Haeckel, 1860, emend. Schewiakoff, 1926
Skeleton of 20 radial spicules usually of equal size and same shape, linked at base into single homogeneous either spherical or star-like mass. Endoplasm and ectoplasm separated by thin peripheral capsular wall. Periplasmic cortex as network of elastic threads between which narrow channels lead to internal space. These splits often with electron dense material which may correspond to defecation vesicles or degraded organelles. Six to 12 short flat, or 16-32 thread-like myonemes.
Four genera:
Genus Acantholithium
Genus Astrolithium
Genus Astrolonche
Genus Heliolithium