|
|
LTHOUGH not generally considered attractive,
for two reasons the earthworm has an important
place in nature-study: it furnishes an
interesting example of lowly organized creatures,
and it is of great economic importance
to the agriculturist. The lesson should have
special reference to the work done by earthworms
and to the simplicity of the tools with
which the work is done.
The earthworm is, among lower animals, essentially the farmer. Long before man conceived the idea of tilling the soil, this seemingly insignificant creature was busily at work plowing, harrowing, and fertilizing the land. Nor did it overlook the importance of drainage and the addition of amendments—factors of comparatively recent development in the management of the soil by man.
Down into the depths, sometimes as far as seven or eight feet, but usually from twelve to eighteen inches, goes the little plowman, bringing to the surface the subsoil, which is exactly what we do when we plow deeply. To break up the soil as our harrows do, the earthworm grinds it in a gizzard stocked with grains of sand or fine gravel, which act as millstones. Thus it turns out soil of much finer texture than we, by harrowing or raking, can produce. In its stomach it adds the lime amendment, so much used by the modern farmer. The earthworm is apparently an adept in the use of fertilizers; it even shows discrimination in keeping the organic matter near the surface, where it may be incorporated into the soil of the root zone. It drags into its burrows dead leaves, flowers and grasses, with which to line the upper part. Bones of dead animals, shells, and twigs are buried by it, and, being more or less decayed, furnish food for plants. These minute agriculturists have never studied any system of drainage, but they bore holes to some depth which carry off the surplus water. They plant seeds by covering those that lie on the ground with soil from below the surface—good, enriched, well granulated soil it is, too. They further care for the growing plants by cultivating, that is keeping fine and granular, the soil about the roots.
It was estimated by Darwin that, in garden soil in England, there are more than 50,000 earthworms in an acre, and that the whole superficial layer of vegetable mold passes through their bodies in the course of every few years, at the rate of eighteen tons per acre yearly.
This agricultural work of the earthworm has been going on for ages. Wild land owes much of its beauty to this diminutive creature which keeps the soil in good condition. The earthworm has undermined and buried rocks, changing greatly the aspect of the landscape. It has preserved ruins and ancient works of art. Several Roman villas in England owe their preservation to the earthworm. All this work is accomplished with the most primitive tools, a tiny proboscis, a distensible pharynx, a rather indeterminate tail, a gizzard and the calcareous glands peculiar to this lowly creature.
An earthworm has a peculiar, crawling movement. Unlike the snake, which also moves without legs, it has no scales to function in part as legs; but it has a very special provision for locomotion. On the under side of a worm are found numerous setae—tiny, bristlelike projections. These will be seen to be in double rows on each segment, excepting the first three and the last. The setae turn so that they point in the opposite direction from which the worm is moving. It is this use of these clinging bristles, together with strong muscles, which enables a worm to hold tightly to its burrow when bird or man attempts its removal. A piece of round elastic furnishes an excellent example of contraction and extension, such as the earthworm exhibits. Under the skin of the worm are two sets of muscles the outer passing in circular direction around the body, the inner running lengthwise. The movement of these maybe easily seen in a good-sized, living specimen. The body is lengthened by the contraction of circular and the extension of longitudinal muscles, and shortened by the opposite movement.
The number of segments may vary with the age of the worm. In the immature, the clitellum, a thick, whitish ring near the end, is absent. The laying of the earthworm's egg is an interesting performance. A saclike ring is formed about the body in the region of the clitellum. This girdle is gradually worked forward and, as it is cast over the head, the sac-ends snap together enclosing the eggs. These capsules, yellowish-brown, football-shaped, about the size of a grain of wheat, may be found in May or June about manure piles or under stones.
Earthworms are completely deaf, although sensitive to vibration. They have no eyes, but can distinguish between light and darkness. The power of smell is feeble. The sense of taste is well developed; the sense of touch is very acute; and we are not so sure as is Dr. Jordan, that the angleworm is at ease on the hook.
Any garden furnishes good examples of the home of the earthworm. The burrows are made straight down at first, then wind about irregularly. Usually they are about one or two feet deep, but may reach even eight feet. The burrow terminates generally in an enlargement where one or several worms pass the winter. Toward the surface, the burrow is lined with a thin layer of fine, dark colored earth, voided by the worm. This creature is an excavator and builder of no mean ability. The towerlike "castings" so characteristic of the earthworm, are formed with excreted earth. Using the tail as a trowel, it places earth, now on one side and now on the other. In this work, of course, the tail protrudes; in the search for food, the head is out. A worm, then, must make its home, narrow as it is, with a view to being able to turn in it.
An earthworm will bury itself in loose earth in two or three minutes, and in compact soil, in fifteen minutes. Pupils should be able to make these observations easily either in the terrarium or in the garden.
In plugging the mouths of their burrows, earthworms show something that seems like intelligence. Triangular leaves are invariably drawn in by the apex, pine-needles by the common base, the manner varying with the shape of the leaf. They do not drag in a leaf by the footstalk, unless its basal part is as narrow as the apex. The mouth of the burrow may be lined with leaves for several inches.
The burrows are not found in dry ground nor in loose sand. The earthworm lives in the finer, moderately wet soils. It must have moisture since it breathes through the skin, and it has sufficient knowledge of soil texture and plasticity to recognize the futility of attempts at burrow building with unmanageable, large grains of sand.
These creatures are nocturnal, rarely appearing by day unless "drowned out" of the burrows. During the day they lie near the surface extended at full length, the head uppermost. Here they are discovered by keen-eyed birds and sacrificed by thousands, notwithstanding the strong muscular protest of which they are capable.
Seemingly conscious of its inability to find the way back to its home, an earthworm anchors tight by its tail while stretching its elastic length in a foraging expedition. It is an omnivorous creature, including in its diet earth, leaves, flowers, raw meat, fat, and even showing cannibalistic designs on fellow earthworms. In the schoolroom, earthworms may be fed on pieces of lettuce or cabbage leaves. A feeding worm will show the proboscis, an extension of the upper lip used to push food into the mouth. The earthworm has no hard jaws or teeth, yet it eats through the hardest soil. Inside the mouth opening is a very muscular pharynx, which can be extended or withdrawn. Applied to the surface of any small object it acts as a suction pump, drawing food into the food tube. The earth taken in furnishes some organic matter for food; calcareous matter is added to the remainder before being voided. This process is unique among animals. The calcareous matter is supposed to be derived from leaves which the worms eat. Generally the earth is swallowed at some distance below the surface, and finally ejected in characteristic "castings." Thus, the soil is slowly worked over and kept in good condition by earthworms, of which Darwin says: "It may be doubted whether there are many other animals which have played so important a part in the history of the world as have these lowly organized creatures."
References—The Earthworm, Darwin; The Natural History of Some Common Animals, Latter.
"Fly fishing is an art, a fine art beyond a doubt, but it is an art and, like all art, it is artificial. Fishing with an angleworm is natural. It fits into the need of the occasion. It fits in with the spirit of the boy. It is not by chance that the angleworm, earthworm, fishworm, is found in every damp bank, in every handy bit of sod, the green earth over, where there are races whose boys are real boys with energy enough to catch a fish. It is not by chance that the angleworm makes a perfect fit on a hook, with no anatomy with which to feel pains, and no arms or legs to be broken off or to be waved helplessly in the air. Its skin is tough enough so as not to tear, not so tough as to receive unseemly bruises, when the boy is placing it on the hook. The angleworm is perfectly at home on the hook. It is not quite comfortable anywhere else. It crawls about on sidewalks after rain, bleached and emaciated. It is never quite at ease even in the ground, but on the hook it rests peacefully, with the apparent feeling that its natural mission is performed."
Leading thought—The earthworm is a creature of the soil and is of much economic importance.
Method—Any garden furnishes abundant material for the study of earthworms. They are nocturnal workers and may be observed by lantern light. To form some estimate of the work done in a single night, remove the "casts" from a square yard of earth one day, and examine that piece of earth the next. It is well to have a terrarium in the schoolroom for frequent observation. Scatter grass or dead leaves on top of the soil, and note what happens. For the study of the individual worm and its movements, each pupil should have a worm with some earth upon his desk.
Observations—
1. How does the earthworm crawl? How does it turn over? Has it legs? Compare its movement with that of a snake, another legless animal. What special provision for locomotion has the earthworm?
2. Compare the lengths of the contracted and extended body. How accounted for?
3. Describe the body—its shape and color, above and below. Examine the segments. Do all the worms have the same number? Compare the head end with the tail end of the body. Has every worm a "saddle," or clitellum?
4. Does the earthworm hear easily? Has it eyes? Is it sensible to smell or to touch? What sense is most strongly developed?
5. Describe the home of the earthworm. Is it occupied by more than one worm? How long does it take a worm to make a burrow? How does it protect its home? How does it make a burrow? In what kind of soil do you find earthworms at work?
6. Is the earthworm seen most often at night or by day? Where is it the rest of the time? How does it hold to its burrow? When is the tail end at the top? When the head end?
7. What is the food of the earthworm? How does it get its food?
8. Look for the eggs of the earthworm about manure piles or under stones.
9. What are the enemies of the earthworm? Is it a friend or an enemy to us? Why?
10. The earthworm is a good agriculturist. Why?