47. _The Ground-Forms of the Spumellaria._--The SPUMELLARIA, being the oldest and most primitive Radiolaria, have for the most part either indifferent or multistable equilibrium; _e.g._, all #Colloidea# and #Beloidea# which have a spherical central capsule, and also most #Sphæroidea#. Among these primitive Centrostigma true spheres and endospherical polyhedra are represented in the utmost variety, and the regular polyhedra in particular. By the development of a vertical main axis these Centrostigma have also given rise to very numerous Centraxonia, which are usually isopolar, very rarely allopolar. Sometimes they are Monaxonia (circular in transverse section), sometimes Stauraxonia (polygonal in transverse section). The vertical main axis is longer in the #Prunoidea#, shorter in the #Discoidea# than any of the other axes. The #Larcoidea# are distinguished by their lentelliptical or triaxial ellipsoid form; the three different but isopolar axes corresponding with those of the rhombic octahedron; but even among the #Sphæroidea#, #Prunoidea#, and #Discoidea#, this form is sometimes produced by the differentiation of two different transverse axes at right angles to each other. Whilst these ground-forms (Centraxonia and Centrostigma) occur in the utmost variety among the SPUMELLARIA, the centroplanar (or true bilateral) ground-form is entirely wanting. 48. _The Ground-Forms of Acantharia._--In the small family Astrolophida, which contains the most archaic forms of the legion (_Actinelius_, _Astrolophus_), the ACANTHARIA show a direct relation to the most primitive SPUMELLARIA (_Actissa_), and like these have indifferent equilibrium; their central capsule is a sphere, their calymma an endospherical polyhedron, whose angles are indicated by the distal ends of the numerous {xxiii}equal radial spines. In the great majority of ACANTHARIA, however (all #Acanthonida# and #Acanthophracta#), twenty radial spines are present, regularly distributed, according to Müller's icosacanthan law, in five parallel circles, each containing four crossed spines (p. 717). Usually the twenty spines are equal, and the ground-form is the quadratic octahedron, or a regular double pyramid with sixteen sides. But in some groups (the Amphilonchida and Prunophracta) two opposite equatorial spines are much more strongly developed than the other eighteen, and therefore the hydrotomical axis in the equatorial plane is larger than the geotomical axis (p. 719); the isopolar stauraxonian form passes over into the allopolar, and the ground-form becomes the rhombic octahedron or the amphithect double pyramid (compare §§ 33 and 34, and p. 720). The centroplanar ground-form is entirely wanting in the ACANTHARIA.