life cycle of sheep liver fluke fasciola hepatica with intermediate mollusk and main hosts human, sheep on white

animal background biology cercaria cycle development diagram egg fasciola flatworm fluke helminth hepatica human illustration isolated larva larval life lifecycle liver man medicine metacercaria metamorphosis mollusk parasite parasitic pest redia reproduce reproduction scheme science sequence sheep silhouette snail sporocyst stage stages transformation trematoda trematode vector white worm zoology