Light micrograph of a group of different species of unicellular protozoa, Radiolaria sp, found in marine plankton. The inner part of the cell, containing the nucleus and part of the protoplasm, is divided from the rest of the body cell by a thin-walled porous central capsule. The radiolaria skeletons consist of strontium sulphate for the Achantaria species and silicates in the remaining radiolarians. The radiolaria skeletons are among nature's most beautiful structures. Plankton provides food for fish and whales.