Description of Polycystina
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.