28 



INTRODUCTION TO PROTOZOA 



neurm.ip. 



(str. retr oes.) 



mic.n. «»;.-i^.- 



inic.n 



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 



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



a nucleus. S o m e- 

 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- 

 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. 



-— cytop. 



Fig. 1. A complex ciliato, Diplodinium ecaudatuiii , 

 ehowinc liip;hly dovcloijod orRaiiollcs; ca;c., csecuin or 

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

 cytop., cytopjge or cell anus; cytost., cytostome or cell 

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

 end., endoplasm; mac. n., marromicleus; mic. n., nii- 

 cronucleus; myon. (str. retr. oes.), myonemes, strands 

 U)T retracting oesophagus; ces., ce.sophagus; or. cil., 

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

 Sharpc.) 



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 



