Radiolarians are marine organisms that secrete elaborate and delicate shells made of glass( silica) and other materials. Typical shells are spherical with radiating spines, though the structure varies.

Thin, needle like pseudopodia capture food. Most radiolarians are microscopic, but some form sausage-shaped colonies that reach 9 ft in length, making them true giants among tiny sea creatures. Radiolarian shells are more resistant to dissolving under pressure than those of forams.