GENERATIONS, ALTERNATION OF, a phrase devised by Steenstrup, a Danish natu ralist, about the year 1840, to signify " the remarkable and till now inexplicable natural phenomenon of an animal producing an offspring, which at no time resembles its parent, but which, on the other liana, itself brings forth a progeny which returns in its form and nature to the parent animal, so that the maternal animal does not meet with its resemblance in its own brood, but in its descendants in the second, third, or fourth degree or generation; this always taking place in the different animals which exhibit the phenomena in a determinate generation, or with the intervention of a determinate number of generations." The phenomenon has been observed in many of the hydrozoa, in various entozoa, in annelids, in mollascoids (salpce). and in insects (aphides).
We commence with the development of the medusce or jelly fishes, which belong to the class hydrozoa. The medusa discharges living young, having burst the of the egg, swim about freely for some time in the body of the mother. When first discharged or born, they have no resemblance whatever to the perfect medusw, but are little cylindrical bodies, covered with cilia, moving with considerable rapidity, and resembling iufnsoria. After moving freely in the water for some days, each little animal'fixes itself to some object by one extremity, while at the opposite extremity a depression is gradually formed, the four cornets becoming elbngated, and gradually transformed into tentacles. These tentacles increase in number till the whole of the upper margin is covered with them. Transverse wrinkles are then seen on the body at regular intervals, appearing first above, and then extending downwards. As these wrinkles grow deeper, the edge of each segment presents a toothed appearance, so that the organism resembles an artichoke or pine-cone, surmounted by a tuft of tentar Iles. The segments gradually become, more separated, until they are united by only a very slender axis, when they resemble a pile of shallow cups placed within each other.
At length the upper segment disengages itself, and then the others in succession. Each segment continues to develop itself until it becomes a complete medusa; while the basis or stalk remains, and produces a new colony. Here, then, we have the egg of the medusa gradually developed into a polypoid organism, to which the term strobila (from strobilos, a pine-cone) has been given. This polype, by gemmation and fission, yields medusa: with reproductive organs.
The phenomenon of alternation of generations in the cestoid worms (q.v.), and in certain trematoid worms (see FLUKE), has already been noticed, and will be further discussed in the article.TApEwonms. The fission of certain annelids (Syllis and Myria• nida), (see RErnonuerfox), presents an example, although at first sight a less obvious one, of alternation of generations; the non-sexual parent worm yielding, by fissure, progeny containing spermatozoa and ova, from which again a non-sexual generation is produced.
The sace (mollusca or molluscoids belonging to the family tunicata) are usually regarded as affording a good illustration of the phenomenon under consideration. it was in these animals that it was originally noticed by Chamisso, who accompanied Kotzebue in his voyage round the world (1815-18). The salme (from twenty to forty in number) are united together by special organs of attachment, so as to form long chains, which float in the sea, the mouth, however, being free in each. The individu als thus joined in chains produce eggs; one egg being generally developed in the body of each animal. This egg, when hatched, produces a little mollusc, which remains solitary, differs in many respects from the parent, does not produce an egg, but propa gates by a kind of internal gemmation, which gives rise to chain already seen within the body of the parent, which finally bursts and liberates them. These chains, again, bring forth solitary individuals.