I. NUDA.--Protoplasmic body without any pellicle or shell save in the resting encysted condition, sometimes forming colonial aggregates by coalescence of pseudopods (_Myxodictyum_), or even plasmodia (_Protomyxa_). Brood cells at first uniflagellate or amoeboid from birth. Fresh-water and marine genera _Protogenes_ (Haeckel), _Biomyxa_ (Leidy), _Myxodictyum_ (Haeckel), _Protomyxa_ (Haeckel) (fig. 1B).
This group of very simple forms includes many of Haeckel's Monera, defined as "cytodes," masses of protoplasm without a nucleus. A nucleus (or nuclei) has, however, been demonstrated by improved methods of staining in so many that it is probable that this distinction will fall to the ground.
[Illustration: FIG. 16.--_Heterostegina_.]
II. ALLOGROMIDIACEAE (figs. 1A, 2).--Protoplasmic body protected in adult state by an imperforate test with one or two openings (pylomes) for the exit of the stylopod; test simple, gelatinous, membranous, sometimes incrusted with foreign bodies, never calcareous nor arenaceous; reproduction by fission alone known. Fresh-water or marine genera _Allogromia_ (Rhumbl.), _Myxotheca_ (Schaud.), _Lieberkühnia_ (Cl. & L.) (fig. 1A), _Shepheardella_ (Siddall) (fig. 2, 3-10), _Diplophrys_ (Barker), _Amphitrema_ (Arch.) (fig. 2, 11), Diaphorophodon (Arch.) (fig. 2, 12), are possibly Filosa. This group differs from the preceding in its simple test, but, like it, includes many fresh-water species, which possess contractile vacuoles.
III. ASTRORHIZIDIACEAE.--Simple forms, rarely polythalamous (some _Rhabdamminidae_), but often branching or radiate; test arenaceous, loosely compacted and traversed by chinks for pseudopodia (_Astrorhizidae_), or dense, and opening by one or more terminal pylomes at ends of branches. Marine, 4 Fam. The test of some _Astrorhizidae_ is so loose that it falls to pieces when taken out of water. _Haliphysema_ is remarkable for its history in relation to the "gastraea theory." _Pilulina_ has a neat globular shell of sponge-spicules and fine sand. Genera, _Astrorhiza_ (Sandahl) (fig. 22), _Pilulina_ (Carptr.) (fig. 19), _Saccammina_ (Sars) (fig. 19), _Rhabdammina_ (Sars), _Botellina_ (Carptr.), _Haliphysema_ (Bowerbank) (fig. 22).