On the problem of variable numbers of hosts in the life-cycles of helminths

The authors analyze the phenomena underlying the variation of host numbers in the life-cycle of several helminth species. Variation of host number in the cycle is effected by alternative shortening of the cycle (with this the cycle turns out either without or with a polyvalent obligatory host or, in rarer cases, without or with appearing of neogeny) or by facultative widening of the cycle (without or with participation of additional hosts, namely para-paratenic, paradefinitive, metaparatenic, or euparatenic hosts). The term of metaparatenic host is made precise by limitation to a "hemistadiogenous" additional host (sensu Barus and Rysavý 1977).