GENERAL PHYSIOLOGY 183 



enough to bring about the motor response and the individual con- 

 tinues through the drop until it strikes the farther limit. Here the 

 stimulus is sufficiently strong to cause the motor response which 

 is manifested as a backward swimming, due to reversal of cilia, 

 turning on the long axis and recovery of normal forward swimming 

 movement. Repetition of this procedure keeps the individual in 

 the acid drop. Others enter in a similar way and are similarly 

 trapped until many are confined in the acid drop where they are 

 ultimately killed. Such motor responses unquestionably play an 

 important role in food-getting and in vital activities generally. 



The stereotyped nature of the motor response in any specific 

 organism may be due in the main to the characteristic silver line 

 and neuromotor systems which the higher types of flagellates and 

 ciliates possess. The observations of Sharp (1914), Yocom (1916) 

 and McDonald (1922) on ciliates, of Kofoid on flagellates, and the 

 experiments of Taylor (1920) in cutting different regions of the 

 neuromotor complex of Euplotes, indicate that the motor response 

 of Protozoa is bound up with coordinating systems possessing some 

 of the attributes of coordinating systems in Metazoa (Fig. 96). 

 Knowledge of these complex systems and their reactions is quite 

 sufficient to dispel any lingering belief in tropisms as due to stimu- 

 lation of special motile elements acting independently in such a 

 way as to orient the organism in respect to the path of the stimulus. 

 Through coordinating fibrils all parts work together; cutting the 

 system at any point leads to inharmonious or uncoordinated move- 

 ments of the motile organs as Taylor has demonstrated. All reac- 

 tions depend upon the organism as a whole; enucleated fragments 

 are unable to react as do nucleated fragments (Hofer, 1890, Willis, 

 1916). Jennings' careful observations, which led him to the con- 

 clusion that the protozoon organism always acts as a whole is fully 

 confirmed by these later observations and experiments. 1 



D. Nutrition. — Under the heading nutrition are included all 

 physiological processes involved in the replacing of substances 

 exhausted by destructive metabolism. Groups of activities includ- 

 ing: (1) food-getting; (2) secretion and digestion; (3) assimilation; 

 (4) defecation, find their place here. Certain specialized structures 

 adapted for these various activities have been described for the 

 most part in the preceding chapters, and the following is supple- 

 mentary in nature dealing with the functions which these structures 

 perforin. 



1. Food-getting.— The varied methods by which Protozoa acquire 

 the needed materials for replenishing protoplasmic substances 

 reduced by oxidation are all correlated with the phenomena of 



1 For discussion of different types of stimuli and the resulting reactions by Pro- 

 tozoa see Minchin (1912), Khainsky (1910), Mast (1910-1918), Putter (1900, 1903), 

 Jennings (1904, 1909). 



