Haeckel, 1862, emend. Riedel, 1967
Discoidal or cylindrical, spongy or finely chambered skeleton, with or without surficial pore-plate, often with radiating arms or marginal spines. The members of this family are characterized by possessing skeletons which are partly or entirely spongy in appearance. However, as opposed to the Actinommidae, which can also have spongy skeletons, the Spongodiscidae are not spherical. Their overall shape can be lenticular (biconvex discs, lentelliptical shell 2), cylindrical (Spongocore cylindrica ov), quadrangular or subquadrangular in outline (Spongaster tetras tetras ov), or Y-shaped (spongy shell 4; spongy shell 5). With the exception of the cylinders, all others are depressed or flattened (rather than circular in cross-section, lentelliptical shell 2). Lenticular, quadrangular, and Y-shaped forms may be entirely composed of a spongy mass with no discernible structure (in which case the central part of the skeleton is often thicker and/or denser, and therefore appears darker in the light microscope; spongy shell 2; Spongodiscus resurgens ov), or may posses a small central chamber surrounded by concentric or spiral, continuous or interrupted bands (spongy shell 5). The surface of some forms may be partly or totally covered with a very thin, porous sieve-plate, which in lenticular forms may extend beyond the central spongy mass forming a delicate equatorial girdle around the periphery of the shell (spongy shell 3) (these morphotypes were formerly included in the family Porodiscidae).
The following species of this family are treated in the present volume (arranged in four groups according the outline):
[1] Circular outline [e.g. Stylodictya aculeata 3]
Spongodiscus resurgens
Spongopyle setosa
Spongotrochus glacialis
Stylochlamydium asteriscus
Stylodictya aculeata
Stylodictya multispina
[2] Elongated outline [e.g. Spongurus pylomaticus 2]
Spongocore cylindrica
Spongurus pylomaticus
Spongurus spp.
[3] Quadrate outline [e.g. Spongaster tetras irregularis]
Spongaster tetras
[4] Trigonal outline [e.g. Amphirhopalum ypsilon]
Amphirhopalum ypsilon
Dictyocoryne profunda
Dictyocoryne truncatum
Euchitonia elegans/furcata