[Foraminifera] Thin calcite projections anchored to the wall; they leave characteristic holes after spines are shed during gametogenesis.

[Acantharia] See Accessory spines.

[Radiolaria Polycystina] In Nassellaria and Spumellaria: any internal or external rod or needle with one free end. According to their relative size and number, spumellarian spines can be primary (larger, usually fewer in number;Cromyechinus icosacanthus intro) and secondary (smaller, more numerous, sometimes bristle- or thorn-shaped;Cromyechinus icosacanthus intro). In cross-section spines can be triangular (3-bladed) with smooth or serrated edges, or cylindrical; straight, or twisted (types of spines). The distal end of spines can be unbranched or branched (types of spines). Club-like spines have a swollen or thickened distal end (types of spines). Radial spines are those radiating centrifugally from the center of the shell (Cromyechinus icosacanthus intro).