Polycystines

Circumscription: A group of marine radiolaria (protists with axopodia, skeletons, and no flagella in trophic state), usually with a skeleton of hydrated amorphous silica, developing from a four pronged spicule which may be dissolved by at least some species. Cytoplasm separated into an inner area by a capsule having differentiated regions (the fusules) through which the axopodia pass. Frequently with symbiotic dinoflagellates. Distributive stages have two flagella and some have strontium sulphate crystals, suggesting relationship with Acantharea. Ultrastructural identity: Mitochondria with tubular cristae. Microtubules of axopodia form branching arrays or wide hexagons. Capsule a discrete organic structure with fusules as a pore with electron-dense material enveloping the microtubules of the axoneme. Microtubule organizing centre amorphous/fibrous/granular. Mitosis with spindle inside nucleus but microtubule nucleating material external to nuclear envelope. Synapomorphy: Tubulocristate eukaryotes, with capsule with fusules separating endoplasm and ectoplasm.

The polycystines are a group of radiolarians. They include the vast majority of the fossil radiolaria, as their skeletons are abundant in marine sediments, making them one of the most common groups of microfossils. These skeletons are composed of opaline silica. In some it takes the form of relatively simple spicules, but in others it forms more elaborate lattices, such as concentric spheres with radial spines or sequences of conical chambers. Two of the orders belonging to this group are the radially-symmetrical Spumellaria, dating back to the late Cambrian period, and the bilaterally-symmetrical Nasselaria, whose origin is placed within the lower Devonian.

An illustration of polycystines of the subclass Spumellaria, from Ernst Haeckel's 1904 Kunstformen der Natur (Artforms of Nature)