1a. Skeleton of siliceous spicules (SiO2) (make Thick Sections, cf. Introduction module).

1b. Skeleton of calcareous spicules (CaCO3) (make Thick Sections, cf. Introduction module).

1c. Skeleton lacks spicules of silica or limestone, but consists of spongin fibres (a horn-like protein forming amber-coloured fibres which may or may not have inclusions such as sand grains or broken spicules) (make Thick Sections, cf. Introduction module).

1d. There is no skeleton (no spicules, no fibres) (make Thick Sections, cf. Introduction module).