Complex sentence: Mesogastropoda was for many years a traditional taxonomic group of snails, an order, composed mostly of sea snails, but also including some land snails and freshwater snails, all of which were prosobranch gastropod mollusks.
Split into simpler sentences: Mesogastropoda was for many years a traditional taxonomic group of snails, an order. The order was composed mostly of sea snails, but it also including some land snails and freshwater snails, all of which were prosobranch gastropod mollusks.