46. _Primary and Secondary Ground-Forms._--The geometrical sphere must be regarded as the original ground-form of the Radiolaria; it being understood that its monophyletic derivation from a single stem-form, _Actissa_, is correct. The simplest forms of _Actissa_ (_Procyttarium_, Pl. 1, fig. 1) are in fact geometrically _perfect spheres_; indeed even the individual parts which compose their unicellular bodies (nucleolus, nucleus, central capsule and calymma) are concentric spheres. But in addition the central capsules of most other SPUMELLARIA, especially the #Sphæroidea#, as well as of many ACANTHARIA {xxii}are true spheres. Furthermore the simple or concentrically composed lattice-spheres of #Sphæroidea#, #Sphærophracta#, and #Phæosphæria# may be regarded as spheres, although strictly speaking they are endospherical polyhedra. From the primary spherical form of the Radiolaria all other secondary forms may be derived in the following order:--1. By the development of a main axis the Monaxonia arise. 2. By the development of transverse axes the Stauraxonia arise. 3. In both groups (Monaxonia and Stauraxonia) the two poles (or upper and lower halves of the body) are at first similar (Isopola). 4. By differentiation in the two poles or halves of the body (distinction between the basal pole and the apical) the forms with different poles (Allopola) arise. 5. The transverse axes of the Stauraxonia are at first equal (regular pyramids and double pyramids). 6. By differentiation in the transverse axes (distinction between the sagittal and the frontal axis) the amphithect pyramids and double pyramids arise. 7. From the amphithect pyramids the Amphipleura arise by differentiation of both poles of the sagittal axis. 8. The zygopleural ground-form appears last, as the simplest form of the Amphipleura.