Dear Dave,
I'll try and give you a short uncomplicated version of what is quite a complex physiological story.

Firstly, all life on land has evolved from the sea. Body fluids, such as blood, have much the same salt concentration as sea water, because the primitive animals we evolved from weren't very good at stopping salts and water in the sea water passing in and out of their bodies through their skin. Which brings us to a physical phenomenon called osmosis. If you put salty water of two different salt concentrations together, water from the less salty solution will move to the saltier solution until they are both the same saltiness. Now, if we use freshwater fish as an example, their blood and body fluids are much 'saltier' than freshwater. If osmosis was allowed to occur, their bodies would be flooded with freshwater rushing in to lower the salt concentration so that it matched the surrounding freshwater. In effect the fish would inflate with freshwater and burst! However freshwater fish have evolved ways of preventing this - skin that doesn't allow water through easily, and kidneys that can pump excess water out - a bit like a boat's bilge pumps.

Which brings us back to land slugs. They have an even greater problem because they live out of water and their skin is not particularly good at preventing water movement in and out. That is why  in air they have to be careful to stay damp so they don't dry out. That is one of the reasons they are most active at night, and why they secrete 'slime' from skin glands..

Now to the salt treatment. In terms of osmosis, damp salt is a very highly concentrated solution, so when salt is placed on a land slug, water from inside the slug quickly moves out through the skin to try and balance the salt concentration inside and outside the body. The result is that most of the water is sucked out of the slug and it dies. Sea Slugs and land slugs have much the same salt concentration in their blood as sea water, so Sea Slugs in sea water are ok. However if you keep sea slugs in an aquarium, and you don't look after the aquarium, often water will evaporate and the salt concentration in the aquarium water will rise and the animals will die. Not quite as dramatic has covering a slug with salt, but the same basic principle.

Best wishes,
Bill Rudman

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