Creatures could develop some protection from the sun in the water, but the land was wide open, and predators were yet to get anywhere the efficiency of ocean based predators.
They can understand why shelled arthropods had the protection to survive, but can't understand the survival of soft bodied things like worms and Molluscs? then perhaps they can explain why a slug like creature evolved a shell and branched unto the Snail genotype.
i don't imagine life is safe anywhere for exposed slugs and worms, and solar radiation, particularly UV Light can accelerate development. and change. Perhaps they were moving to the place that was the least dangerous.