Encased in Silica: Phaeodaria (Phaeodarea)

Behold Phaeodaria, close relative to Radiolaria, a diverse group of unicellular plankton whose intricate, silica-based skeletons first entranced the great 19th-century German biologist Ernst Haeckel. Besotted with the astonishingly variable symmetries of radiolarians he encountered on a trip to Messina, Italy, Haeckel went on to study and draw over 3,000 species, of which he counted (mistakenly, as it turns out) Phaeodaria as one. This particular illustration is included in his 1904 masterwork, Kunstformen der Natur (Art Forms in Nature).

Radiolarians, diatoms, sponges, and other minute marine creatures rely on silicon for their skeletal structures: they absorb silicic acid from ocean water and convert it into biogenic silica, which is essentially glass. Fossilized radiolarian skeletons abound on ocean floors in a diatomaceous sediment known as siliceous ooze, and help constitute a sedimentary rock called chert, which can be found locally in San Francisco and along the Marin Headlands of the San Francisco Bay.