Fig. 1. Megascleres of M. chuni (A–D,G–I) and M. intermedia (E,F); scanning electron microscopic analysis. (A) The tauactins are covered by a collagen (col) sheath, which is interspersed with 5 µm holes (h). (B) Cross-section through a giant basal spicule showing the outer lamellar zone (la) and the central axial cylinder (cy), which surrounds the axial canal (ac). (C,D) The tauactins are composed of a maximum of 20 lamellae. (E) A stack of lamellae of the outer zone of a giant basal spicule. (F) Cross-section through a giant basal spicule; the surface has been etched with HF, disclosing the composition of the lamellae by silica nanoparticles. (G) The tip of a tauactin, allowing inspection of the axial canal with its axial filament (af). (H) Cut-through section of a tauactin at the position where two rays/spines originate, exposing two axial canals with their axial filaments. (I) Tip of a tauactin, harbouring three axial canals.