ADDITIONAL COMPLEXITY. “Many parasite species, however, in addition have asexual multiplication in the first intermediate host and free-living stages actively penetrating the second intermediate host (most digenean trematodes, comprising several thousand species). This alternation between asexual and sexual reproduction may have important consequences for shaping the life cycle.”
http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2005.00895.x/full