Radiolarian skeletons are amazingly complex. They can either be spherically symmetrical, or bilaterally symmetrical (like the one below). Having spines extending from the main body is a way of increasing surface area (and thus resistance to falling in the water column), without greatly increasing overall weight. Some have oil globules as well.

Radiolarians have been around since Cambrian times (540 million years ago). Since then, different types and amounts of radiolarians have dominated different eras. Geologists are now using this information in geological mapping. Radiolarians have such a good fossil record because their silicon skeletons preserve so well.

Limestone and chalk deposits around the world are largely formed of radiolarian deposits from the Mesozoic and Early Cenozoic eras (250 - 65 million years ago). The quarries used for the Egyptian pyramids are mostly radiolarian and foraminiferan (another shelled protozoan) shells.