conical freshwater gastropod limpet limpets marine nature ocean patella saavem sea shape shell snail snails vulgata water background barnacle Devon England life mollusc texture animals Balanus barnacles coast creature crustacea Great Britain Mollusca natural Patella vulgata Pembrokeshire rock rocks sealife united kingdom wales welsh chitin India krayker macro malacology mantle mantle cavity Marine organism Molluscs mollusk mother of pearl plates shells spicules spiral Spirally-coiled shells