REPORT ON THE RADIOLARIA. 



XIX 



Principal Groups of 

 Ground-Forms. 



II. Centrasonia. 

 The geometrical centre of 

 the body is a straight 

 line (the vertical main 

 axis). 



Constant transverse axes 

 (perpendicular to the 

 main axis) are want- 

 ing in the Monaxonia 

 (which have circular 

 transverse sections); on 

 the contrary they are 

 differentiated in the 

 Stauraxonia (which 

 have polygonal trans- 

 verse sections). 



Subsidiary Groups of 

 Ground-Forms. 



r 



III. Monaxonia. 



Uniaxial ground-forms or 

 centraxonia without 

 transverse axes. The ' 

 transverse planes (per- 

 pendicular to the main 

 axis) are circles. 



Geometrical Type. 



Monaxonia isopola. 



(Spheroids and ellipsoids ; 

 both poles of the main axis 

 similar. ) 



Examples. 



Central capsule and lattice-shell of 

 of many ]) i s c o i d e a (lenses) 

 , and Pruno idea (ellipsoids), 

 t Belonaspida, &c. 



IV. stauraxonia. 



Pyramidal ground-forms 

 or centraxonia with 

 transverse axes. The 

 transverse planes (per- 

 pendicular to the main 

 a.xis) are either regular 

 or amphithect poly- 

 gons. 



L 



r 10. 



u. 



Monaxonia allopola. ( Central capsule and lattice-shell of 



(Cone,ovoidandhemisphere ; J many Na.ssellakia, especially 

 the two poles of the axis 1 the Cyrtoidea eradiata 

 dissimilar.) I (Cyrtocalpida, &c.). 



Dipyramides rcgularcs. f Acanthaei.a with twenty radial 



(Quadratic octahedron, or J spines, the four equatorial being 

 quadrilonchial forms and 1 equal. Multiradial D i s c o i d e a 

 regular double pyramids.) L and Staurospherida. 



Dipyramides amphithectm. { Acantharia with twenty radial 



(Rhombic octahedron, len- J spines, whose four equatorial 

 tellipsoid, and amphithect 1 spines are unequal but paired, 

 double pyramids. ) L Many Larcoidea. 



12. 



Pyramides rcgularcs. 

 (Regular pyramids.) 



13. Pyramides amjihithcctos. 

 *- (Rhombic pyramids.) 



Many Nassellakia (triradial and 

 multiradial). Medusettida and 

 Tuscarorida. 



Phseoconchia. Bipedal S p y- 

 r i d e a and Stephoidea. 



III. Centroplana. 



The geometrical centre of 

 the body is a plane (the ' 

 sagittal plane). 



IV. ACENTRA. 



V. Bil^erali. f ''■ ^^SS^ radial ground- j ^2^:^^ and Spyroidea 



(or Zeugita). i form. ) \ 



Bilateral forms in the i 



general sense with lb. Zygoplcura ,) Most Nassellaria (primitively at 



right and left halves. I (Bilaterally symmetrical j^ast), many Challengerida. 



'- ground-torm.) ) " j a 



VI. Anaxonia. 



( 16. Irregiilaria. 

 There is no geometrical I No definite axes can be I (Absolutelyirregularground- 



centre. ( determined. ( forms.) 



Collodastnim, OollospJicera, Phor- 

 ticida, Soreumida. 



40. Mechanical Causes of the Geometrical Ground-Forms. — The great variety of 

 ground-forms exhibited by the Radiolaria is of special interest, since in most instances 

 their causes admit of recognition, and since they are so intimately related to each other 

 that even in the remaining cases the assumption that they have arisen by purely 

 mechanical causce efficientes seems justified. In this respect the first rank is taken by 

 statical conditions, especially the indifferent or stable equilibrium of the whole organism, 

 which floats freely in the water. With regard to these fundamental statical relations, 

 three principal groups of ground-forms may be distinguished, pantostatic, polystatic, 

 and monostatic. 



41. Pantostatic Ground- Forms. — By pantostatic or indifferently stable ground-forms 

 are meant those in which the centre of gravity coincides with the centre of the body, so 

 that they are in equilibrium in any given position. Strictly speaking, the only form 

 which possesses perfectly indifferent equilibrium is the sphere, that being the only truly 

 homaxon and perfectly regular form. Nevertheless, in a somewhat wider sense many 

 Polyaxonia, especially the endospherical polyhedra with very numerous sides, may be 



