[1] The organisms of this order are characterized by a skeleton cross link with a cone or ring.

These organisms are unicellular eukaryotic heterotrophic plankton typically with a siliceous cone-shaped skeleton.

Both spumellarians and nassellarians are common chert-forming microfossils and are important in stratigraphical dating, as the oldest radiolarians are Precambrian in age.

[2] Nassellarians have been and continue to be some of the most remarkable and aesthetically interesting protists both alive and in the fossil record.

The holotype morphology of the Nassellarian order as described by Anderson[2] and Boltovsky et al.[3] is of an egg-shaped central capsule (the part of the cell containing one or more nuclei, Golgi bodies, mitochondria, lysosomes, and other bodies important for cellular function) located within a porous conical skeleton made of silica.

This material is primarily alveoli, gas-filled bubble-like structures that regulate the buoyancy of the organism.

Small subunit ribosomal DNA analysis also shows evidence that no coevolution of the dinoflagellates and radiolarians has occurred.

These organisms instead form a wide terminal cone, which they cast out behind them from the basal aperture like a fishing net.

[5] These “fishing net” nassellarians are much easier to see feeding in the wild and present quite a spectacular sight.

[2][3][5] Early Paleozoic radiolarian fossil history is dominated by Spumellaria until the Carboniferous period, during which nassellarian fauna experienced a sharp increase in diversity.