ALEXIS ROSENFELD / SCIENCE PHOTO LIBRARY ALEXIS ROSENFELD / SCIENCE PHOTO LIBRARY
Nudibranch (Felimare picta) head. Nudibranchs, or sea-slugs, are shell-less gastropod marine molluscs that live in seas around the world. They have external gills (blue) that give the nudibranch its name, meaning naked gills. Photographed in the Mediterranean Sea, France.
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