In traditional classifications, a class of parasitic flatworms (see platyhelminthes) comprising the flukes, such as Fasciola (liver fluke) and Schistosoma mansoni (blood fluke). Flukes have suckers and hooks to anchor themselves to the host and their body surface is covered by a protective cuticle. The whole life cycle may either occur within one host or require one or more intermediate hosts to transmit the infective eggs or larvae (see cercaria; miracidium). Fasciola hepatica, for example, undergoes larval development in a land snail (the intermediate host) and infects sheep (the primary host) when contaminated grass containing the larvae is swallowed.