20 minutes

Flatworms, with their three clearly defined cell layers and bilateral symmetry, represent an important advance in early animal evolution. This program provides detailed observations on structure, behaviour and life cycles of planarians and other free-living flatworms, some so small they can only be studied with a microscope (Class Turbellaria). It examines the bizarre life cycles of flukes (Class Trematoda) and tapeworms (Class Cestoda) with revealing shots of these parasites at home in the organs of their vertebrate hosts. (2007).