10 



have remained at much the same level of organisation, while 

 the main line continued upwards to Actissa. A side branch 

 from this point leads to Actinelius, the ancestral form of the 

 Acantharia. This was derived from Actissa, according to 





Fig. 5. Actissa (after Haeckel). 



Haeckel, by the hardening of the firmer axial part of the 

 radiating pseudopodia into spicules of acanthin, the only 

 form of skeleton found in the Acantharia. The central cap- 

 sule in this group remains simple and spherical, and is 

 pierced on all sides by fine pores. 



The Spumellaria are more nearly in the direct line 

 of development than the other three groups, and may be 

 traced back to Actissa as an ancestral form. The central 

 capsule remains in its simple form, but most of the 

 Spumellaria have acquired a siliceous skeleton, or shell, 

 which serves to distinguish them from the Acantharia. The 

 Nassellaria have diverged from the primitive Spumellaria, 

 their probable ancestor Cystidium being derived from 

 Actissa by the pores of the capsule membrane becoming 



