QUEER LODGERS.
Scientific research, especially when directed to the more obscure and remote conditions of animal life, has often a twofold interest. In itself, and in the marvellous structural adaptations revealed by the microscope, the pursuit has its own special attraction; while, in addition, the information thus obtained may be so practically utilised as to minister to the preservation of health, and to the improved rearing and cultivation of animals and plants. An inquiry, conducted three years ago, by Professor A. P. Thomas, at the instance of the Royal Agricultural Society of England, is noticeable in both these respects. The inquiry extended over a period of more than two years, and the object in view throughout was the discovery of the origin and possible prevention of a well-known and destructive disease affecting sheep and other grazing animals, both in this country and abroad; and during the course of the inquiry, which was a painstaking and exhaustive one, facts of no small interest, from the view-point of natural history alone, have been elicited.
By this disease—Liver-fluke, Fluke Disease, Liver-rot, as it is variously termed—it has been estimated that as many as one million sheep perished annually, in this country alone, from the effects of the malady—a loss which was doubled, if not sometimes trebled, by the advent of a wet season such as 1879, and which does not include the large percentage of animals annually dying in America, Australia, and elsewhere from the same cause. It was known that the disease was due to the presence of a parasitic flat worm in greater or lesser numbers, together with its eggs, in the entrails of infected sheep, and also that flocks grazing habitually in low and marshy pasture-grounds were generally more liable than others to be attacked; but it was not known precisely in what manner the disease was incurred.
It was not until 1882 that careful experiment finally succeeded in tracing throughout the wonderful life-career of the liver-fluke, and shedding light upon the possibility of the prevention of the scourge. Into this latter question of prevention, we do not enter at present. Those who are interested, practically or otherwise, in this branch of the subject may consult for full particulars the scientific journals in which the results of this inquiry first appeared. (See Journal of Royal Agricultural Society, No. 28; also Quarterly Journal of Microscopical Science for January 1883. For the history of the disease, see The Rot in Sheep, by Professor Simonds; London: John Murray, 1880.) Even from a dietetic point of view, it is for the public good that the disease should be extirpated, as it is well known that unwholesome dropsical meat, from the bodies of fluke-infested sheep, is frequently pushed on the market. Nor is this parasite exclusively confined to the lower animals. It has been communicated to human beings, doubtless from the consumption of infected meat producing cysts in the liver, &c.
But it is the initial results of Professor Thomas’s experiments, those which trace the progress of the fluke from the embryo to the adult stage, with which we have to do at present.
Starting from the previously observed but obscure relationship said to exist between the larval forms of certain snails or slugs and the liver-fluke, as found in the carcases of sheep and other infected back-boned animals, it was discovered, after much careful examination, that a certain connection did exist between them, with this remarkable circumstance in addition—that the minute cysts, or bags, which contain the embryo fluke, and which are to be found adhering to grass stalks in some sheep-pastures, emanated, indeed, from the body of one particular description of snail, but that this embryo parasite was undoubtedly derived—several generations previously, and in quite another form—from the sheep itself!
The original embryo—not that which clings to grass stalks, but the embryo three or four generations before, born of the adult fluke’s egg—is hatched after the egg drops from the sheep’s body, in marshy ground, ditches, or ponds. It then attaches itself to the snail, produces in the snail’s body two, and sometimes three generations of successors, all totally dissimilar from the original fluke. The last generation alone quits the snail, and, assuming the ‘cyst’ form, waits to be swallowed by the grazing animal, in order to become a full-grown fluke. The fluke’s progeny again go through the transformation changes of their predecessors.
Once more, in order to render the process clear. Taking the adult fluke—laying its eggs principally in the bile-ducts of the sheep, which it never leaves—as the original parent, its children, grandchildren, and great-grandchildren, inhabiting the snail, are all totally different in appearance from their original progenitor—most of the generations differing also from each other. It is only the fourth, though sometimes the third generation, which, changing its form to a migratory one, is enabled thereby to leave the snail, and ultimately to assume the cyst form, adapted to produce in time the veritable fluke once more. Naturalists term this process, one not unknown in other forms of life, ‘alternation of generation,’ or metagenesis.
The appearance of the full-grown fluke (Fasciola hepatica) is well known to sheep-farmers and others. It is of an oval or leaf-like shape, not unlike a small flounder or fluke (hence the name of the worm), pale brown in colour, and ranging in size from an inch to an inch and a third in length—though occasionally much smaller, even the twenty-fourth of an inch—and in breadth about half its own length. A projecting portion is seen at the head, with a mouth placed in the centre of a small sucker at the tip, by which the fluke attaches itself. Over two hundred flukes have been found in the liver of a single sheep. Each one is estimated to produce some hundreds of thousands of eggs. Each of the eggs contains one embryo, which when full grown is nearly the length of the egg—the spare egg-space up to that time being filled with the food-stuff to support it till hatched. As long as the egg continues in the body of the sheep, it remains inert. It is only when dropped—as they are from time to time in great numbers by the animal—and alighting upon wet ground, or on water in ditches or drains, that, under favourable conditions of heat, &c., the embryo at length comes forth. The time which elapses before the egg is hatched is extremely variable.
Viewed through a microscope, the egg, which is only the two-hundredth of an inch in diameter, may be seen to contain the embryo, which is unlike its parent in every way, and will never show any trace of family likeness to it. It is in the shape of a sugar-loaf, with a slight projecting point at the broader end, and two rudimentary eyes near the same. When hatched on damp ground or in water, it swims freely about with the broader end forward, like a boat propelled stern foremost. The whole of its body, except the projecting horn, which is drawn in when swimming, is covered with long waving hairs, or cilia, which, being moved backwards and forwards, serve as oars, or paddles, to propel it through the water.
Swimming with a restless revolving motion through the water, the embryo begins to search for suitable quarters—in other words, to find a snail wherein to quarter itself. It is not easily satisfied, although snails, generally speaking, are plentiful enough. Indeed, it has been definitely ascertained that of all the known descriptions of snails there are only two which the embryo ever attacks. Of these two species, only one is apparently suitable as a dwelling, those who enter the other perishing shortly after admittance. The only suitable snail is a very insignificant fresh-water one, Limnæus truncatulus, with a brown spiral shell. It is only from a quarter to a half inch in size, and seems to have no popular name. It is to be found very widely distributed through the world. Said to breed in mud of ditches and drains, it is so far amphibious as to wander far from water. It can also remain dry for a lengthened period; and even when apparently quite shrivelled up for lack of moisture, revives with a shower of rain.
The embryo knows this snail from all others; placed in a basin of water, with many other species of snails, it at once singles this one out, to serve as an intermediate host. Into the soft portion of the snail’s body, the embryo accordingly begins to make its way. Pressing the boring horn or tool of its head against the yielding flesh of the snail, the embryo advances with a rotary motion like a screw-driver, aided by the constant movement of the cilia. The borer, as it pierces the snail, grows longer and longer, and finally operating as a wedge, a rent is eventually made sufficiently large to admit the unbidden guest bodily to the lodgings it will never quit. It settles at once in or near the lung of the snail, there to feed on the juices of the animal. The paddle-like cilia, now useless, are thrown off; the eyes become indistinct; it subsides into a mere bag of germs, as it changes to a rounder form, and becomes in other words a sporocyst, or bladder of germs—for this animal, unlike its egg-laying parent, produces its young alive within itself.
This, then, is the first stage—the embryo, from the fluke’s egg, migrates to, and becomes a sporocyst in the snail’s body.
The germs inside the sporocyst in time come to maturity, commencing the existence of the second generation, which are called rediæ. These germs number from six to ten in each sporocyst; they grow daily more elongated in form, and one by one, leave the parent by breaking through the body-walls, the rent which is thus made closing up behind them. These rediæ thus born, never leave the snail. They are, however, different from the sporocyst, being about the twentieth of an inch, in adult size, sack-like in shape, furnished with a mouth, and also with an intestine. Two protuberances behind serve the animal for legs; for, unlike the sporocyst, the redia does not remain in one part of its house, but travels backwards and forwards, preying chiefly on the liver of the snail, and generally doing a great deal of damage. Finally, indeed, these parasites destroy their host altogether.
In the bodies of the rediæ—so called after Redi, the anatomist—the third generation again is formed in germ fashion. The nature of this third generation varies. Rediæ may in turn produce rediæ like themselves, tenants of the snail for life; or they may produce another form, totally dissimilar, one which is fitted for quitting the snail and entering on another mode of existence. This change, however, takes place either in the first generation produced by the rediæ, or, at latest, in the second, more frequently in the latter. At first, this new form appears like the young of the sporocyst. But when either in the children or the grandchildren of the first rediæ, this stage is reached, the animal undergoes a remarkable change, to fit it for new surroundings. It is to be an emigrant, and dons for that purpose a tail twice as long as itself. It is then termed a cercaria, and is shaped like a tadpole.
To recapitulate, then. A cercaria may thus be the young of the rediæ, either of the first or second generation; and the rediæ again sprang from the sporocyst, which is the after-formation of the fluke’s embryo. These cercariæ or tadpole-shaped animals are flat and oval in the body, about the ninetieth of an inch in length, and tail more than twice as long. They escape from the parent rediæ by a natural orifice, crawl out of the snail, and enter on a new life. Its existence as a cercaria in this style will much depend on the locality of the snail for the time being. If it should find itself in water when quitting the snail, the cercaria attaches itself when swimming to the stalks of aquatic plants; or if in confinement, to the walls of the aquarium. If the snail is in a field or on the edge of a ditch or pool, the cercaria on leaving proceeds to fix itself to the stalks or lower leaves of grass near the roots. In every case the result is the same. Gathering itself up into a round ball on coming to rest, a gummy substance exudes from the body, forming a round white envelope; the tail, being violently agitated, falls off, and the round body left, hardening externally with exposure, the cyst or bladder—measuring about the hundredth of an inch across—is complete. Every cyst contains a young fluke, ready to be matured only when swallowed by some grazing animal, such as a sheep. Till that happens, the fluke within remains inert; and if not swallowed thus within a few weeks, the inmate of the cyst finally perishes. Of this remarkable family, however, a sufficient number outlive the changes and risks of their life-history to render the disease caused by the survivors a serious scourge.
It is to be hoped that the further results of careful inquiry into the habits of these parasites will have the effect of reducing the evil to a minimum.