(Selenka, 1867)
H. (Cystipus) rigida is a white to grey species, often covered with sand, with a double row of dark spots along upper surface, and a lateral row of blunt papillae on each side. It is a stout, flattened and rigid species (size to 75 mm), with 20 tentacles (H. rigida photo[/l]). [l][m]Glossary[/m][r]Spicule[/r]Spicules are low tables, fenestrated spheres and knobbed buttons (H. rigida spicules[/l]). It may be found on [l][m]Glossary[/m][r]Reef[/r]reef flats below rocks.
Remarks: A second species in this subgenus, H. (Cystipus) inhabilis recognised by a lack of fenestrated spheres, simpler tables and more irregular buttons has been reported from northern reefs.