animal animals coiled eye eyes feeler foot gastropoda molluscan mouth muscle muscular nature pavement saavem shell slug snail snails street calcium carbonate coiled shells crawling exoskeleton eye stalks friction garden snail helix aspersa herbivorous land snails locomotion logarithmic spiral mantle mollusk movement mucus muscular foot olfactory organs ommatophores radula retractable retracting ripple rippling skin Slime slimy spirally tentacles terrestrial snails textured surface unsegmented