Trematode transmission patterns.

The primitive transmission pattern in the Trematoda is a 1-host pattern in which an aspidocotylean matures in a mollusc. The progeny from the parent aspidocotylean typically disseminate from the mollusc to establish in others, but the possibility that some progeny remain in the same mollusc as did the parent and produce a sequential generation exists. It is this potential for sequential generations in the mollusc that may be the key to understanding the complex transmission and life cycle patterns in the Digenea.