28 



INTRODUCTION TO PROTOZOA 



nearm. 



-cytost. 



protozoans and green algae, makes such a group as the Protista 



appear both natural and convenient. 



Structure. A protozoan, in its simplest form, conforms to I 



the usual definition of a cell a bit of protoplasm containing 1 



a nucleus. S o m e- 

 -- ? oncil. times there are two 

 or more similar nuclei 

 and in the majority 

 of ciliates there are 

 two nuclei which dif- 

 fer from each other 

 both in form and func- 

 tion, a large " macro- 

 nucleus " which is 

 associated with the 

 ordinary vegetative 

 processes of the cell, 

 and a small " micro- 

 nucleus " which ap- 

 parently is concerned 

 only with sexual re- 

 production. In some 

 protozoans nuclear 

 material is extruded 

 from the nucleus itself 

 into the protoplasm 

 outside where it floats 

 about in the form of 



FIG. 1. A complex ciliate, Diplodinium ecaudatum, minute particles Or 



showing highly developed organelles; case., caecum or granules known as 

 rectal canal; cut., cuticle; c.v., contractile vacuole; , . ,. , , 



cy top., cytopyge or cell anus ; cytost., cytostome or cell cnromidia, tne latter 



mouth; d.m., dorsal membranelle; ect., ectoplasm; sometimes having the 



end., endoplasm; mac. n., macronucleus ; mic. n., mi- k , 



cronucleus; myon. (str. retr. CBS.), myonemes, strands power, under certain 



for retracting oesophagus; CBS., cfisophagus; or. cil., circumstances, of 



oral cilia; sk. lam., skeletal laminse. X 750. (After , . 



Sharpe.) forming new nuclei. 



In some Protozoa 



there is no nucleus as such, though the essential substance of 

 the nucleus, chromatin, is always present, but in scattered par- 

 ticles. 



The protoplasm of a protozoan is usually more or less clearly 



