Almost all ruminants harbor the liver fluke (Fasciola hepatica). This worm has been found in every variety of the common ox and zebu (Bos taurus, var. Indicus), in the sheep, goat, and argali (Ovis aries, Capra hircus, and G. argali), in the antelopes and gazelle (A. dorcas), in red-deer, roe, and fallow (Cercus elaphus, C. capreolus, and C. dama), and in the two-humped camel (Camelus bactrianus). A closely-allied but much larger species of fluke (F. gigantea) infests the giraffe (Camelopardalis). All these animals are more or less liable to suffer from the “rot” which is produced by these flukes. Into the history of the affection the space at my command does not permit me to enter, but as regards the development of the common fluke I believe the following conclusions to be tolerably well founded. I had long entertained the opinion that our common Planorbis plays the rôle of intermediate bearer, and this view has at length received confirmation.
1. The liver fluke, in its sexually-mature state (Fasc. hepatica), gives rise to the disease commonly called rot; this affection being also locally termed coathe (Dorsetshire, Devon), iles (Cornwall), and bane (Somersetshire). In France it is known as the Cachexie aqueuse, and more popularly as pourriture. In Germany the epidemic disease is called egelseuche, and in a more limited sense either die Fäule or die Leberkrankheit.
2. The rot is especially prevalent during the spring of the year, at which time the fluke itself and innumerable multitudes of the free eggs are constantly escaping from the alimentary canal of the bearer. The germs are thus ordinarily transferred to open pasture-grounds along with the fæces of the bearer.
3. As it has been shown by dissections that the liver of a single sheep may harbor several hundred flukes, and as, also, a single adult fluke is capable of throwing off several thousand eggs, it is certain that any rot-affected flock is capable of distributing millions of fluke germs.
4. Such flukes as have escaped the host per anum do not exhibit active powers of locomotion. Their slight contractile movements, however, serve the purpose of concealing them in the grass, and probably aid in the further expulsion of eggs, which pass from the oviduct in single file.
5. After the death of the escaped flukes the further dispersion of the eggs is facilitated by the subsequent decomposition of the parent worm, and also by its disintegration, partly occasioned by the attacks of insects. It has been calculated that the uterus of a full-grown fluke may contain upwards of forty thousand eggs.
6. By the agency of winds, rains, insects, the feet of cattle, dogs, rabbits, and other animals, as well as by man himself, the freed ova are dispersed and carried to considerable distances; and thus it is that a considerable proportion of them ultimately find their way into ponds, ditches, canals, pools of all kinds, lakes, and running streams.
7. At the time of their expulsion the eggs exhibit a finely segmented condition of the yolk. The egg-contents continue to develop whilst outside the parent’s body, the granular matrix finally becoming transformed into a ciliated embryo, which when set free follows the habit of infusorial animalcules in general by swimming rapidly in the water. The escape of the embryo is effected at the anterior pole of the egg-shell, which is furnished with a lid that opens in consequence of the action of prolonged immersion, aided by the vigorous movements of the contained embryo.
8. The ciliated, free-swimming embryo, at the time of its birth, exhibits the figure of an inverted cone, its anterior extremity, which is broad and somewhat flattened, supporting a central proboscis-like papilla. A small pigment spot placed dorsally, and having the form of a cross, is supposed to be a rudimentary organ of vision. After the lapse of a few days the cilia fall off, the embryo then assuming the character of creeping larvæ (planulæ).
9. Notwithstanding its abridged locomotive powers the non-ciliated larvæ sooner or later gain access to the body of an intermediary bearer, within or upon whose tissues it becomes transformed into a kind of sac or sporocyst. In this condition the larva is capable of developing, agamogenetically, other larvæ in its interior. The sporocysts are highly organised, forming rediæ. According to Willemoes-Suhm, the redia of Fasciola hepatica lives on the body of Planorbis marginata. This organised nurse, which is about a line in length, is the Cercaria cystophora of Wagener. The progeny of this redia consists of armed Cercariæ, which after a time quit the nurse to pass an independent existence in the water.