INTRODUCTION. 5 



It is hardly possible to identify the various species 

 by their nuclei, but there are four distinct types of 

 nucleus distinguished by Penard. The nuclei are 

 always bounded by a definite membrane which in some 

 cases appears as a double line, and they usually 

 contain one or more nucleoli. When an individual 

 assumes the resting-state all the chromatin may be 

 collected into a large nucleolus instead of being dis- 

 seminated in the form of a net-work. 



FIG. 178. Diagram of a Heliozoon with central nucleus (Actinophrys- 



(After Penard.) 



nucleus is placed centrn/h/ (Text-fig. 178) in 

 the following species : 

 Actinophrys vol. Clathrulina ?lri/<nt*. 



vesiculatai. ,, cienJcowsJcii. 



Astrodisculus ::oua(<ix. stuhlmanni. 



,, araneiformis. 



The nucleus is eccentric (Text-fig. 179), and a 

 centrosome or central granule is present in all other 

 freshwater species except in the genus Actinosphserium 

 which is multi-nuclear as are also some marine 

 genera. 



In those species of Heliozoa which have the nucleus 

 placed eccentrically in respect to the envelope the 

 centre of the organism is occupied by the central 

 granule which is surrounded by a zone of clear 

 plasma, the whole apparently functioning as a centre 

 of kinetic activity; the axes of the pseudopodia arise 



