bleached cluster coiled shells colored coloured exoskeleton faded gastropods land snails metal fence post Molluscs slimy small snail snails spirally coiled shells terrestrial snails white calcium carbonate coiled crawling eye stalks eyes friction garden snail gastropoda helix aspersa herbivorous locomotion logarithmic spiral mantle mollusk movement mucus muscular foot olfactory organs ommatophores radula retractable retracting ripple rippling shell skin Slime spirally tentacles textured surface unsegmented animal beige brown close-up grass green grey l gravel nature sand soi spring summer gravel Soil spiral