Fig. 9From: Insights into the structure and morphogenesis of the giant basal spicule of the glass sponge Monorhaphis chuniStructure of the annular layer (AL) and its relation to the underlying tuberculate layer (TL) and plain glassy silica (PG). a General view of all 3 zones in the spicule: PG, TL and AL. b, c Contact between AL and underlying TL (note that tubercles fit perfectly into depressions in the lower surface of the AL), it resembles tenon and mortise or dovetail joints. d Granular characters of silica in the AL (note layering). e, f Details of the AL lower surface showing complex regular depressions into which fit tubercles from the underlying layerBack to article page