68 



ELEMENTS OF BIOLOGY 



trial and error (Fig. 19) it also finds the most suitable temperature, 

 salt content, acidity, and other features of the environment. Fur- 

 thermore, when attacked, its trychocysts are instantly thrown out. 

 So, in general, both the property and mechanism of irritability, as 



MOUTH 



ORAL CILIA 



OESOPHAGUS 



. ORAL DISK 



MOTOR MASS 



OPERCULAR FIBERS 

 .OPERCULUM 



CIRCUMOESOPHAGEAL RING 

 VENTRAL MOTOR STRAND 



VENTRAL 



MEMBRANELLES 



DORSAL DISK 



DORSAL MOTOR STRAND 



ANT. CONTRACTILE 

 VACUOLE 



MICRONUCLEUS 



MACRONUCLEUS 



POST CONTRACTILE 

 VACUOLE 



CAECUM 



RECTUM 



ANUS 



Fig. 18. — The neuro-motor apparatus of a diplodinium. An upper portion of 

 the animal has been cut away to show the relations and paths of the conducting 

 fibres. (After Sharpe.) 



well as those of response, or contractiUty, are much more distinct 

 and better developed in paramcecium than in amoeba. This fact 

 may be taken to mean that the chemical processes in the proto- 

 plasm which build up and maintain such structures are more com- 

 plicated in paramcecium than in amoeba. The term differentiation 



