xxii THE VOYAGE OF H.M.S. CHALLENGER. 



are true spheres. Furthermore the simple or concentrically composed lattice-spheres of 

 Sphseroidea, Sphserophracta, and Phseosphseria may be regarded as 

 spheres, although strictly speaking they are endospherical polyhedra. From the primary 

 spherical form of the Kadiolaria all other secondary forms may be derived in the follow- 

 ing 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 Amphi2jleura arise by differentiation of both poles 

 of the sagittal axis. 8. The zygopleural ground-form appears last, as the simplest form 

 of the Amphipleura. 



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 sj^herieal central 

 capsule, and also most Sphseroidea. 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 rai'ely allopolar. Sometimes they are Monaxonia (circular in transverse 

 section), sometimes Stauraxonia (polygonal in transverse section). The vertical main axis 

 is longer in the P r u n o i d e a, 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 rombic octahedron ; 

 but even among the Sphseroidea, 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. TJie 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 ; theii- central capsule is a sphere, their calymma an endo- 

 spherical polyhedron, whose angles are indicated by the distal ends of the numerous 



