November 1, 1895.] 



KNOWLEDGE. 



259 



EARTHWORMS. 



By C. P. Maeshall, M.D., B.Sc, F.E.C.S. 



THE common earthworm, despised by man and 

 heedlessly trodden under foot, fulfils a part in 

 nature that would seem incredible but for the 

 facts revealed by the patient and long-continued 

 researches of Darwin. " Worms," says Darwin, 

 " have played a more important part in the history of the 

 world than most persons would at first suppose." Let us 

 follow Darwin, and see how this apparently insignificant 

 creature has changed the face of nature. We will first 

 consider the habits and mode of life of the earthworm. 

 As everyone knows, the worms live in burrows in the 

 superficial layer of the ground. They can live anywhere 

 in a layer of earth, provided it retains moisture, dry air 

 being fatal to them. They can, on the other hand, exist 

 submerged in water for several months. They live chiefly 

 in the superficial mould less than a foot below the surface, 

 but in long-continued dry weather and in very cold seasons 

 they may burrow to a depth of eight feet. The burrows 

 are lined by a thin layer of earth, voided by the worms, 

 and end in small chambers in which they can turn round. 

 The burrows are formed partly by pushing away the 

 earth, but chiefly by the earth being swallowed. Large 

 quantities of earth are swallowed by the worms for the 

 sake of the decomposing vegetable matter contained in it, 

 on which they feed. The earth thus swallowed is voided 

 in spiral heaps, forming the worm castings. In this way 

 the worm obtains food and at the same time excavates its 

 burrows. 



In addition to the food thus obtained, half-decayed 

 leaves are dragged into the burrows, mainly for food, but 

 also to plug the mouths of the burrows for the sake of 

 protection. Worms are also fond of meat, especially 

 fat ; they will also eat the dead bodies of their relatives. 

 They are nocturnal in habit, remaining as a rule in 

 the burrows diu-ing the day and coming out to feed at 

 night. 



The leaves dragged into the burrows are moistened by 

 a fluid secreted by the worm, of a digestive nature, and 

 the food is thus partly digested before being swallowed. 

 The digestive fluid of the earthworm resembles the 

 pancreatic juice of higher animals, and only acts when 

 alkaline, ^'arious acids are produced by decaying vegetable 

 matter, and similar changes occur in the leaves swallowed 

 by worms. Now if some of this acid was not neutralized, 

 digestion could not take place, because the digestive fluid 

 is alkaline. This is avoided by the action of some small 

 glands, called the calciferous glands, opening into the 

 alimentary canal. These glands secrete carbonate of lime, 

 which neutralizes the acids generated in the decaying 

 leaves. 



^ The earthworm has no eyes, but is affected by strong 

 light if exposed to it for some time. It has no sense of 

 hearing, but is sensitive to the vibrations of sound. The 

 whole body is sensitive to touch. There appears to be 

 some sense of smell, but this is limited to certain articles 

 of food, which are discovered by the worm when buried in 

 earth, in preference to other bodies not relished. The 

 worrn appears to have some degree of intelligence from the 

 way in which it draws the leaves into its burrows, always 

 judging which is the best end to draw them in by. This 

 is remarkable in so lowly organized an animal, being a 

 degree of intelligence not possessed by many animals of 

 more complex organization. For instance, the ant can 

 often be seen dragging objects along transverselv, instead 

 of taking them the easiest way. 



As we have seen, vast quantities of earth are continually 

 being passed through the bodies of worms and voided on 

 the surface as castings. When it is stated that the number 

 of worms in an acre of ordinary land, suitable for them to 

 live in, is fifty-three thousand, we can imagine the great 

 eflect which they must have on the soil. They are, in fact, 

 continually ploughing the land. At one part of the alimen- 

 tary canal of the worm is a gizzard, or hard muscular organ, 

 capable of grinding food into fine particles ; it is this gizzard 

 which is the main factor in triturating the soil, and it is 

 aided by small stones swallowed with the earth, which act 

 as mill-stones. The earth is thus continually passing 

 through the mill formed by the gizzards of worms, and is 

 reduced to fine mould. Again, from the collapsing of the 

 old burrows the mould is in constant slow movement, and 

 its particles rubbed together. Fresh surfaces are thus 

 exposed to the action of the carbonic acid in the soil and 

 to the humus acids, agents which act in the destruction 

 of stones and rocks. Moreover, the acids produced in the 

 digestive tract of the worms is not all neutralized, for the 

 castings have an acid reaction, and this acid acts further 

 in the disintegration of rocks. 



Thus all the mould covering a field passes every few 

 hours through the bodies of worms, and the same fragments 

 are probably swallowed and brought to the surface many 

 times over in the course of centuries. Changes are also 

 produced in the slopes of hills by the flowing down of 

 moist castings and the rolling down of dry ones, thus 

 reducing the slope of the hills by accumulations at the 

 bottom. The castings are also blown repeatedly in one 

 direction by the prevalent winds. Now as a layer of earth 

 one-fifth of an inch thick, or ten tons by weight, has been 

 calculated in many places to be brought annually to the 

 surface per acre, if only a small part of this flows down 

 every incUned surface, or is blown by the wind repeatedly 

 in one direction, it is easy to see that a great change may 

 be produced in the surface of the land in the course 

 of ages. 



In consequence of the immense amount of earth con- 

 tinually being brought to the surface by worms, it is not 

 diflicult to understand how objects, such as stones, rocks, 

 etc., lying on the surface will in course of time become 

 gradually buried in the ground. The worms, undermining 

 the stones, bring up the earth to the surface, and so raise 

 the ground round the edge of the stone till the latter 

 sinks and is eventually buried in the soil, provided the 

 soil is suitable for worms to live in. Darwin showed 

 that in a field covered with flints of various sizes, 

 the smaller ones disappeared in a few years, and in 

 thirty years all had become buried owing to the action of 

 worms. 



Owing to the burial of stones and other objects by the 

 action of worms, ancient monuments, portions of Roman 

 villas, and other objects of antiquity have been preserved. 

 These have been gradually buried by the worms, and so 

 preserved from the destructive eflect of rain and wind. 

 Many Roman remains were studied by Darwin, and traces 

 of the action of worms found, to which action their 

 preservation was mainly due. The sinking of the 

 foundations of many old buildings is due to the action 

 of worms, and no building is safe from this unless 

 the foundations are laid lower than the level at which 

 the worms can work, viz., about eight feet below the 

 surface. 



Another useful effect produced by worms is the prepara- 

 tion of the soil for the growth of seedlings. By their 

 agency the soil is periodically sifted and exposed to the 

 air, and in this way is able to retain moisture anl absorb 

 soluble substances of use for the nutrition of plants. 



