Do slugs all live in sea water?

Dear Song,
Modern studies of Gastropod evolution consider that land snails and land slugs [Pulmonata] and the Sea slugs [Opisthobranchia] are closely related in a clade called the Heterobranchia. This clade also contains a few  problematic 'snail' groups including the Valvatoidea, Architectonicoidea, Risoelloidea and the Pyramidelloidea. There are a number of anatomical features which link these groups together which I won't go into here. Suffice to say that Land snails, Land slugs and Sea slugs are closely related to each other but we cannot say that Land slugs evolved from Sea slugs. The slug form is a 'shape' or grade of evolutionary development which has occurred many times in the molluscs, but in each case the 'slug' has evolved from a 'snail' through the reduction in size and gradual loss of the shell.

So the land slugs you see around your campus have evolved from land snails. Like land snails they have had to evolve ways to live out of water.. The reason you see more around during very wet weather is that since they have evolved to live out of water, they are now unable to live in water, and so are probably having to abandon their normal 'home' in the soil or under rocks, or they will drown! Concerning eggs and development. Land snails could only become true land dwellers if their eggs and larvae were able to develop out of water. So today Land snails and Land slugs have what is called intracapsular direct development, where the larvae develop within the egg capsule and hatch out as small crawling snails or slugs.

I have prepared a few pages to illustrate some of these points.
Land slugs - Introduction
Land slugs - the Semi-slugs
Land Slugs - eggs & development

Best wishes,
Bill Rudman