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.
CHEWTON-ABBOT.
BY HUGH CONWAY.