Of all of the species Haeckel described, he is most famous for his illustrations of radiolarians. Planktonic, unicellular marine eukaryotes, radiolarians are found in all of the world’s oceans. The multiple body compartments of a radiolarian are surrounded by an elaborate mineral skeleton called the test, which exhibits the exquisite designs that so captivated Haeckel. Haeckel first encountered these beauties while at Messina in Sicily, and it was this experience that prompted him to pursue doctorate studies in zoology. Haeckel helped popularize these animals with the public through his 1862 monograph Die Radiolarien, his 1887 report as part of the H.M.S. Challenger expedition, and his illustrations in Kunstformen der Natur.