Description of Acantharea
Circumscription: One of several groups previously referred to as radiolaria;-large marine unicells with radiating axopodia and an inorganic skeleton (in the case of Acantharea composed of 10/20 spicules made of strontium sulphate). Cell with central endoplasm and peripheral ectoplasm separated by a fibrous capsular wall, outside face of cytoplasm coated with a fibrous cortex that is joined to spicules by contractile myonemes. Often with symbionts. No propulsive organelles in trophic form but may have flagellated stage and/ or amoebas and/or cysts in life cycle. Ultrastructural identity: Not well established because of difficulty of good preservation. Mitochondria with tubular cristae. Axopodia arising from unspecified sites in the cytoplasm but having an open hexagonal or larger polygonal arrays of microtubules; periplasmic cortex, axopodial details, extrusomes, and so forth. With concentric extrusomes. Mitosis involves an eccentric spindle located inside an intact nuclear envelope. Synapomorphy: Tubulocristate protists with strontium sulphate skeleton with symmetry based on 20 radial elements.