Show here in Darkfield, Polycystina are one of two types of Radiolaria. The Polycystines, have solid skeletal structures of opaline silica. As such, they do not dissolve easily in sea water and remain as fossils on the sea floor. The second type, the Phaeodarians, have skeletons made of amorphous silica and organic material, which rarely fossilize, so there are many fewer found in sea bed sediments. Polycystina are amoeboid protozoa (diameter 0.1-0.2 mm) that live in the ocean. They produce the intricate and beautiful silica skeletons shown in the photomicrograph, and are prized specimens for microscopists.
The background is black because the polycyctina are lit not from directly below as is normally done, but from the sides using a hollow cone of light from a special kind of microscope condenser, called appropriately enough a darkfield condenser. This makes them appear to glow against the black background.
Since they are made of silica – glass essentially – the polycystina/radiolarian skeletons are quite transparent and very difficult to see well with normal microscope illumination, so Darkfield (sometimes called Darkground), Phase Contrast, or Oblique illumination is necessary to be able to see them clearly.
The Challenger Expedition from 1872-1876, produced a treasure trove of samples from the bottom of the world’s oceans. Many of theses samples contained polycystina. Upon return at the end of the voyage, experts spent years examining the sediment specimens identifying the organisms they found. Ernst Haeckel was a biologist who examined and named many of the polycystina they found. His book of drawings, Kunstformen der Natur (known in English as Art Forms in Nature), was originally published in sets of ten between 1899 and 1904 and collectively in two volumes in 1904. It consists of 100 prints of various organisms, many of which were first described by Haeckel himself. It remains one of the most extraordinary and beautiful books ever produced.
Below are a couple pages from the book showing the extremely intricate structure of both Polycystina and Radiolaria, and Haeckel’s highly detailed illustrations. Reprints of the book are rather inexpensive (about $18) on Amazon.