffice to render of no effect the contents of the upper part of the alimentary canal, it is probable that the lime is aggregated into concretions, in the anterior pair of glands, in order that some may be conveyed to the posterior parts of the intestine, where these concretions would be rolled about among the acid contents. The concretions found in the intestines and in the castings often present a worn appearance, but whether due to attrition or chemical corrosion it is impossible to say. That they are formed for the sake of acting as mill stones, as Claparède believed, and of thus assisting in the trituration of food, is not at all likely, as this object is already attained by the stones that are present in the gizzards and intestines.

In dragging leaves into their burrows worms generally seize the thin edge of a leaf with their mouths, between the projecting upper and lower lip, the thick and strong pharynx at the same time being pushed forwards within their bodies, so as to afford a _point de resistance_ for the upper lip; but in the case of broad and flat objects the pointed anterior extremity of the body, after being brought into contact with an object of this kind, is drawn within the adjoining rings, so that it becomes truncated and as thick as the rest of the body. This part is then seen to swell a little, seemingly from the pharynx being pushed a little forwards. By a slight withdrawal of the pharynx, or by its expansion, a vacuum is produced beneath the truncated, slimy end of the body whilst in contact with the object, and by this means the two adhere firmly together. Worms can attach themselves to an object in the same manner under the water.

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.