1. The existence of distinct genetically determined forms of the same species; such as, distinct male and female forms or distinct young and mature forms.
2. Characterized by a regular alternation of parasitic and nonparasitic phases, as in the life cycle of gordian (long slender worms found in water whose larva live as parasites on arthropods) or horsehair worms (nematode worms that live as parasites in the digestive tracts of domestic animals).