Marine flatworms and planarians are ______ in mostly aquatic environments, while tapeworms and flukes are ______, living within a host animal.

Marine flatworms and planarians are free-living in mostly aquatic environments, while tapeworms and flukes are parasites, living within a host animal.

Flatworms such as planarians can be commonly found in a range of aquatic habitats, including freshwater and marine environments, where they play a role in the ecosystem as scavengers and predators of smaller organisms. In contrast, tapeworms and flukes have adapted to a parasitic lifestyle, where they depend on the host for survival, obtaining nutrients at the expense of their host's health. This divergence in lifestyle highlights the diversity of flatworm adaptations to their environmental niches.