Among the variations, some of which have been described in the function of generation, that of alternate generation is the most remarkable. It has been aptly defined as the production, by an animal, of an offspring which at no time resembles its parent, but which itself brings forth a progeny which gives rise to other forms still differing from the parent animal, so that the original maternal animal does not meet with its resemblance in its own brood, but in its descendants of the second, third, or fourth generation. This paradoxical position of the reproductive function is not exceptional nor even rare. Vertebrate animals are the only class in which it has not been observed. In bell-shaped Polypes, Claviform Polypes, Medusae, Salpae, Vorticellae, and Entozoa it is well known. The last-named class, Entozoa, and Insects, furnish not only most striking, but also easily recognized examples.
Fig. 544. - Alternate Generation.
1. Chrysaora (Medusa): a, egg; b, Hydratuba stage; c, Hydra undergoing subdivision ; d, young medusa detached. 2. Distomum hepatieum: e, adult; f, egg; g, larva (1st generation); h, redia stage (2nd generation); i, cercaria stage (3rd generation).
Flukes, which occur in the ducts of the liver of various animals, horse, sheep, and cattle, and tape-worms, which inhabit the intestines, are among the most instructive instances of alternation of generations.
A few lines will suffice to describe the curious metamorphoses, the elucidation of which has occupied scientists for years of patient labour.
Stock-owners are well aware of the effects of the invasion of the liver of the sheep by the common fluke. This parasite is in form something like a flounder or minute sole, about an inch in length when fully grown; its digestive tubes are usually filled with bile. The reproductive system is highly developed, male and female organs existing in the same creature. Millions of eggs are deposited in the ducts of the liver of the sheep and other animals, and carried into the intestines along with the bile, finally being expelled along with the excreta. Falling on moist ground, the eggs are hatched, and from them emerge - not young flukes, but long, ciliated embryos, as much unlike the parent as the most erratic imagination can realize. Now the changes begin; the long embryo swimming about finds a snail, the shell of which it pierces, and lodges itself in the body of the animal, and becomes a " sporocyst", which means a cell full of germs. This is the first generation. The germs are developed, and become more highly organized than the embryos were, and are called "Rediae" (second generation). The Rediae escape from the parent cysts and lodge themselves in various parts of the snail; meanwhile, inside these Rediae long-tailed Cercaria are developed (third generation). Some of the Cercaria, which are tadpole-like creatures, wriggle out of the snail and enjoy for a brief space a free life of swimming in the pools and puddles of wet grounds. Soon, however, they fix themselves on grasses and other plants growing in water, exude a gummy substance, and form little cysts, in which the Cercaria, the inchoate fluke, is enclosed. In this state they remain until they are swallowed by a sheep or other warm-blooded animal, when they escape from their slight prison, find their way to the liver ducts, and assume the form of minute flukes (fourth generation, from the egg of the parent fluke).
To put the case in one view, one fluke egg gives exit to one embryo, which becomes one sporocyst, in which many Rediae are developed. In each Redia sac numerous Cercaria of the tadpole shape, the fluke of the next generation, are formed and set free. Thus a single fluke egg is calculated to be responsible for at least 200 Cercaria. Leuckhart has estimated that the oviduct of a fluke may contain 45,000 eggs; it is only necessary to multiply that number by 200 to arrive at the total number of young flukes which one fluke may produce. The whole story sounds like a fairy tale; it is, however, a true story every whit.
Only less marvellous is the history of the generation of the tape-worm, which in every mature joint produces myriads of eggs, each containing a living embryo, globular in form and armed with six minute booklets, which have a purpose presently to be divulged. Mature joints or segments are constantly being expelled from the intestines of infested animals, lambs, for example, and are, as a matter of course, eaten with herbage by other grazing animals. Reaching the digestive organs of the warm-blooded animal, the eggs are set free, the armed embryos find their way, by the aid of their booklets, to certain organs, lungs, liver, or brain, fix themselves securely by the little hooks, and grow into water-bladders (hydatids), sometimes of great size. In the interior of the hydatid there are to be found numerous minute germs, - sometimes the numbers cannot be estimated, - which arc in reality tape-worm heads and necks, ready to grow joint by joint, until they reach several feet or yards in length. The. hydatid is swallowed by a dog or other carnivorous animal, and tape-worms are again developed.
These two examples of alternation of generations may suffice. Obviously the scheme has enormous advantages on the side of multiplication of species with an abnormal rapidity, and it is a startling reflection that the creatures thus liberally distributed over the world are destined in their struggle for existence to inflict disease and death on creatures higher in the scale of creation than themselves, undiscovered, even unsuspected.