Radiolarians are tiny protozoans that live exclusively in the ocean. Although they seem small to us, they are quite large compared to other protozoans.
Some species are several millimeters in diameter.
Radiolarian skeletons are amazingly complex. Some have oil globules as well. They form an important part of the marine food web. They act as decomposers, and when they die, their silicone-based skeletons rain down onto the sea floor and make up a lot of the substrate material. Since then, different types and amounts of radiolarians have dominated different eras.