Abstract The analysis of population systems is carried out on the basis of the spatial and functional classification of populations developed by V.N. Beklemishev. The population system is a functional part of a particular community. Steady interrelationships between population systems of different species within the community (referred to as "community links'") appear to be a prerequisite for the formation of a complex of population systems. A prominent example of this is the parasitic system. The parasitic system is the population system of a parasite with all the connected populations of its hosts. The complexity of a parasitic system depends on: (1) peculiarities of the life cycle of the parasite, since its population system is the organizing component of the parasitic system and (2) subdivision of the environment for the parasites. The first trait is discussed from the standpoint of the phase structure of populations, which is clearly seen in parasites. The second one comprises the organization of the parasites' environment according to the scale of variability (interspecies, interpopulation or infra-population) of hosts. These make it possible to recognize spatial and functional parts in the framework of the parasitic system. A critical review of the terminology is presented together with a list of the pertaining vocabulary.

Key words Parasitic systems • Parasite populations • Population structure • Hemipopulations