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life cyclesee LIFE COURSE.
or cycle of development, the group of developmental phases through which an organism passes in attaining maturity and becoming capable of giving rise to the next generation (thus completing the life cycle).
The length of a life cycle is determined by the number of generations that develop during the course of a year or the number of years required to complete the cycle; it also depends on the length of the period of dormancy or diapause. In animals one may distinguish a simple life cycle, in which there is direct development of individuals, and a complex life cycle, with metamorphosis or alternation of generations. In development with metamorphosis the life cycle can be observed in the development of a single individual. For example, in the swine tapeworm, ovum → oncosphere → cysticercus → adult tapeworm; in the cockchafer, ovum → larva → pupa →imago.
In development with alternation of generations or of the modes of reproduction the life cycle can be observed in two or more individuals belonging to different generations, until the initial form reappears. For example, in the Scyphozoa, ovum → planula → scyphistoma → ephyra → medusa; in the liver fluke, ovum → miracidium → rediae → cercaria → adolescaria → adult worms; in aphids, ovum → stem-mother → migrants sexupara → bisexual insects. Thus, the unit in studying the life cycle may be either the individual ontogeny or a series of alternating ontogenies. In higher plants one may distinguish annual, biennial, and perennial life cycles. The alternation of the gametophye and sporophyte generations is characteristic of the life cycles of many lower plants and ferns. In parasitic fungi the life cycle is similar in its complexity to the cycles of certain parasitic worms. In rust molds the life cycle is complex; there are forms that yield aeciospores, urediospores, and teleutospores, as well as a basidial stage. In the protozoans the most complex life cycles are found in the Sporozoa (for example, in Gregarinida and Haemosporidia).
M. S. GILIAROV