Reproduction and host-location among the parasitic platyhelminthes.

This review examines briefly the reproductive capacity of representatives of the 4 principal groups of platyhelminths, the "Turbellaria", Monogenea, Digenea and Cestoda. Of the flatworms, 3 main groups are wholly parasitic (monogeneans; digeneans; cestodes). Among the largely free-living "Turbellaria", there are several parasitic representatives in some families (Umagillidae; Graffillidae; Pterastericolidae; Fecampiidae; Acholadidae). Endoparasitic platyhelminths with complex life-cycles produce large numbers of eggs and numbers of offspring are increased further in the digeneans and a few cestodes by asexual multiplication. Like their free-living relatives, most ectosymbiotic and ento- and ectoparasitic flatworms ("turbellarians" and monogeneans) produce, on the whole, far fewer eggs and progeny but are still successful organisms in terms of their numbers of species and diversity. Estimates of parasite fecundity from in vivo experiments are needed for representatives from all flatworm groups. For those parasites that are host-specific, the particular species of host provides a predictable target to be located. Adaptations displayed by the eggs and infective stages of some flatworms increase their chances of finding and recognising their specific host and these are reviewed: attachment of eggs to their "host"; egg hatching in response to host chemicals; rhythmical emergence; special behaviours of infective stages; host recognition.