Spicules of (A, D–F) S. domuncula and (B, C) G. cydonium. (A) The skeletal tissue of S. domuncula only forms long spicules called megascleres which are either tylostyles (spicules with swelling at one end) or styles (pointed at both ends); the axial canal <(ac) visible where one spicule is broken. (B) Microscleres (mis) are composed both of megascleres (mes) and microscleres, consisting of many thin rays radiating from a globular center and with the axial canal clearly visible in cross sections (C), sometimes with the axial filament (af) visible. (D) The tylostyle (sp) swelling has a terminal knob (k) atop a collar. (E) The axial filament may be seen more clearly in a spicular cross section. (F) The underlying axial filament is more clearly seen when the spicule is partially dissolved in hydrogen fluoride. Reproduced with permission from [38].
