REPORT ON THE RADIOLARIA. XXI 



44. Principal Axes. — From the foregoing consideration of the statical conditions 

 and their direct causal connection with the geometrical ground-forms of the Radiolaria, 

 the great mechanical significance of the differentiation of definite axes in these unicellular 

 free-swimming organisms will be manifest. The most important of these is the primary 

 main axis (axis principalis, or protaxon), which in all cases has a vertical direction. 

 It is wanting in the Centrostigma (spheres and endospherical polyhedra), and in the 

 Anaxonia (acentra). It is isopolar in the phacotj'pic forms (Monaxonia isopola), and in 

 the double pyramids (Stauraxonia isopola). It is aUopolar in all monastatic ground- 

 forms, in the conotypic forms (Monaxonia allopola), pyramids (Stauraxonia allopola), 

 and the Centroplana (or bilateral forms). 



45. Secondary or Transverse Axes. — In contrast to the vertical main axis all the 

 other constant axes differentiated in the body may be called " secondary axes," or 

 "transverse axes," since they cross the former at definite points. All ground-forms 

 whose vertical axis is crossed by a fixed number of such axes at definite angles may be 

 called " Stauraxonia." They are divided into two groups, double pyramids and single 

 pyramids ; in the former the two poles of the main axis (or the two halves of the body 

 separated by the equatorial plane) are similar (Stauraxonia homopola), in the latter 

 dissimilar (Stauraxonia heteropola). If all the secondary axes be equal, the stauraxon 

 ground-form is regularly radial. If some of them be unec[ual they are arranged in 

 certain relations towards two primary transverse axes, perpendicular to each other, to 

 which all the other secondary axes are subsidiary ; the ground-forms are then either 

 amphithect or bilateral. The two primary transverse axes, which may also be designated 

 " ideal transverse axes " (euthyni), divide the vertical main axis in its centre ; one of 

 them is the sagittal, the other the frontal. These three dimensive axes give the factors 

 which accurately determine the ground-form and the dimensions in most Radiolaria ; 

 the vertical main axis determines the length (principal axis) ; one horizontal transverse 

 axis determines the thickness (sagittal axis), and the other the breadth (frontal axis). 

 Those ground-forms in which the transverse axes are isopolar are termed "amphithect," 

 and those in which the one (frontal or lateral) is isopolar and the other (sagittal or 

 dorso-ventral) is aUopolar, are termed " bilateral," or better " zeugitic." 



46. Primary and Secondary Groitnd- 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 

 oi Actissa {Procytta,rium, PL 1, fig. 1) are in fact geometrically pe^yeci 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 S p h se r o i d e a, as well as of many Acanthaeia 



