These calcareous sponges have a skeleton composed exclusively of free spicules, without hypercalcified non-spicular reinforcements or spicule tracts. Spicules are usualy regular triradiate, with the rays and angles between the rays being equal, and sometimes include parasagittal or sagittal triradiate spicules. The basal system of tetractines is similar to a triactine bearing au fourth actine perpendicular to teh plane of the basal system. Young sponges may have only triradiate spicules. Choanoytes have a peculiar structure with the spherical nucleus of the cell in the basal region, and the basal body of the flagellum is not adjacent to the nucleus. Larve are entirely ciliated hollow blastula (cocloblastula).