The primary organisms categorized within this class are called the radiolarians, a type of zooplankton. Radiolarians are amoeboid marine protozoans with actinopods and usually produce a skeleton of silica. They have central nuclear regions surrounded by an external membrane known as the capsule. This separates the inner nuclear region from the outer nuclear region. The capsule is usually perforated with few to many pores. Because of these pores, actinopods are able to pass through and form the calymma. This is a vacuolated area that is within the outermost cytoplasm of a radiolarian. The calymma typically secretes the internal skeleton of silica that is characteristic of these organisms. Since silica does not dissolve in water, even under immense pressure, the skeletons of radiolarians accumulate in vast amounts on the seafloor as radiolarian ooze.