Alternation of Generations. In most cases the progeny of an animal resembles in structure that of the parent; thus, the young of dogs are dogs. But with many of the lower animals and plants this is not the case; the parent is succeeded by one or more generations totally unlike itself, and from these are produced the original parent form. Thus, the plant-like colony of a Zoophyte such as Campanularia produces buds which are detached from the parent, and swim about as Jelly-fish; these produce embryos which ultimately grow into the plant-like colonies of the first generation. Instances are also found among the Aphides, Barnacles, Tape-worms, Mosses, Ferns, etc.