Page:The Cornhill magazine (Volume 1).djvu/84

 *duction, 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. 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:—

In this watchglass of water we have several Cercariæ swimming about. To them we add three or four of those darting, twittering insects which you have seen in every vase of pond-water, and have learned to be the larvæ, or early forms, of the Ephemeron. The Cercariæ cease flapping the water with their impatient tails, and commence a severe scrutiny of the strangers. When Odry, in the riotous farce, Les Saltimbanques, finds a portmanteau, he exclaims, "Une malle! ce doit être à moi!" ("Surely this must belong to me!") This seems to be the theory of property adopted by the Cercaria: "An insect! surely this belongs to me!" Accordingly every one begins creeping over the bodies of the Ephemera, giving an interrogatory poke with the spine, which will pierce the first soft place it can detect. Between the segments of the insect's armour a soft and pierceable spot is found; and now, lads, to work! Onwards they bore, never relaxing in their efforts till a hole is made large enough for them to slip in by elongating their bodies. Once in, they dismiss their tails as useless appendages; and begin what is called the process of encysting—that is, of rolling themselves up into a ball, and secreting a mucus from their surface, which hardens round them like a shell. Thus they remain snugly ensconced in the body of the insect, which in time develops into a fly, hovers over the pond, and is swallowed by some bird. The fly is digested, and the liberated Cercaria finds itself in comfortable quarters, its shell is broken, and its progress to maturity is rapid.

Von Siebold's description of another form of emigration he has observed in parasites will be read with interest. "For a long time," he says, "the origin of the threadworm, known as Filaria insectorum, that lives in the cavity of the bodies of adult and larval insects, could not be accounted for. Shut up within the abdominal cavity of caterpillars, grass-*hoppers, beetles, and other insects, these parasites were supposed to originate by spontaneous generation, under the influence of wet weather or from decayed food. Helminthologists (students of parasitic worms) were obliged to content themselves with this explanation, since they were unable to find