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 chitin India krayker limpets macro malacology mantle mantle cavity Marine organism mollusc Mollusca mollusk mother of pearl plates shell shells spicules spiral Spirally-coiled shells calcium carbonate coiled crawling eye stalks eyes friction garden snail gastropoda helix aspersa herbivorous locomotion logarithmic spiral movement mucus muscular foot olfactory organs ommatophores radula retractable retracting ripple rippling skin Slime spirally tentacles textured surface unsegmented