He roused from his trance, looked round him, then clasping hands at his back, walked dreamily after his son.
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.