Complex sentence: Some species of gastropods listed here (for example "Peringia ulvae") live in habitats that are intermediate between freshwater and saltwater, or between land and saltwater, and therefore snails or slugs with these intermediate habits are often also included in lists of marine species.
Split into simpler sentences: Some species of gastropods listed here (for example "Peringia ulvae") live in habitats that are intermediate between freshwater and saltwater. Snails or slugs with either of these kinds of intermediate habits are often also included in lists of marine species.