Complex sentence: The rediae, still developing inside of the snail, become larger, and begin to produce cercariae, which exit from the snail and penetrate the skin of a freshwater fish and then into its muscle and become metacercarial cysts.
Split into simpler sentences: The rediae, still developing inside of the snail, become larger, and begin to produce cercariae. The individual cercaria exit from the snail and penetrate the skin of a freshwater fish and then into their muscle and become metacercarial cysts.