42 



THE INVERTEBRATA 



nosphaerium and Paramecium caudatum in less highly organic infusions, 

 Volvox and Amoeba proteus in much purer waters, Haematococcus in 

 rain water. As a rule the marine and freshwater faunas are restricted by 

 conditions of salinity, but Polystomella ranges from the sea into brack- 

 ish waters. For many holophytic protozoa the amount of sunlight is 

 important. Others, as Euglena gracilis, bleach in the absence of light, 

 but can still flourish if the presence of organic matter in solution makes 



c.vac. 



f.vac. 



Fig. 33. Actinosphaerium eichhorni, x 180. From Leidy. The endoplasm is 

 crowded with food vacuoles containing diatoms, and nuclei are represented 

 in the figure by the dark areas, c.vac. contractile vacuole ;/.z;<2c. food vacuole 

 which has just swallowed a rotifer; ps. pseudopodia. 



saprophytic nutrition possible. Holozoic species must of course have 

 their proper food; in infusions they appear as this becomes plentiful, 

 first, after the bacteria, those whose diet is purely bacterial, such as 

 Monas and Colpoda, then those, such as Stylonichia, that feed upon the 

 first comers, and so on ; though some bacterial feeders, as Paramecium, 

 are rather late to appear. Temperature has also an influence upon 

 protozoan faunas. The powers, possessed by freshwater protozoa, of 

 distribution across inhospitable regions and of surviving unfavourable 



