Complex sentence: The siphonariids are in the informal group Basommatophora and as such are somewhat closely related to freshwater air-breathing snails such as Lymnaeidae, whereas the trimusculids are in the clade Eupulmonata, and are quite closely related to air-breathing land snails.
Split into simpler sentences: The siphonariids are in the informal group Basommatophora and as such are somewhat closely related to freshwater air-breathing snails such as Lymnaeidae. The trimusculids however are in the clade Eupulmonata, and are quite closely related to air-breathing land snails.