Complex sentence: Neritoida is an order of sea (and freshwater) snails, mostly marine but with some aquatic gastropod mollusks in the superorder Neritopsina within the order Orthogastropoda.
Split into simpler sentences: Neritoida is an order of sea (and freshwater) snails, mostly marine but including some aquatic genera. Neritoida are gastropod mollusks in the superorder Neritopsina within the order Orthogastropoda.