Part 26

FIG. 134.

(For Description, see note, next page.) ]

Footnote 4:

FIG. 134.—The Beet Eelworm (Heterodera Schachtii): 1, A beet root with adult females attached (natural size); 2, a lateral rootlet of beet which Eelworm larvæ (a) are penetrating (enlarged); 3, a lateral rootlet of beet with swelling (a) of the outer skin caused by the excessive development of larvæ which have previously entered it, and which have now become nearly mature females (enlarged); 4, a just-hatched larva; 5, a larva which has penetrated a root and swollen up into a club-shaped form; 6–8, development of the male; 9, a nearly adult and, 10, an adult female. The following letters have reference to Figs. 4–10: a, mouth spine,—b, sucking stomach (pharynx),—c, d, intestine,—e, f, rectum,—f, anus,—g, excretory organ; h (in Figs. 4–7), rudiments of the sexual organs in the undeveloped larvæ; h (in Fig. 8), testis; i (in Figs. 7 and 8), copulatory spicules of the male; k (Figs. 9 and 10), ovary; l (Figs. 9 and 10), female sexual opening; m, nerve ring; w{1} (Figs. 6–8), larval skin; w__{2} (Figs. 6–8), new skin which the developing male forms within the larval skin. Figs. 4 and 5 are magnified more than Figs. 6–10.

The =Beet Eelworm= (Heterodera Schachtii)

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.

The =Root-knot Eelworm= (Heterodera radicicola).

This second species of the genus Heterodera develops much like the beet eelworm, living like it on the roots of plants, but more deeply within them, so that the body of the female (which swells till it becomes pear- or flask-shaped) does not cause the outer part of the infested root to project, and does not reach the exterior. Where the eelworms collect, galls are formed which decay later on, when the eel-shaped larvæ escape into the soil, very soon, however, to enter the roots again at their slender tips. The galls are thickenings in the course of the root, and are never lateral appendages, like, for instance, the well-known “tubercles” in the roots of leguminous plants (e.g. clover, pea). The root eelworm lives in the roots of more than fifty plant species in the most various parts of the world, and infests both weeds and cultivated plants. It is especially damaging to clover and lucerne, in which not only the galls die, but also all parts of the root below them. In this country the roots of cucumber and tomato are sometimes attacked (E. A. Ormerod).

CLASS: =PLATYHELMIA= (FLAT WORMS).

Flat worms have a flat, generally leaf-like body, and usually no body-cavity, the space between the internal organs and the muscular body-wall being filled up with connective tissue.

I will only deal here with the orders of Tapeworms (Cestoda) and Flukes (Trematoda).

ORDER: =Cestoda= (TAPEWORMS).

FIG. 135.—Tænia saginata: head, and a number of joints, represented natural size. ]

FIG. 136.—The Common Tapeworm (Tænia officinalis): a, head and neck, strongly magnified; b, joints, natural size; c, eggs, strongly magnified. ]

FIG. 137.—Tapeworm Larva (of Tænia solium), much enlarged. ]

FIG. 138.—The three types of Bladder-worm, diagrammatically represented. ]

FIG. 139.—Measle of T. solium, with head thrust out (× 6). ]

The external characters of an ordinary tapeworm may be gathered from Figs. 135 and 136. A very small “head” continued behind into a small “neck” can be distinguished, and a very large number of joints, of which the first are very small and short, while those which follow get larger and larger the further they are from the head. The tapeworm at first consists only of a head which holds on to the lining of the intestine by means of suckers or similar structures. The hinder part of the head next elongates to form the so-called “neck,” and a cross-wall is quickly developed a little way in front of the hind end of this neck. By this division is formed the first joint, which, though forthwith tolerably independent, remains attached to the neck of the tapeworm head, until a new joint is separated off from the hinder part of the neck, immediately in front of the first joint. Every new joint is similarly formed by the constriction of the hinder part of the neck. Each joint possesses a complete set of hermaphrodite reproductive organs; it becomes sexually mature without reference to other joints, breaks off, and creeps about, even for considerable distances, and finally ruptures, so that the numerous eggs are liberated. Each joint may therefore be regarded as an individual, though the tapeworm, considered as a whole, also possesses a certain individuality. The tapeworm is a species of animal which exists in two forms. One of these is the sexually reproducing joint, which leaves the original host, and from the eggs of which are developed larvæ, which become, in another host, bladder-worms, or measles. These develop a head, which is the asexual second form of the tapeworm. This, therefore, is a case of metagenesis (p. 16). It appears, from what has been said, that the head can in no sense be compared to the head of an ordinary animal; it has neither mouth nor jaws, for tapeworms do not possess a gut. The head and joints take up osmotically through their walls the nutritive matter found in the intestine of their host. There are suckers and, in several species, a circlet of hooks on the head (Fig. 136, a), by which the animal holds fast to the intestinal wall. Thousands, or even tens of thousands, of eggs may be produced by a single joint. The joint is liberated when the eggs begin to ripen, often before. In some species several adjacent joints are pinched off at the same time. The tapeworm joints leave the intestine of their host either alone or with the dung. After falling to the ground they creep along like snails, and attach themselves to various plants. It is obvious that a tapeworm joint or a number of the eggs produced by it may easily be taken up by a grazing animal. The eggs of a particular kind of tapeworm are only able to develop further if they reach the intestine of a particular kind of host. It is undoubtedly true that, as a rule, only a relatively small number of the eggs produced by a tapeworm develop further, but the great powers of reproduction prevent the species from dying out. If, now, either a joint or the eggs from one are taken into the gut of a suitable host, the joint and the egg-shells, or the latter only, as the case may be, are digested in the stomach or intestine, so that the larvæ are set free. These (Fig. 137) are spherical, glass-like, of microscopic size, as might be expected, and bear three pairs of hooklets, by means of which they quickly perforate the wall of the gut, and make a way through the tissues of the host. They may also be carried further by the blood-stream. The larva of a particular species settles down finally, not only in some particular kind of host, but also in some definite organ or tissue. It then loses its hooks, and is rapidly transformed into a hollow bladder-worm (measle), which grows till it reaches a definite size, depending on the species. Meanwhile it gradually develops one (Cysticercus) or several (Cœnurus) ingrowths, or (in Echinococcus, Fig. 138) several new bladders are developed inside the bladder-worm, and ingrowths are usually formed within these, similar to those of the simpler kinds of bladder-worm. These ingrowths resemble tapeworm heads in all respects, possessing suckers, and, it may be, a circlet of hooks, but these are inside, and not outside. Later—either in the animal originally inhabited by the bladder-worm, or after it has been transferred in the flesh of this to the gut of some carnivorous animal—the bladder-worm contracts, so that it can no longer hold the fluid which is present, and the ingrowth is turned inside out, the suckers thus becoming external. The tapeworm head is formed in this way, but the bladder still remains attached to its hinder end (Fig. 139). If a host inhabited by bladder-worms is not devoured by another animal, the bladder-worms, after reaching their full size, remain for a long time in the same stage of development without being able to develop further. But if the host is devoured by another animal, the bladder-worms enter its stomach and intestine, where the bladder is digested, while the tapeworm head remains uninjured. This thrusts out its suckers and attaches itself by means of them to the wall of the intestine, the process of joint-formation already described beginning soon afterwards.

The following tapeworms, which are important to the owner of stock, will be briefly dealt with.

The =Common= or =Armed Tapeworm of Man= (Tænia solium).

FIG. 140.—Measles in Pork; the white spot is the head (natural size). ]

Six to ten feet long, with seven to eight hundred joints, and a circlet of hooks (Fig. 136), lives in the human intestine. Several of its sexually mature joints are commonly pinched off at a time, and if these are picked up from a field or dung-heap by a pig, the contained eggs develop into “pork measles” (Cysticercus, Fig. 140), which are especially abundant in the connective tissue between the muscles, and make the meat “measly.” They may also occur in the connective tissue of other parts (fat, brain, spinal cord). Young pigs not more than six months old are most in danger of becoming measly. A tolerably large number of measles can be borne very well, but if very abundant they cause emaciation, weakness, lameness, and poverty of blood. The flesh of an animal badly infested with measles may be pale, watery, or even greasy, while pork not so much diseased may seem all right, though it is really unfit for food, since if eaten in an insufficiently cooked state by human beings it would produce tapeworms. The measles are most abundant in the muscles of the breast and neck, and next to them in those of the hams and shoulders. From twelve to twenty thousand measles may be present in a single pig.

The =Unarmed Tapeworm of Man= (Tænia saginata)

lives in the intestine of man, and, as a bladder-worm (Cysticercus), in the connective tissue of the muscles of the ox (and especially in the calf). It is from twelve to twenty feet, with as many as a thousand joints, but does not possess a circlet of hooks. The joints generally leave the intestine one by one, and creep on to grass and herbs, where they burst, so that the eggs are set free, and therefore get widely distributed. The ox scarcely ever takes in several joints of Tænia saginata, and consequently only a certain number of eggs. Beef never, therefore, contains so many measles as pork, and there is not a special measle disease of oxen. As, however, the eggs of Tænia saginata are more widely distributed, calves and oxen are more frequently infected with bladder-worms than is the case with swine.

The =Cœnurus Tapeworm of the Dog= (Tænia cœnurus)

is the cause of the bladder-worm, producing sheep-gid, or sturdy (Cœnurus cerebralis). This tapeworm, which may live in the intestine of other animals besides the dog, has a circlet of hooks, is from fourteen to sixteen inches long, and consists of about two hundred joints. It generally lives in the intestine of the sheep-dog, in correspondence with which is the fact that the bladder-worm lives in the brain of young sheep. If a sheep-dog harbours Tænia cœnurus in its stomach, a sheep may very easily take up a joint with grass or heath. The joint is then digested in the sheep’s intestine, and the larva escapes from the egg. It bores through the wall of the sheep’s intestine, gets into the circulation, and is carried by the blood-stream to various parts of the body; it can only develop further, however, in the brain (or spinal cord). After the minute larvæ have reached the cavity of the cranium they move about on the outer surface of the brain, and dig out channels there till they find a suitable place for further growth. The larva now becomes a bladder-worm, which gradually grows, until it attains a size varying from that of a nut to that of a hen’s egg. It develops numerous tapeworm heads, even from three to four hundred. These become tapeworms if a sheep is killed by the parasite, and its head devoured by a dog or fox.