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.
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