Agricultural zoology

Worms^[3] are bilaterally symmetrical animals, in which the body is enclosed in a “dermo-muscular tube.” Under the delicate epidermis there is found a layer, which does not, as in the higher animals, consist exclusively of dermis, but is partly composed of muscle-fibres, which form a distinct coat internally. Worms are able to move by contracting the various components of the dermo-muscular tube thus formed. In some worms limbs assist in the movements, but in others this is not so; in any case, however, the limbs play a relatively subordinate part. These limbs, which are only present in the bristle-worms, are small, always unjointed, foot-stumps, which bear bristles. Leeches and some other worms possess suckers by which they can attach themselves, and move by alternately contracting and extending their bodies.

There are segmented and unsegmented worms (thread worms, liver-flukes); the degree of segmentation is also very various. In many tapeworms each joint may be regarded as an individual animal. The joints of other segmented worms (earthworms) do not become detached, like those of tapeworms, for the purpose of reproduction. The nervous system is absent in the lowest worms; in the higher worms ganglia always constitute its central parts, and in annelids these are arranged in pairs on the ventral side of the body, and united by nerve fibres into a ventral cord, which is connected in front with a nerve ring surrounding the gut and thickened above into cerebral ganglia. In many worms (e.g. tapeworms and flukes) the nervous system is of much simpler structure. The sub-kingdom of worms contains a very large number of forms. It is only necessary to mention here representatives of three classes: Segmented worms (Annelida), round worms (Nematelminthes), and flat worms (Platyelminthes).

CLASS: ANNELIDA (SEGMENTED WORMS).

Segmented worms with rounded (earthworm) or flattened (leech) bodies, and possessing both mouth and anus. They fall into two sub-classes: I. Leeches (Discophora), II. Bristle-worms (Chætopoda). The first possess suckers as organs assisting locomotion, the latter foot-stumps, which bear longer or shorter bristles; but these foot-stumps may be absent, and the bundles of bristles are then simply imbedded in pits. The latter is the case in the only members of the class which interest us here, i.e. the earthworms.

The Earthworms (Lumbricus).

There are several species in the genus, but all agree in their habits. The body of the earthworm is slender and cylindrical, tapering in front and somewhat flattened behind. On the ventral side of the second segment is found the opening of the mouth. There are no eyes, though these animals are sensitive to the action of light. The earthworm is hermaphrodite (p. 16); when two individuals pair they mutually fertilize each other. On a warm summer evening two adjacent worms creep half out or nearly out of their burrows, and apply the front parts of their bodies together, especially a reddish, swollen part found at about the middle of the anterior half of the body. In this region the glands in the skin are very strongly developed, and secrete a substance which surrounds the eggs as a capsule as they are being laid. Earthworms are chiefly found in damp humus, or, at any rate, not in very poor sandy soil or clay. From time to time they carry their burrows up to the surface in order to get rid of the undigested remains (“worm castings”) of the humus and vegetable matters which have been taken into the body. The burrows run down obliquely into the soil, or more rarely vertically, to the depth of eight feet or more; they end in an enlargement, where the worm remains coiled up during the winter, after having closed the mouth of the burrow with a plug of leaves, twigs, paper, straw, etc. Although the earthworm chiefly subsists on the organic matters found in earth rich in humus, it also devours the leaves of cabbage, onion, and other plants, and especially seedlings (particularly those of beet). It draws these parts of plants about four-fifths of an inch into its burrow, and moistens them there with an acid fluid it secretes, and which acts upon them before they are taken into the body. Earthworms may effect considerable damage by destroying seedlings, particularly in damp fields, though this damage is always local. A not inconsiderable amount of benefit is to be set against this. By means of the burrows which they dig earthworms cause air to penetrate into the soil much better than it could otherwise do, which is known to be of the greatest importance for plant life. Earthworms are extremely important owing to the large numbers in which they are present in the soil, and for another reason besides the one just given. Darwin shows that in many parts of England a weight of ten tons of earth per acre passes through the bodies of earthworms, and is brought to the surface by them, so that in a few years the entire humus-containing surface layer of earth has passed through their bodies. They therefore prepare the soil in an excellent manner for the growth of plants, by continually exposing it to the air. They cause stones to sink in the soil by throwing out at the top earth which naturally consists only of particles small enough to pass through their intestines. They play the part of gardeners by thoroughly mixing together the particles of soil, and bury under their castings, in a shorter time than one would imagine, objects found on the surface of the soil (bones, oyster-shells, dead animals, leaves). Plantfood is quickly formed again from these buried matters. Earthworms, therefore, play a very important part in the economy of nature; indeed, many regions, now occupied by luxuriant pastures or fertile cornfields, would be a waste, had it not been for them. Where, however, in damp spots they injure seedlings, they may be collected, either during the day after a warm rain, or in the evening, at which times they lie half out of their burrows. If a decoction of walnut leaves is poured into these, they will crawl out. Enemies: Mole, shrews, hedgehog, toads and frogs, ground beetles, rove beetles, mole-crickets, centipedes, etc.

CLASS: NEMATELMINTHES (ROUND WORMS).

Body cylindrical, unsegmented; body-wall tough, hooklets or spines may be present on it, but deeply imbedded bristles are always absent. The gut may be absent (some parasites), but this is exceptional. There are no special organs for circulation and respiration. Sexes distinct in the large majority. I will deal with only one of the orders belonging here.

Order: Nematoda (Thread Worms).

Elongated, thread-shaped or spindle-shaped, unsegmented. A gut, terminating in a ventral anus, is present. Outer investment of the skin smooth, often weakly ringed or striated. The muscle layer underlying the true skin is interrupted here and there, where the skin itself stretches further inwards. In this way are distinguished the lateral lines or fields (Fig. 128, a), which run right along the sides of the body, dividing it into a dorsal and a ventral region. There are often present as well two smaller but similar dorsal and ventral lines, respectively situated above and below (Fig. 128, b). Mouth usually surrounded by lip-like folds, but more or less distinct jaws may also be present. The anus either lies at the apex of the tapering posterior end of the body (trichina), or further forward, on the ventral surface. Among the Nematodes are included a number of relatively small species, living free in the earth, and sucking plant parts, while other forms fairly closely related live parasitically in plants, and often cause very serious plant diseases (e.g. Beet Eelworm); a larger number of species are parasitic in animals. The free forms and those parasitic in plants lay relatively few, but very large, eggs, although, in several species, increase is furthered by a succession of many generations in the year. The Nematodes parasitic in animals lay very numerous eggs, even several millions. Harmful species are known from the following families: 1. Palisade Worms (Strongylidæ), 2. Whip Worms (Trichotrachelidæ), 3. Slender Thread Worms (Filaridæ), 4. the Round Worms (Ascaridæ), 5. Eelworms (Anguillulidæ).

I will successively treat of the harmful forms, but since many species belonging to different families infest the digestive organs of the different domesticated animals, causing similar symptoms of disease which may be dealt with in much the same way, the following general sketch is first given:—

Nematodes in the Gut (more in young animals than old) cause the following symptoms: (1) Appetite variable. (2) Nutrition in general affected, even if sufficient food is taken; constipation or diarrhœa; belly much drawn in or else, and usually, swollen out. The animal itself is thin, and has (except horse) a tendency to vomit. (3) An itching all over the body, especially at the nostrils and anus. The animal rubs and bites the sides of its body. (4) Tongue covered with a thick, soft, yellowish coating. A sweet smell from the mouth. (5) The skin is tense, lacking its usual elasticity. (6) The animal suffers from spasmodic colic, and (7) disturbances of the nervous system (whining or crying; unrestrained or suppressed fits). Remedies: Horses and cows infested with thread worms may be given chopped carrots, beets, and turnips, previously mixed with sugar or crushed sugarcandy. Roasted oats are also good. For sheep finely crushed glass, kneaded into pills with bread, will always suffice. Gritty sand in the food may also be of use. Pigs should be given sour milk, acorns, unripe cheese, horseradish. For dogs, sausages containing garlic, as much flesh food as possible, strongly salted food, milk boiled with garlic. In all cases iron may be given. The advice of a veterinary surgeon should be sought in the matter of medicines (tansy, male fern root, tartar emetic, arsenious acid, etc.).

Family: Strongylidæ (Palisade Worms).

Spindle-shaped; anus on the tip of the hind end of the body; in all thread worms the rectum and male sexual organs open by a common cloacal opening, which, in the palisade worms, is surrounded by an umbrella, or cup-shaped apparatus (bursa), kept expanded by means of muscular ribs (Fig. 129).

Here belong—

The Giant Palisade Worm, or Strongyle, (Eustrongylus gigas). Female from a foot to thirty-nine inches long, and as much as two-fifths of an inch thick. Male six to sixteen inches long; reddish. Lives in the cavity (pelvis) of the kidneys in horses, oxen, dogs, and man; causes degeneration of the kidneys, with blood in the urine, nervous diseases, and disturbance of the feelings and intelligence.

The Armed Palisade Worm, or Strongyle (Strongylus armatus), four-fifths of an inch to two inches long, one twenty-fifth to one-twelfth of an inch thick; reddish brown. Taken into the gut of the horse with the drinking water as a young, minute worm, it bores through the walls of this organ into the blood-vessels branching there. Later on it bores into the walls of the larger arteries of the hinder part of the body, especially into those which carry blood to the wall of the gut (particularly anterior mesenteric artery). At those parts of the arterial walls where it collects in large numbers, swellings (aneurisms) are produced, varying from the size of a pea to that of a hen’s egg. A constriction is often developed quite close to this swelling, and a plug of fibrin may also be formed within the artery, so that the circulation in the wall of the gut is greatly hindered. As a result of this the glands and muscles in the wall of the digestive tube refuse their office, so that regular digestion and onward movement of the food do not take place. This leads to colic. As soon as the strongyloid larvæ have become adult, they pass from the walls of the arteries into their cavities, and are carried by the blood-stream to the walls of the gut, which they penetrate in order to reach its cavity, where (cæcum and large intestine) they pair, after which the female lays eggs, which are carried to the exterior in the dung. The quickly hatched young seek water, mud, or damp earth, where they live until, by some means (e.g. with the drinking water) they manage to enter the gut of a horse or (more often) ass. Preventive Measure: Horses and asses must not be allowed to drink stagnant water.

The Stomach Palisade Worm, or Strongyle of the Sheep (Strongylus contortus). Male one-half to two-thirds of an inch, female three-quarters to four-fifths of an inch long; whitish or reddish, somewhat coiled at both ends. The disease of the stomach, and diarrhœa, which affect lambs, usually in spring, and result from the presence of these worms in large numbers in the true or fourth stomach, lead to emaciation, weakness, and poverty of blood. Development unknown. Nutritious food will cure lambs which are not very badly infested, but hastens the death of those which are.

The Lung Worm of Lambs (Strongylus filaria). Male an inch, female as much as three and a half inches long, thread-shaped, white or yellowish. It appears that the young worms are taken into the stomach of a lamb (or sheep) with the drinking water; they are found there in May, June, and July. They quickly climb back into the throat, from which they get into the windpipe and its branches. There they penetrate the mucous membrane, where, until they have reached the sexual stage, they are found imbedded in small swellings, which they quit at the end of winter or the beginning of the next spring, if their host lives so long. The female bears living young, which as very minute worms may be met with by the hundred in the mucous lining the windpipe and its branches. A direct transference of the parasite from one sheep to another does not take place, since the young born in the mucus secreted by the walls of the air-tubes must first live and grow for some time in water or slime before they are able to undergo further development in the body of a sheep or lamb. The fate, however, of the young worms which pass to the exterior in the mucus is not yet sufficiently known. Symptoms of Disease: Running of slime from the nose, difficulty of breathing, cough, emaciation, poverty of blood. Death ensues in from two to four months. Recovery only of very strong individuals infested by comparatively few parasites, most of which they cough out. Preventive Measures: In regions where the evil is very prevalent the young sheep must not be allowed to go into the meadows. If it is impossible to keep them penned in, a little food and water may be given before sending them to pasture in the morning. In this way the danger of infection will be somewhat lessened, since it is then less likely that the animals will eagerly betake themselves to pools and ditches in order to quench their thirst. Since during the months May to July the young worms (most probably taken in with the water drunk) are found in the stomach, it is good during this time to give vermifuges to the lambs and sheep. For medicines a veterinary surgeon should be consulted.

The Small-tailed Palisade Worm, or Strongyle (Strongylus micrurus). Thread-shaped; male one and two-fifths, female two and two-fifths to two and four-fifths of an inch long; lives in oxen (especially calves) in swellings of the arteries, also in the windpipe and its branches. Severe bronchitis.

Family: Trichotrachelidæ (Whip Worms).

Small, slender, elongated worms, with the anus or, in the male, cloacal aperture at the hinder end of the body; the latter, however, is not—as in palisade worms—surrounded by an umbrella or cup-shaped expansion. Here belongs:

The Trichina (Trichina spiralis).

In the sexual condition this lives as a minute worm (male one-seventeenth, female one-eighth to one-sixth of an inch long), the so-called “intestinal trichina,” in the gut of human beings and carnivorous mammals. In this situation it brings forth living young (some fifteen hundred in number), and the multiplication is enhanced by the fact that of the very numerous trichinæ found in the gut there are, on the average, about twelve females to every male. The young trichinæ bore into the walls of the gut, and are carried by the blood-stream into the muscles. At first each little worm extends itself longitudinally in a muscle fibre, but, as it gets larger, curves and becomes spirally twisted, so that the sheath (sarcolemma) of the fibre is bulged more and more outwards. The growth of the young trichina is very rapid—an individual only ¹⁄₂₅₀ of an inch long before leaving the gut may attain the length of one twenty-fifth of an inch in fourteen days. It then grows no more, and the sheath of the muscle fibre gradually thickens to form a capsule or cyst. Later on, in about a year, lime is deposited in the capsule. The “muscle trichinæ” retain their vitality for several years. If now the host of the muscle trichina is devoured by any other mammal, the capsule is digested in the stomach of the latter, and the once more liberated worm becomes a sexually mature “intestinal trichina” in a few days. The pig is the ordinary host of trichinæ, which (without the intervention of any other host) can be permanently parasitic in this domestic animal, since pigs often devour swine’s flesh. They often enough devour even their own young, and, especially in large slaughterhouses, swine are often fed with the offal of their companions. The trichinæ also spread through the body of any living being (men, rats) which eats trichinous pork. The trichinæ thus introduced cause, in the human subject, a dangerous or even fatal disease known as trichinosis. The size of this book does not permit me to describe the symptoms of this complaint in man. Pigs suffer much less from the parasite; they may even contain an enormous number of trichinæ in their muscles without being noticeably ill. Symptoms of disease, however, often appear more or less clearly. A short time after eating the trichinous meat the appetite of the pigs is bad; they appear in bad spirits; the tail straightens out, and they often remain standing with bent limbs and arched backs. There may be, in addition, pains in the abdomen, diarrhœa, and fever. Later on, when the trichinæ have settled down in the muscles, the pigs suffer from stiffness in the legs and tenderness in the loins; they often cry out in pain. After this, however, health and appetite may return, and they can be fattened. Trichinous swine therefore appear quite sound, and are slaughtered in due course. The trichinæ in the pig are found most abundantly in the diaphragm, the masticatory, eye-, and other muscles of the head, also in those of the neck, larynx, abdomen, and loins. The fore part is more infested by trichinæ than the hinder part, but in very bad cases they are found everywhere, even in the hams. Remedies: (1) The swine must be prevented, as far as possible, from taking up trichinæ, and therefore must never be given the offal from other slaughtered swine. The flesh of trichinous swine should be burnt, and not buried, lest dogs or other animals should eat it and spread trichinosis. (2) Care must be taken that no trichinous pork is eaten by human beings (microscopic examination; meat inspection). Pork (or sausages) must never be used raw or half cooked. Trichinæ are killed by a temperature of 140° to 150° F. The inside of the piece of meat must be exposed to this temperature.

Family: Filaridæ (Slender Thread Worms).

Elongated and thread-shaped. Round mouth. Do not infest the gut or other cavities, but the tissues of the body, chiefly connective tissue. None of the species require notice here.

Family: Ascaridæ (Round Worms).

Relatively shorter than the worms of the preceding family. Mouth triangular, surrounded by three teatshaped lips. In the male the hinder end of the body is usually bent in a hook-like manner. The anus is not placed quite at the hind end of the body. All the species inhabit the alimentary canal (Cf. p. 211).

Horse Worm (Ascaris megalocephala). Male six to eight, female twelve to seventeen inches long; sometimes occurs in great balls, causing stoppage.

Round Worm of Cat (Ascaris mystax). Male two to two and two-fifths, female four and four-fifths to five and one-fifth inches long; in the intestines of the cat. Ascaris marginata infests the dog.

Pinworm (Oxyuris curvula). Male one-fourth to one-third, female one and four-fifths of an inch long. In the rectum of the horse, causing persistent itching.

Family: Anguillulidæ (Eelworms).

Extremely small, thin-skinned; lay only a few relatively large eggs, which develop very quickly. With few exceptions either earth eelworms (i.e. live in decomposing organic matter or humus soil) or as plant parasites leading to characteristic diseases of wild and cultivated forms. All these parasitic forms have a “mouth spine.” This structure, found in the mouth cavity, is very sharp and pointed in front, and can be worked forwards and backwards so as to penetrate the cell walls of plants. A mouth spine is not only found in the species which infest plant tissues (several species of Tylenchus and Aphelenchus, as well as all the known species of Heterodera), but also in those forms living free in the earth which bore into the exterior of plant roots (the remaining species of Tylenchus and Aphelenchus, Dorylaimus, etc.). In all cases an eelworm, devoid of a spine, is not a plant parasite. I will only deal with those parasites by which well-known destructive diseases of cultivated plants are caused. These belong to the genera Tylenchus and Heterodera. The first remain eel-shaped throughout life, but the adult females of the latter swell out considerably and become lemon- or pear-shaped.

The Stem Eelworm (Tylenchus devastatrix).

Length one-thirtieth to one-fifteenth of an inch long, usually of intermediate size; the two sexes of approximately equal length. Live and reproduce in various cultivated plants (e.g., rye, oats, stored onions, hyacinths, buckwheat, potatoes, clover, fuller’s teasel) and wild plants (e.g., Poa annua, Anthoxanthum odoratum, Dipsacus silvestris, Polygonum persicaria), but not to the same extent in all. It must also be added that eelworms of which the progenitors have developed for a considerable number of generations in the same plant, are not easily transferred to another kind of plant, or at any rate do not multiply vigorously there. Eelworms, of which the ancestors have lived for many years exclusively in rye, or alternately in rye and buckwheat, do not readily pass over to seedling onions, and first only reproduce in them to a small extent. The eelworms live only in stems, branches, and leaves, never in roots. In the places where they have penetrated the plant tissues an abnormal growth in thickness of the parts involved takes place, while the growth in length is either much diminished or even entirely stopped. Also the chlorophyll disappears sooner or later from the attacked spots, and rapid death generally ensues. Since only those parts of an organ which are inhabited by a large number of eelworms swell much, it is obvious that cracks are often developed in the stems and leaves concerned. It is further easily seen that the species and constitution of the infested plants will have an important influence on the progress of the disease caused by the eelworms. But since several generations of this parasite succeed one another during the same year, the resulting malformation is usually very considerable. “Clover sickness” is probably due to this eelworm.

I will treat in somewhat greater detail the Eelworm Disease of rye, which is largely prevalent in Westphalia and the Rhine provinces, and had already made its appearance, in the latter at least, at the beginning of the century.

The eelworms producing the disease migrate into the soil on the ripening or death of the grain, and later on go back to the new young rye plants or to other plants in which they can live, as the case may be. It is therefore clear that (1) where this disease exists the soil is infected for a time, and (2), the eelworm disease of rye principally appears in regions where the culture of rye is carried on to excess. Germination of the seed corn goes on quite normally in infected fields, and it is only exceptionally that anything particular is noticed in the young plants of winter rye during autumn and winter. The disease appears at the beginning of spring. Some plants soon become yellow and die: others appear to grow very luxuriantly; they possess a beautiful bluish-green colour and seem very healthy; later on they develop enormously in breadth, and each separate plant covers a relatively large area. The base of the stem swells abnormally, so that the plant looks as if it bore below an onion projecting above the ground. This is caused by the lower joints of the haulm remaining very short and thickening considerably, causing the leaf-sheaths which surround the base of the stem to become thicker and broader than usual. The feeble development of roots is also characteristic. The leaves generally remain short, but get very thick; they often become wavy, and may even appear frilled. All the leaves, however, are not crumpled in this way; a few remain quite normal, while others are small and grass-like, but thick (Fig. 132). Later on, the tip of the haulm and the ear often do not come out of the leaf-sheaths; in other plants the ear may appear, but remains small and deformed as well as the rest of the haulm, while the grains which develop are small. A number of shoots, however, may develop normally and bear fruit. Badly infested plants quickly die, some at the very beginning of spring, others later.

Where the disease is very bad it may easily be recognized by its characteristic distribution. A number of bare places are noticed in spring on the infected fields; round about these places still living but badly diseased plants may be noticed, and the symptoms of attack are less obvious the further one goes from the bald spots.

On the death of the rye plants the eelworms mostly travel back to the soil, but sundry eggs and larvæ stop in the dried-up remains. Rye straw may further infect a diseased field by getting into farmyard manure and being brought back again with it; for the eelworms (at any rate as eggs and larvæ) are killed neither by drying nor by the action of dung and other decaying substances. Strongly infected spots (bare patches) on an infested field are usually the places where dung containing diseased rye-straw has lain for some time. From such centres the spreading of the eelworms takes place: (1) actively by the migration of the worms, (2) passively by rain, the feet of labourers, the hoofs of horses, field implements, etc.; also (on loose soils) by the wind, which not only blows about particles of earth, but also the dried-up larvæ which are always found on the surface of the ground. As the eelworms multiply very rapidly whenever they are in the plants, and as the means of distribution are very numerous, the disease spreads with great rapidity.

Remedies.—Proper rotation; limitation of the culture of rye, growing in place of it carrots, turnips, or lupines. Now and then (but not too frequently), potatoes, buckwheat, clover, and oats may be cultivated in the fields affected, at any rate before sowing rye again. Abundant manuring, especially with sulphates of potash, ammonia, and iron. Deep cultivation of the soil, since the eelworms find no food in the deeper, damper layers of the soil, nor can they pass into the dried condition, and so must die.

Oats suffer from eelworms just in the same way as rye.

Clover Sickness is marked by short stunted shoots, and whitish rounded buds, often remaining closed; also by the whitish colour and abnormal thickening of the shoots and buds.

Eelworm Disease of Potatoes.—Crumpling and small growth of the parts above ground. In many cases there may even be no potatoes at all, or only small ones, poor in starch, though sometimes they may be tolerably large. On the surface of the tubers there are discoloured rotten spots, not penetrating deeply, and usually (but not always) at the attached end. These dark patches contain the eelworms.

Eelworm Disease of Buckwheat.—Joints of the stem for the most part much thickened, but abnormally short. In many cases a large amount of branching in the lower part of the stem, usually at a place where it bears a nodular swelling. Branches generally short. There may be twists and bends in the stem and branches. Often, but not always, the formation of flowers and fruit is stopped. The thickened parts of the stem are brittle; they contain the eelworms in their interior.

The Wheat Eelworm (Tylenchus scandens = T. tritici).

Length of the male about one-twelfth of an inch, of the female one-tenth to one-fifth of an inch (according to the size of the galls in which the eelworms develop). The wheat eelworm is the cause of “ear cockles,” also known as “peppercorns” and “purples.” In several parts of the ear short thick dark brown galls (Fig. 133), resembling the seeds of corn-cockle, are found instead of wheat grains. Inside the thick brown shell there is a yellowish white mass, containing hundreds or even thousands of eelworm larvæ (one thirty-first to one twenty-seventh of an inch long). These are quite dry and rigid, but gradually revive on moistening, even if the black galls have remained twenty years in the dried-up condition. When the wheat is ripe the dark-walled galls are gathered in with the crop, and in many cases are sown again with the sound grains. The brown shell then decays, and the eelworm larvæ leave the gall, travelling to a neighbouring wheat seedling, where they live between leaf-sheath and haulm, also penetrating into the terminal bud. The haulm of a wheat plant infested by many eelworms remains relatively short, the leaves are often sharply bent and have wavy margins. Wheat plants thus infested closely resemble rye plants diseased in a similar way, but are much less deformed. This is because the wheat eelworms do not reproduce till they reach the ear, and there is consequently only one generation per year, while several generations of eelworms succeed one another in the same rye plant. The eelworms quickly travel from all parts of the plants into the ears, and get into the rudiments of the flowers, causing them to swell up like bladders, and their walls to become first dark green, and then dark brown. Sixteen to twenty eelworms are present in the lowest flowers of the ear, ten to twelve in those higher up (and therefore smaller), and four to six in the topmost (smallest) ones. Soon after entering the flowers the eelworms become sexually mature, and lay eggs (600 to 1600), from which are developed the larvæ that inhabit later on the cockle-seed like galls. The disease is known in England, Germany, France, and Italy, and is especially harmful in Saxony, where it sometimes attacks a quarter of the wheat crop. Remedies: Either there should be no ear cockles in the seed corn, or else the eelworms should be destroyed in them. The infested grain may either be put through a sieve, or else soaked for twenty-four hours in weak sulphuric acid (one pint strong acid to thirty-three gallons of water), when many of the galls float and can be skimmed off, while the eelworms are killed in those which sink.

The Beet Eelworm (Heterodera Schachtii)^[4]

is the cause of the “beet sickness” of the soil. The course of the disease is as follows. At the end of July light-coloured patches are found here and there among the normally developed beet. The leaves are weak and limp, and the outer ones especially get yellow, spotted, and die off. Later on the inner leaves die as well, after which the top of the beet becomes black and the whole root gradually decays. In less severe cases the beet may recover towards autumn and develop new heart leaves, but the roots remain small and the crop is poor, often being only one-third of its normal amount. Kühn has proved by infection experiments that the sole cause of beet sickness is a nematode, of which the life history is as follows. The female is found fixed to the branches of the root; it is citron-shaped, about one twenty-fifth of an inch long (Fig. 134, 1 and 10), and contains on an average three hundred and fifty eggs. Some few of these, together with a jelly-like substance making up an “egg-sac,” may pass out to the exterior, but the large majority develop in the body of the female, which ultimately becomes a mere sac enclosing the eel-like larvæ. The female is killed by the process. The liberated larva (Fig. 134, 4) seeks out a root (about one twenty-fifth of inch thick), and bores into it. Here it lives as a parasite, causing the disease of the attacked beet plant. The larva quickly sheds its old skin, assumes a thicker form (Fig. 134, 5), ceases to move, and gradually causes the outer skin of the root to bulge out externally (Fig. 134, 3, a). The distinction between the sexes now rapidly makes its appearance. A thick motionless larva, destined to become a male, temporarily ceases to feed, shrinks within its old skin, develops a thin new one, and ultimately becomes a long eel-like worm (Fig. 134, 6, 7, 8), which grows into an adult male (8). In the stage represented in Fig. 134, 8, the animal still lies under the outer skin of the root, which never bursts during its development, but the mature male bores out of its larval skin and out of the root, passing into the soil, where it finds and fertilizes the female, which in the meantime has developed but remains attached to the root. The female develops in a simpler way, by the gradual distension and growth of a larva (not by a process of re-formation) and gradual development of the female sexual organs. As the larva passes from the stage of Fig. 134, 5, into that of Fig. 134, 9, and later on into the adult condition, 10, the outer skin of the rootlet is ruptured, and the female comes out from its tissues, remaining, however, attached to its outside (Fig. 134, 1). The entire development from egg to sexual adult takes four or five weeks, and there may be six or seven successive generations, the reproduction is consequently very rapid.

It also obviously follows that “beet sickness” of the soil is especially prevalent in fields where there has been an excessive amount of beet culture. The disease, however, may suddenly appear in fields which have been hitherto “safe” for beet, and in many such cases it has been proved to result from manuring with artificial compost rich in refuse from affected fields. The disease frequently appears, too, in fields where beet have never been cultivated, but where cabbage has been grown for a long time. It has been shown, especially by Kühn’s investigations, that the beet eelworm can live in many plants both cultivated and wild, e.g. of the former, cabbage, rape, mustard, garden cress, chickling peas, mangold, oats; of the latter, charlock, spurrey, couch grass. These researches are of the greatest importance both for understanding the way in which beet sickness spreads and in combating it.

Preventive Measures: Manufactured compost must not be used as a manure on beet-fields. The refuse from infested beets, if used on other fields, must be mixed with one-sixth its bulk of quicklime. The boots of labourers employed in beet-sick fields, the hoofs of horses working in them, and also the implements used, must be carefully cleansed lest infected earth should be carried to other fields. Remedies: Kühn has recommended the use of plants which attract the eelworms (“lure-plants”). He sows on beet-sick land rapidly germinating plants, of kinds which the worms readily attack, and weeds them out again when they have become infested by the parasites, but before these have had time to mature and re-enter the soil. The eelworms are thus allured into the plants grown, and destroyed with them. Such lure-plants must be sown as thickly as practicable, so that the soil may be penetrated by as many slender rootlets as possible. After these plants have been dug up, a second lot should be grown, since all the eelworms will not have attacked the first lot; and it is even advisable to grow a third batch. Kühn used as lure-plants the various kinds of cabbage, also summer rape (Brassica rapa), since this plant has a great attractive power for the beet eelworms, and can hold a large number of them in its numerous, much branched rootlets. I cannot go into all Kühn’s researches here, and will only mention the following. In the course of the year 1880 part of a beet-sick field had three successive crops of lure-plants grown upon it, each being dug up from thirty to forty days after sowing. The field was ploughed in autumn, suitably manured the next spring, and sown with beet in the middle of April. The other part of the piece of land was treated in exactly the same way, except that no lure-plants were grown upon it. A difference was very soon seen between the two plots, and there was a very great difference at the time the crop was ready to be gathered in. The plants on all parts of the first plot were in a flourishing condition, but those on the second plot were in many places either killed outright or else small and misshapen. The crop succeeding the lure-plants was three times as great as it had been before, and almost equal to that of a healthy field. Later on, Kühn made an important discovery; he found that larvæ which have reached the thickened motionless stage, depicted in Fig. 134, 5, require a considerable amount of food to keep them alive, and enable them to develop further. If the plants are disturbed in such a way as to kill the rootlets containing the larvæ in this stage, these are unable to develop any further. Kühn caused a kind of horse machine to be made, adapted for rapidly destroying the lure-plants in the fields. For further details, his original memoirs may be consulted.