4 BRITISH I'BESHWATEIt HELIOZOA. 



absorption of CO^ and the liberation of oxygen from 

 the included and living chlorophyll cells rendering the 

 vesicles nnnecessary ; in some species, e. g. Heterophrys- 

 fochei and Aciinophrys Sol, the vesicles are particularly 

 noticeable either for their size, number, or rapid 

 pulsation. 



Only one pulsating vesicle is normally present and 

 its period of dilatation and contraction is usually 

 between 40 and 100 seconds. The non-contractile 

 vacuoles are as a rule numerous and disseminated 

 throughout the ectoplasm. 



The food of the Heliozoa consists of living animal- 

 cula or algge, the smaller organisms being ingested by 

 the ectoplasm whilst the larger are captured by means 

 of the pseudopodia. Digestion takes place in vacuoles 

 containing a clear secretion having digestive functions. 



Some species of Heliozoa are normally infested by 

 parasites ; these may be species of bacteria as found 

 in Acanthocystis clisetoplioru, A. spinijera, BhajAi- 

 diophrys viridis, and Reterophrys iniiriop)oda; and even 

 Ciliates or Rotifers in the" case of Bliaphldiophrys 

 viridis and Acanthocystis cJisetophora.. 



The more typical Heliozoa, that is those belonging 

 to the order Chalarothoraca, have the nucleus and 

 endoplasm placed eccentrically in the body whilst 

 the centre of the spherical envelope is occupied by 

 the " central granule " which appears to act as a 

 true centrosome with the addition of certain kinetic 

 functions; in those species in which the nucleus 

 occupies the central position the centrosome is con- 

 tained within it. >) 



Nucleus. 



The nucleus is present and singife in all freshwater 

 Heliozoa, except in the genus Actinosphairium which 

 is multi-nuclear. Two or more nuclei may sometimes 

 be found but this is due either to a process of division 

 or to a merging together of two or more individuals. 



