The lancet fluke (Dicrocoelium lanceolatum) belongs to the order Trematoda. Like other flukes, scientifically called trematodes, it as well shows a generation cycle with a change of reproduction as well as a change of hosts. The terminal host are mammals, usually sheep or goats. The distribute the fluke's eggs by their excrements on plants, on which snails feed. Those snails again are the intermediate host.
Eastern or white heath snail (Xerolenta obvia) on a wire mesh
fence in Vienna. [RN]
Those snails, mostly hygromiid snails of the heath snail kind or bulin snails (Zebrina), take in the flukes eggs together with their food. In the snail's digestive gland the fluke' first larval generation hatches. That generation is called miracidium. A miracidium is a simply built larva with few resemblance to the adult fluke. It is able to swim or at least to float by a ciliar crest. The miracidium makes a metamorphosis inside the snail, resulting in the next larval stage, that more resembles the adult fluke.
The so called cercaria has got a tail that propels it in the water. The cercariae change themselves in endurance stages called cysts, that the snails surrounds with a mucus layer and excretes them.
Red forest ant (Formica rufa). Picture: Richard Bartz.
Ants eat those mucus balls (obviously the mucus contains also certain pheromones that makes it highly attractive to ants) and then are afflicted by the cercariae. The cercariae grow in the ants' abdomen (terminal body part) and then one of them moves into the head.There they afflict the ant's nervous system and cause a change of behaviour, that makes Bernard Werber *) call them "zombie ants". The ants now, very atypically, climb up plant stems and attach themselves with their mandibles to exposed positions such as leaves.
Grazing mammals, such as sheep or goats, eat the plants together with the ants and then infect themselves with the lesser liver fluke. Inside this terminal host the cercariae change into the fluke's adult stage which reproduces sexually and then lays eggs that are excreted by the mammal - the generation cycle is closed when the next snail eats the eggs.
*) "Les fourmis", a three volume story on red ants.