28 



INTRODUCTION TO PROTOZOA 



(sfr. i^trocs.) 



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



appear both natural and convenient. 



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



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



a nucleus. S o m e- 

 times theie arc two 

 or more similar nuclei 

 and in the majority 

 of ciUates there are 

 two nuclei whidi dif- 

 fer from each other 

 bot h in form and func- 

 tion, a large " macro- 

 nucleus " which is 

 associated with the 

 ordinary \-egetativc 

 processes of the cell, 

 and a small " micro- 

 nucleus " which ap- 

 parentlj' 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 

 minute particles or 

 granules known a s 

 chromidia, the latter 

 sometimes having the 

 power, under certain 

 circumstances, of 

 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 



Fig. 1. A complex ciliatc, DipMiniiim ccaudalum, 

 showing highly developed organelles; esse., ctecum or 

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

 c>'top., c>'topyge or cell anus; c>'tost., c>'tostome or coll 

 mouth; d.m., dorsal membranelle; ect., ectoplasm; 

 end., eudoplasm; mac. ii., macronucleus; mie. n., nii- 

 cronucleus; myon. (str. rctr. oes.), myonemcs, strands 

 for retracting oesophagus; oes., oesophagus; or. cil., 

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

 Sharpe.) 



