X THE VOYAGE OF H.M.S CHALLENGER. 



18. The Principal Groups of Geometrical Ground-Forms. The great variety of the 

 geometrical ground-forms which are actually realised in the variously shaped bodies of 

 the Eadiolaria, renders it desirable to classify these in as small a number as possible of 

 principal groups and a larger number of subdivisions. As extensive principal groups 

 four at least must be distinguished ; the Centrostigma or Sphserotypic, the Centraxonia 

 or Grammotypic, the Centroplana or Zygotypic, and the Acentrica or Atypic. The 

 natural centre of the body, about which all its parts are regularly arranged, is in the first 

 group a point (stigma), in the second a straight line (principal axis), in the third a plane 

 (sagittal plane), in the fourth a centre is of course wanting. 



19. The Centrostigma or Sphcerotypic Ground-Forms. The first group of geometri- 

 cal ground-forms, here distinguished as sphaerotypic or the Centrostigma, is undoubtedly 

 the most important among the Radiolaria, inasmuch as if these be considered mono- 

 phyletic, it must be the original one from which all the other ground-forms have been 

 derived. The common character of all these sphserotypic ground-forms is that their 

 natural centre is a point (stigma) ; thus there is no single principal axis (or protaxon) 

 such as is characteristic of the two following groups. The sphserotypic ground-forms are 

 subdivided into two important smaller groups, the sjiheres (Homaxonia) and the endo- 

 spherical polyhedra (Polyaxonia). The spherical ground-forms, fully developed in the 

 central capsule and calymma of Actissa and the Sphseroidea as well as in many 

 ACANTHARIA, present no different axes ; all possible axes passing through the centre of 

 the body are equal (Homaxonia). In the endospherical polyhedra, on the contrary, 

 numerous axes (three at least) may be distinguished, which are precisely equal to each 

 other and different from all the remaining axes (Polyaxonia). If the extremities of these 

 axes, or the poles, which are all equidistant from the common centre, be united by 

 straight lines, a polyhedral figure is produced whose angles all lie in the surface of the 

 sphere. According as the poles of the axes are at equal, subequal, or at different 

 distances from each other, we may divide the endospherical polyhedra into regular, sub- 

 regular and irregular. (See Gener. Morphol., Bd. i. pp. 404-416.) 



20. The Centraxonia or Grammotypic Ground-Forms. The second principal group 

 of organic ground-forms, here called grammotypic or Centraxonia, is characterised by the 

 fact that a straight line (gramma) or a single principal axis (protaxon) forms the natural 

 centre of the body. This important and extensive group is divided into two subgroups, 

 those with one axis (Monaxonia) and those with crossed axes (Stauraxonia) ; in the latter 

 different secondary transverse or cross-axes may be distinguished, but not in the former. 

 In the Monaxonia, therefore, every transverse section of the body perpendicular to the 

 principal axis is a circle, in the Stauraxonia, on the contrary, a polygon. The Monaxonia 

 are further subdivided into two groups, in one of which the two poles of the principal axis 



