IX] 



OF THE RADIOLARIAN SKELETON 



709 



nor what seems the very perfection of hexagonal symmetry in 

 Aulonia are as we are wont to conceive them; hexagons indeed 

 predominate in both, but a certain number of facets are and must 

 be other than hexagonal. If we look carefully at Carnoy's careful 

 drawing we see that both pentagons and heptagons are shewn in 

 his reticulum, and Haeckel actually states, in his brief description 

 of his Aulonia hexagona, that a few square or pentagonal facets are 

 to be found among the hexagons. 



Such skeletal conformations are common: and Nature, as in all 

 her handiwork, is quick to ring the changes on the theme. Among 



Fig, 323. Actinomma arcadophorum Hkl. 



its many variants may be found cases (e.g. Actinonima) where the 

 vesicles have been less regular in size; and others in which the 

 meshwork has been developed not on an outer surface only but at 

 successive levels, producing a system of concentric spheres. If the 

 sihceous material be not limited to the linear junctions of the cells 

 but spread over a portion of the outer spherical surfaces or caps, 

 then we shall have the condition represented in Fig. 324 (Eihmo- 

 sphaera), where the shell appears perforated by circular instead 

 of hexagonal apertures and the circular pores are set on shght 

 spheroidal eminences; and, interconnected with such types as this, 



