HETERACTINIDA Hinde, 1888

Order HETERACTINIDA Hinde, 1888

Remarks. Heteractinids are a group of Palaeozoic sponges with anomalous spicules containing a non-actualistic number of rays ( de Laubenfels, 1955; Rigby and Nitecki, 1975; Kozur et al., 2008). In almost all cases, the spicules are based on a hexaradiate pattern, sometimes with an additional pair of rays in the perpendicular plane (octactins), or additional radial rays ( Kozur et al., 2008). The triradiate or hexaradiate symmetry mirrors that of calcite crystals, which appears to influence the growth form ( Jones, 1970), although there is some evidence for the existence of a mineralogically distinct core, as in certain other early sponge groups ( Botting and Butterfield, 2005). They are historically and currently regarded as representing the early stem group of Calcarea ( Botting and Muir, 2018).