194 



REPRESENTATIVE SINGLE-CELLED ANIMALS 



I. 



— >or.ci 



— cytost. 



— ocs. 





Sk.lima 



tnac.n. 



mic.n. 



show an astonishing complexity. Comparisons can be made even 

 in forms as simple as paramoecium, in which, although there is no 

 digestive canal, there is a region in the cytoplasm along which 



vacuoles move from 

 "mouth" to "anus." 

 Again, the paramoe- 

 cium " behaves as a 

 whole " in the action 

 of its cilia during the 

 avoiding reaction and 

 in swimming forward 

 or backward. These 

 considerations have 

 led some students of 

 the group to empha- 

 size the individuality 

 of the protozoan in- 

 stead of its cellular 

 state, and to disre- 

 gard the comparisons 

 that may be made 

 between colonial pro- 

 tozoa and metazoa 

 — caec, as colonies of cells. 



This idea of the in- 

 dividual as more im- 

 portant than the cell, 

 has been called the 

 Organismal Theory in 

 contrast to the Cell 

 Theory of organisms. 

 The more reasonable 

 position seems to be 

 a recognition of the 



: eruL 



cut. 



cyTop, 



Fig. 102. — A complex ciliate, Diplodinium ecauda- 

 tum, showing highly developed organelles. 



coBC, csecum or rectal canal; cut., cuticle; c.v., contrac- 

 tile vacuole; cytop., cytopyge or cell anus; cytost., cytos- 

 tome or cell mouth; d.m., dorsal membranelle; ect., ecto- 

 plasm; end., endoplasm; mac. n., macronucleus; mic. n., 

 micronucleus; myon. {str. retr. oes.), myonemes, strands 

 ioT retracting uesophagus; neur. m. ap., neuromotor appa- 

 ratus; cEs., CBSophagus; or. cil., oral cilia; sk. lam., skeletal 

 laniime. X750. (After Sharpe.) 



protozoa as physi- 

 ologically balanced and independent cells, which in some instances 

 have undergone extreme specialization of structure within the 

 limits of their unicellular nature; and of the metazoa as multi- 

 cellular organisms in which the cells are physiologically unbal- 

 anced because of their mutual dependence. 



