Nudibranchia

The story

Nudibranchs — shell-less sea slugs that advertise their toxicity (often borrowed from the stinging cells of the anemones and corals they eat) with some of the most saturated coloration in the animal kingdom — gave Haeckel a subject that barely needed his usual compositional embellishment. This plate arranges seven species across the page like specimens pinned for display, their frilled cerata and rhinophores rendered in the coral pinks, acid yellows, and deep violets that make nudibranchs a favorite of macro photographers to this day, more than a century after this plate was drawn. What to notice: Haeckel's argument that beauty in nature followed evolutionary law finds its most literal proof here — the slugs' colors aren't decorative in the way a flower's are, they're a hard-won survival adaptation that happens to also be strikingly beautiful.