This class contains two orders, Heliozoa and Radiolaria, which are superficially rather similar. Individuals of both orders may have siliceous skeletons arranged as spheres with holes in them. Axopodia, pseudopodia stiffened with axial filaments, project through the holes. Food is caught on the axopodia and is drawn by the movement of the cytoplasm to the cell body where it is ingested. The radial arrangement of the axopodia is the same from whichever side the organism is viewed.
Locomotion using the tips of the axopodia is slow; in both orders water currents and sometimes a flotation mechanism help dispersion.
Heliozoans (Order Heliozoa) The “sun animalcules”, beloved of Victorian microscopists, are freshwater organisms found in peat and other acid pools. The various species are generally more than 40μ in diameter and as much as I mm. The cytoplasm of Actinosphaerium is clearly divided into two concentric spheres. The outer is vacuolated and considered to be ectoplasm. The inner sphere is dense and contains the food vacuoles and most other organelles.
The roots of the axial filaments arise in the dense cytoplasm. This and the related genus Actinophrys are both naked, that is, without a stiffening skeleton. Raphidiophrys and Clathrulina have siliceous skeletons. In the genera studied in detail, cysts form in well-fed individuals. Within the cyst, gametes are formed and these fuse with each other. Hatching of the zygote seems to require winter conditions. Repro-duction is also by repeated binary fission.
Radiolarians (Order Radiolaria)
These arcs all marine and mostly pelagic. Structurally they are separated from the heliozoans by having a distinct membrane between the ectoplasm and endoplasm. The ectoplasm in some species appears to be capable of secreting freshwater into vacuoles. The presence of such vacuoles clearly lightens a skeleton-bearing animal and enables it to rise to the surface of the sea. Not all radiolarians have massive skeletons.
The suborder Actipylina has radial spicules and some members of the suborder Peripylaria have four-pronged spicules or none at all. Silica is the principal skeletal constituent. The best-known radiolarians are those with full and persistent skeletons, which sink to the bottom of the ocean to form a thick ‘radiolarian ooze’. The variety and detail make these remains beautiful objects for microscopy.
Heliozoa is an example of Class Actinopoda
Binary fission is shown in several stages. After the nucleus has divided, the cytoplasm becomes narrowed, separation continues but still, the cells are joined by a thick bridge. Still separating, the connection is reduced to a filament and becomes very fine.
Radiolaria is an example of Class Actinopoda
Radiolaria live as plankton in the sea, usually in warm waters. When the organisms die the cell disintegrates and leaves the finely sculptured shells figured here. The shells sink slowly into the oceanic ooze where they may become fossilized if conditions are suitable.