The infection of a snail by a trematode starts with the search for a specific snail suitable as an IH by the miracidium either in water or in mud. Snails produce species-specific substances (pheromones) probably to achieve an even distribution in their biotop. Miracidia orient towards their IH snails by chemotaxis or chemokinesis along gradients of such pheromones.
The penetration of the snails skin, the development of the mother-sporocyst, its production of daughter sporocysts or rediae and, finally, the production of cercariae are steps that can be modified quantitatively by feedback mechanisms. In particular, the production of cercariae is regulated by the metabolic processes of the snail (see summary of chapter 2.9). On the other hand, mortality waves of the snails during the invasion of daughter sporocysts into the liver and, secondly, at the first delivery of cercariae reduce the infected part of the IH population in cases of an overoptimal entry of eggs into the snails biotop. Positive and negative correlations as described for filariae (see summary 2.3) can thus be observed in the trematode-snail relationship.