10 



THE SMALLEST LIVING THINGS 



have tissues, such as epithelial or covering tissue, muscle tissue, 

 secreting or glandular tissue, germinal tissues, and the like. The 

 special parts of an organism which we know as organs, are made 

 up of tissues, and the tissues of cells. The functional activity 

 of the organ as a whole is the sum total of the activities of its 

 constituent cells. Thus, a higher organism is the aggregate of 



specialized cells, each con- 

 tributing its part to the 

 harmonious working of 

 the whole. 



In "lower" metazoa 

 this division of physiologi- 

 .0* cal labor is less complete; 

 each of the constituent 

 cells has more work to do 

 to maintain the fundamen- 

 tal vital functions; each is 

 more generalized. Final- 

 ly, there is the great world 

 of "lower" unicellular or- 

 ganisms in which all of the 

 fundamental vital func- 

 tions are performed by a 

 single cell independently 

 of all others. Such uni- 

 cellular organisms repre- 

 sent physiological gener- 

 alization par excellence, 

 and some are, paradoxi- 

 cally, the most complex of 

 all living cells and at the 

 same time the simplest of 

 living organisms (Fig. 3). 



Fig. 3-DIPLODINIUM ECAUDATUM BeJng single cdls they are 



A complex, ciliated one-celled protozoan parasite of m | nute and as the sma \\. 

 the horse s stomach. Sharpe, who first described it, 



named 48 different parts of which the most important g§t of the living things 



are: or, the mouth; ocs, the gullet; sk, skeletal ele- , . , r 11 



ments; m.m., motorium; an, the anal opening; c.v., treateQ in tne lOHOWing 



the contractile vacuole; mac, macronucleus, and mic, rjjjrrps fhev havP 3. SDCCial 



micronucleus . 



Magnification, 920 importance in many eco- 



mac 1 



