Fig. 5

Monorhaphis basal spicule in transverse and longitudinal thin (a–f) and thick (g–n) sections. a, b Central part of the spicule in transmitted light with regular layering of the plain glassy layer (PG) zone and apparently non-layered axial cylinder (AC); transverse section. c Central part of the spicule in green epifluorescent light; layered axial cylinder displays stronger fluorescence; transverse section. d Marginal part of the spicules showing AL, TL, and PG zones superimposed (transverse section), transmitted light. e Longitudinal thin section showing AL, TL and PG layers superimposed, transmitted light. f Longitudinal thin section showing axial canal (arrowed), axial cylinder AC and PG zone. g–n Thick transverse sections: g showing that most of the spicule consists of PG layers, and asymmetry of spicule; h, i showing conspicuous layering of PG and that AC is also layered but less regularly; note brown hue (organics) of AC, and that transition from AC into PG zone is clearly continuous (i). j Section of the marginal zone of the spicule to show dark (high organics content) AL and TL layers. k Blue epifluorescence light showing various layers with epifluorescence of different intensity. l, m Details of marginal zone of the spicules in blue (l) and green (m) epifluorescence. n Crossed polarized transmitted light; note heterogeneity of apparently homogeneous PG zone