Part 17
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_).
[Illustration:
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.
The disease caused by the presence of bladder-worms in the brain is called “gid,” or “sturdy,” also “staggers.” Two stages can be distinguished in it. During the first period the symptoms of disease are chiefly due to the wounds which the larvæ make on the surface of the brain, and the consequent inflammation of the investing membranes. Even in this stage the disease may be fatal, but the symptoms cease as soon as the larvæ cease to move about. The bladder-worms are only able to do harm by the pressure they exert, which is small at first, though it gradually becomes greater. The first stage of gid manifests itself in late summer, the second in late autumn, winter, or spring, according to the rate at which the bladder-worms develop. The symptoms of the first stage are caused by the increased flow of blood to the brain and head generally, resulting from the irritation which the larvæ exert. (Head very hot. Eyes bloodshot. The sheep are sluggish but restless, and carry their heads in a strange way, to one side, bent back, or turned up. They often run round in a large circle. There may also be gnashing of the teeth, foaming at the mouth, squinting, convulsive movements, and loss of consciousness.) The symptoms of the disease in its second stage are the result of pressure on the brain and some of the arteries of the brain and skull. (The sheep affected separate themselves from the others, and generally hang their heads. Gaze fixed. Occasional loss of consciousness. Movements strange; run or turn round in a circle. The sheep often staggers and falls to the ground. The bones of the skull become thinner at the place under which the bladder-worm is found, sometimes even as thin as paper.)
_Remedies_: Trepanning, as the bladders lie just underneath the roof of the skull. _Prevention_: Reducing the number of sheep-dogs; giving the dogs medicine to rid them of tapeworms in spring and summer; administering a specific to the yearling sheep in July and August, suitable for driving out any tapeworm larvæ which may be present in their intestines; _burning_ (instead of _burying_) the heads of sheep affected with staggers which have died or been slaughtered. It is but rarely that all the larvæ living on the surface of the brain die between the first and second stages of the disease. A sheep affected by gid is therefore almost sure to die, and should be slaughtered as soon as possible. Compare “false gid,” p. 189.
The =Minute Tapeworm of the Dog= (_Tænia eckinococcus_)
is at most scarcely one-sixth of an inch long, and consists of only three or four joints, often lives in large numbers in the intestine of the dog without injuring its host, but causes huge cysts (from the size of a pigeon’s egg to that of a child’s head) in the liver, lungs, or other organs of man, pig, and ruminants. These cysts are of the type represented on the right of Fig. 138. They may prove fatal, owing to the pressure they exert in the organs containing them and the blood-vessels of the same.
The =Expanded Tapeworm= (_Tænia expansa_)
lives in the intestine of the ox, where it may attain the length of two feet, while in the intestine of the lamb it reaches at most the length of about nineteen inches. It is sometimes present in large numbers in lambs, causing difficulty of digestion with resulting emaciation, poverty of blood, and even death. Bladder-worm still unknown.
ORDER: =Trematoda= (FLUKES).
Flattened parasitic worms, tongue-shaped or leaf-shaped, living on or in a host, and provided with one or two ventral suckers. The intestine forks into two branches immediately behind the gullet, and these two divisions in some species (_e.g._ the large liver-fluke) branch again repeatedly in a tree-like fashion. Anus and blood-vessels absent. Almost all flukes are hermaphrodite; they fertilize themselves. All the forms living within a host (_i.e._ endoparasitic forms) lay an immense number of small eggs, from which larvæ are hatched that become asexually reproducing animals. These or their progeny produce once more the sexual form. The development is therefore an example of metagenesis (p. 16), but there is a change of host. Only the two liver-flukes need be dealt with here.
The =Large Liver Fluke= (_Distoma hepaticum_),
(Fig. 141), is flat, but swollen in the middle, when it contains a large number of eggs, under which circumstances the edges only are flat. The last are of a dirty brown colour, while the middle of the body is greyish yellow. There is a triangular projection at the front end of the body with a sucker at the apex, and another at hinder end of the triangle. The outer surface of the body is covered with minute backwardly projecting spines, which present no hindrance to the forward creeping of the body in the bile-ducts, but make backward creeping or gliding impossible.
[Illustration:
FIG. 141.—The Liver Fluke (_Distoma hepaticum_), natural size. ]
Eggs ¹⁄₁₈₀ of an inch long, with a red shell possessing a greenish sheen. They can only develop if they get into water, and in wet years the conditions are very favourable on damp meadows, for under such conditions the sheep-dung, with the contained fluke eggs, is liable to fall into pools of water. But many of the eggs can develop in years that are not so wet and on dryer fields, as they may be carried into a ditch by rain or on the feet of the sheep. The larva hatched from the egg of the liver-fluke is ¹⁄₁₂₅ of an inch long, elongated in shape, and swims about freely in the water by means of a covering of cilia (Fig. 142, _a_). It soon finds its way into the lung cavity of a small water snail (_Lymnæus truncatulus_, Fig. 142, _i_), loses its covering of cilia, and becomes broader, even almost spherical. In the free-swimming larva there are already to be found a collection of germinal cells in the hinder part of the body; later on, these cells repeatedly divide and form little heaps, which ultimately become fresh individuals. The larva has, therefore, passed into a second stage, and this is now known as a _sporocyst_ (Fig. 142, _b_, _c_). The stage developed asexually from the heap of germinal cells is distinguished from the sporocyst by a differently shaped body and by the possession of a gut (with mouth, pharynx, and intestine, but no anus), which is absent in the latter. This stage is termed a “_redia_” (Fig. 142, _d_). From five to eight of them are usually developed within one sporocyst. Germinal cells are also present in the redia, and they develop into little heaps of cells from which, while the weather is warm, new rediæ are developed, that live in the lung cavity of the snail, like their parents. When the weather becomes colder, “_cercariæ_” (Fig. 142, _f_) are developed from the germinal cells, some fifteen to twenty of them within a single redia (Fig. 142, _e_). The cercaria has a long tail, by means of which it can propel itself in the water, after leaving the mother redia and the snail. It possesses two suckers, and a forked intestine without anus. After the cercariæ have moved about in the water for some time they fix themselves, by means of their suckers, to various plants growing in the water. The tail being now superfluous is lost, the cercaria becomes spherical, and contracts somewhat within an investment which it secretes (Fig. 142, _g_). When, later on, the pools in which the cercariæ live dry up, the plants which were before submerged are now on dry land, and may be devoured by sheep, together with the encysted cercariæ found upon them. The wall of the cyst is dissolved in the intestine of the sheep, and the contained cercaria gradually becomes a young liver-fluke, which at first is oval and very small (Fig. 142, _h_), while, later on, its anterior third becomes the triangular projection at the front end of the adult fluke, and its posterior two-thirds grow very considerably. Meanwhile the young liver-fluke has travelled from the intestine of the sheep into its bile-ducts. It appears that the fluke is completely grown within a few weeks after its introduction into the body of the sheep. Infection almost always takes place in summer or autumn. The life history of the liver-fluke exemplifies change of host and metagenesis (p. 16). From the egg of a sexual worm a free-swimming larva is developed, which becomes an asexual _sporocyst_ within the snail. This sporocyst produces asexual _rediæ_, and from these, also asexually, are developed _cercariæ_, which leave the snail, pass into water, attach themselves to plants, and reach the intestine of the sheep later on. The cercariæ become flukes by a process of metamorphosis, and are to be regarded as fluke larvæ.
[Illustration:
FIG. 142.—Liver Fluke: _a_, free-swimming ciliated larva; _b_, young _Sporocyst_, a few days after migration into the snail; _c_, _sporocyst_, containing rediæ in various stages of development; _d_, _redia_, containing balls of germinal cells; _e_, mature _redia_, containing its brood of cercariæ; _f_, _mature cercaria_; _g_, just-encysted _cercaria_; _h_, young fluke, soon after entering sheep; _i_, the water snail, _Lymnæus truncatulus_.
_a_, _b_, _c_, _d_ × 200; _e_ × 150; _f_ × 300; _g_ × 150; _h_ × 25; _i_, natural size and × 3½. ]
It is easily explained why the “fluke disease,” or “liver rot,” caused by the fluke should appear much oftener in some regions than others. Low-lying pastures which are now and then flooded are best suited for the development of the disease, since the fluke eggs have a better chance of developing upon them. This is because at certain times of the year there are in such spots numerous submerged plants on which _cercariæ_ encyst, and which, later on, when the pools dry up, are nibbled by sheep. It seems that the _cercariæ_ mostly attach themselves to certain kinds of plants, hence the local nature of the disease, which of course also depends on the presence of _Lymnæus truncatulus_ (Fig. 142, _i_).
Fluke disease first makes itself apparent in the sheep one or two months after infection. If the sheep does not die the parasites often remain in its liver till the next spring. The liver of a single sheep may contain from 200 to 250 individuals. They live in the various branches of the bile-duct, and the irritation they set up first causes an unusual quantity of blood to flow to the liver. Inflammation and internal bleeding may result, and gall stones are deposited in the thickened walls of the biliary passages. The flow of bile from the liver is made difficult or impossible. Later on, the parasites draw so much blood from the liver that this organ is very insufficiently nourished, so that no more bile is secreted, and the liver substance shrivels up. In consequence of these changes the digestion is incomplete, and the whole body of the sheep is very insufficiently nourished. The diseased sheep are therefore sluggish and enfeebled; they eat little, but drink a great deal, and suffer from poverty of blood. Their wool gets dry, and does not hold together, and they become emaciated. Dropsy and diarrhœa set in, and jaundice often makes its appearance. If a sheep is infested by a tolerably large number of flukes, death is almost always the final result.
“The best _preventative_ is a trustworthy and careful shepherd,”[5] who will not let the sheep feed in spots where previous experience shows the disease is usually contracted during damp summers. Furthermore, sheep badly infested should be slaughtered as soon as possible, so that the flukes they contain may not get to the egg-laying stage. Sick sheep should be fed with nourishing food (various kinds of hay, oats; with addition of cooking salt), so that young sheep containing but few flukes may get back their strength, while this treatment will make the badly infested sheep die more quickly.
Footnote 5:
Spinola.
Oxen are also frequently affected by fluke disease in many regions, other animals to a less extent.
The =Small= or =Lancet-shaped Liver Fluke= (_Distoma lanceolatum_)
is about three-eighths of an inch long, one-tenth of an inch broad, thin, and lancet-shaped. Lives, like the preceding kind, in the liver of the sheep. Development unknown.
=Fourth Sub-Kingdom: MOLLUSCA= (MOLLUSCS).
This sub-kingdom, to which oysters, snails, cuttle-fish, etc., belong, includes unsegmented animals devoid of internal skeleton, and with bilaterally symmetrical (p. 16) embryo, while the adult may be much modified, especially in snails, which are enclosed in spiral shells, and also have their bodies partially coiled. The skin of molluscs possesses a peculiar and characteristic covering. From a definite part of the body a larger or smaller fold of skin (“mantle”) grows out, which encloses a space termed the “mantle-cavity,” and also, from its function, the “respiratory cavity.” In most molluscs this mantle secretes an external or internal calcareous mass. In this way a “shell” is developed (Fig. 143, A, _S_), which is usually external. This is the case, for example, with the edible, or Roman snail, and the common garden snail. In other cases (cuttlefishes), the shell is formed inside the mantle, or it may be replaced by a collection of calcareous granules (various slugs). In most molluscs the chief organ of locomotion is the so-called “foot” (Fig. 143, _F_). This is a very muscular region of the body, covered by skin, and serving either for crawling (snails), or else for digging in the sand, and even springing (bivalve molluscs). Its structure varies according to its function. I mention three classes, those of _Cephalopoda_ (Cuttlefishes), _Gastropoda_ (Snails and Slugs), and _Lamellibranchiata_ (Bivalve Molluscs).
[Illustration:
FIG. 143.—Diagrams to explain the form of the body of a Mollusc. A, a Bivalve Mollusc; B, a Snail (cross sections). In both; _K_, body; _F_, foot; _S_, shell; _m_, mantle; _k_, _k_, gills. ]
CLASS: =CEPHALOPODA= (CUTTLEFISHES).