18 



PROTOZOA. 



of increase of the amcebulse in this stage has not been followed. In the spore 

 formation of Dictyostelium the mass rises up into club-shaped prominences, in 

 the axis of which a septate stalk is formed, and the amcebulse become gradually 

 aggregated at the summit of the stalk, where they encyst. The amcebulse which 

 escape from the spores divide by fission, but they do not pass through a flagellate 



Genera. Copromyxa Zopf., Cynthulina Cienk., Dictyostelium Brefeld, Acrasis 

 Van Tieghem, Polyspondylium Brefeld. 



SUB-CLASS ]JL HELIOZOA.* 



For^ the most part fresh-water Gymnomyxa with stiff radiating 

 pseudopodia, and one or more nuclei ; usually with contractile vacuole. 

 A radial silicious skeleton sometimes present. 



The characteristic pseudo- 

 podia give the name to the 

 group. (Fig. 15.) 



When a skeleton is secreted, 

 it consists either of radially 

 arranged silicious spines ( Acan- 

 thocystis) or of latticed sili- 

 cious shells (Clathrulina), and 

 so closely resembles the skele- 

 ton of the Radiolaria that the 

 Heliozoa have been actually 

 described as fresh-icater Radio- 

 laria. 



They differ from the Radio- 

 laria in the absence of the 

 complicated differentiations of 

 the protoplasm, particularly of 

 the central capsule. One or 

 more nuclei may be present 

 in the central mass. An 

 important distinguishing mark 

 is afforded by the presence of 

 the pulsating vacuoles, which have not been observed in any marine 

 Radiolarian. 



Reproduction is effected by fission. Encystment and subsequent 



* L. Cienkowski, ''Ueber Clathrulina." Archiv. fur mikrosk. Anatomic, 

 Tom III., 1867. R. Greeff, "Ueber Radiolarien und radiolarienahnliche Rhizo- 

 poden des stissen Wassers." Ibid. Tom V. & XI. R. Hertwig und Lesser, 

 "Ueber Rhizopoden und denselben nahe stehende Organismen." Ibid. Suppl. 

 Tom X., 1874. Also Archer and F. E. Schulze, etc. 



FIG. 15. Young Actinosplicerium, still with 

 a single nucleus (after F. E. Schulze). 

 N nucleus. 



