The body of sponges contain a supporting exoskeleton. It is made up of either spicules or spongin fibres or both. Spongin fibres are protein fibres. They are either highly branched or show anastomosis. Spicules are calcareous or siliceous with hard spine-like structures.
Depending on the branching the spicules are
(i) monoaxons (single axis).
(ii) triaxons (three axes forming six rays of hexactinal spicules).
(iii) tetraxons (four rays or tetra acinal). (iv) polyaxons (several rays).