This interesting creature was in a half-shell, and descending on its thread of mucus from a tree, as i was walking to the shops. It appears to be a yellow-shelled semi-slug! What a weirdo...and in technical speak it is Parmarion martensi which is a species of air-breathing land semislug, a terrestrial pulmonate gastropod mollusk in the family Ariophantidae. This was snapped with an iphone.
At night and during rain slugs crawl up trees and into nearby bushes. As the air humidity decreases in the morning, they do not retrace their track but just crawl downwards. If they find the trunk, then they come down that....but if they go along a branch, and if the branch is bent down, the slug will crawl to its end and then continue the downward movement on a thread of mucus. Slugs gained in mobility when they lost their shells, but this one seems to be hedging his bets, and has a bit of both....but when they lost their shells they also lost the means of protecting themselves from drying out (as well as from numerous predators). So their behaviour differs...and there are extensive studies on this. Both go up the trees and bushes at night, and also in any rainy or very humid weather. But when the ambient humidity drops, slugs have to seek shelter to avoid drying out so they start to move downwards, to find a moist place to go. But the snails of many species - with their houses on their backs - just attach themselves to the substratum and stay on the trees, or bushes until its humid and wet enough to move on again.