Fig. 4

Surface of pristine spicules (extracted from the sponge that was alive when collected) of petrosid desmosponge. a Spicule with uncomplete layer of silica showing natural steep faces of newly deposited silica layer; b details of a to show vertical surface limiting the newly deposited silica layer. c Another spicule surface with two superimposed uncomplete layers of newly deposited silica limited by steep faces. d Surface of another spicules with superimposed three new incipient layers of silica deposited on the earlier surface of the spicule that are limited by steep faces