THE RADIOLARIA 



107 



spicules have developed by a hardening of the stiff fibre of certain 

 alternate axopodia which formerly met at the centre of the endo- 

 plasm as in Heliozoa, to which group this order suggests other 

 points of affinity. The peculiar nature of these spicules is the 

 distinguishing feature of the order. They are composed, in the 

 best investigated cases, of strontium sulphate (Biitschli, 1906), and 

 not of a chitinoid organic acanthin-substance, as Haeckel supposed. 



Fio. 5. 



To illustrate the structure of the 

 Nassellarian sub-family. A, Plagonis- 

 cu,s tripodiscus, II., showing the central 

 capsule (c.c) supported by the skeletal 

 tripod. B, Cortina typus, H., showing 

 the tripod and sagittal ring (5) enclos- 

 ing the central capsule, within which 

 are seen the podocone (p), the nucleus 

 above, and three oil -globules. C, Tre- 

 pospyris cortiniscvs, H., to show the 

 formation of the helmet-like type of 

 skeleton from the tripod and sagittal 

 ring. (After Haeckel. ) 



The nucleus is a multiple structure, and the large body frequently 

 mistaken for a nucleus (Fig. 4 (6, &)) is a Suctorian parasite. The 

 Acantharia frequent the upper layers of the ocean (chiefly from 

 the surface down to 300 metres), and are abundant in Arctic and 

 Antarctic seas as well as in the intermediate zones. The yellow 

 cells that in other Radiolaria are confined to the extracapsulum, 

 occur almost exclusively within the central capsule in the 

 Acantharia. 



The Monopylaria or Nassellaria include an immense range of 

 forms. In the simplest the central capsule is supported by a 

 siliceous tripod or tetrad spicule, often accompanied by a sagittal ring. 



