Or take that lazy water-snail (Paludina vivipara), first made known to science by the great Swammerdamm, the incarnation of patience and exactness, and you will find, as he found, forty or fifty young snails, in various stages of development; and you will also find, as he found, some tiny worms, which, if you cut them open, will suffer three or four infusoria to escape from the opening.[27] In your astonishment you will ask, Where is this to end?
The observation recorded by Swammerdamm, like so many others of this noble worker, fell into neglect; but modern investigators have made it the starting-point of a very curious inquiry. The worms he found within the snail are now called Cercaria-sacs, because they contain the Cercariæ, once classed as Infusoria, and which are now known to be the early forms of parasitic worms inhabiting the digestive tube, and other cavities, of higher animals. These Cercariæ have vigorous tails, with which they swim through the water like tadpoles, and like tadpoles, they lose their tails in after life. But how, think you, did these sacs containing Cercariæ get into the water-snails? “By spontaneous generation,” formerly said the upholders of that hypothesis; and those who condemned the hypothesis were forced to admit they had no better explanation. It was a mystery, which they preferred leaving unexplained, rather than fly to spontaneous generation. And they were right. The mystery has at length been cleared up.[28] I will endeavour to bring together the scattered details, and narrate the curious story.
Under the eyelids of geese and ducks may be constantly found a parasitic worm (of the Trematode order), which naturalists have christened Monostomum mutabile—Single-mouth, Changeable. This worm brings forth living young, in the likeness of active Infusoria, which, being covered with cilia, swim about in the water, as we saw the Opalina swim. Here is a portrait of one. (Fig. 4.)
Each of these animalcules develops a sac in its interior. The sac you may notice in the engraving. Having managed to get into the body of the water-snail, the animalcule’s part in the drama is at an end. It dies, and in dying liberates the sac, which is very comfortably housed and fed by the snail. If you examine this sac (Fig. 5), you will observe that it has a mouth and digestive tube, and is, therefore, very far from being, what its name imports, a mere receptacle; it is an independent animal, and lives an independent life. It feeds generously on the juices of the snail, and having fed, thinks generously of the coming generations. It was born inside the animalcule; why should it not in turn give birth to children of its own? To found a dynasty, to scatter progeny over the bounteous earth, is a worthy ambition. The mysterious agency of Reproduction begins in this sac-animal; and in a short while a brood of Cercariæ move within it. The sac bursts, and the brood escapes. But how is this? The children are by no means the “very image” of their parent. They are not sacs, nor in the least resembling sacs, as you see. (Fig. 6.)
They have tails, and suckers, and sharp boring instruments, with other organs which their parent was without. To look at them you would as soon suspect a shrimp to be the progeny of an oyster, as these to be the progeny of the sac-animal. And what makes the paradox more paradoxical is, that not only are the Cercariæ unlike their parent, but their parent was equally unlike its parent the embryo of Monostomum (compare fig. 4). However, if we pursue this family history, we shall find the genealogy rights itself at last, and that this Cercaria will develop in the body of some bird into a Monostomum mutabile like its ancestor. Thus the worm produces an animalcule, which produces a sac-animal, which produces a Cercaria, which becomes a worm exactly resembling its great-grandfather.
One peculiarity in this history is that while the Monostomum produces its young in the usual way, the two intermediate forms are produced by a process of budding, analogous to that observed in plants. Plants, as you know, are reproduced in two ways, from the seed, and from the bud. For seed-reproduction, peculiar organs are necessary; for bud-reproduction, there is no such differentiation needed: it is simply an out-growth. The same is true of many animals: they also bud like plants, and produce seeds (eggs) like plants. I have elsewhere argued that the two processes are essentially identical; and that both are but special forms of growth.[29] Not, however, to discuss so abstruse a question here, let us merely note that the Monostomum, into which the Cercaria will develop, produces eggs, from which young will issue; the second generation is not produced from eggs, but by internal budding; the third generation is likewise budded internally; but it, on acquiring maturity, will produce eggs. For this maturity, it is indispensable that the Cercaria should be swallowed by some bird or animal; only in the digestive tube can it acquire its egg-producing condition. How is it to get there? The ways are many; let us witness one:—