202 THE PROTOZOA 



The occurrence of a conducting nervous apparatus is more 

 doubtful ; it has been affirmed for Stentor by Neresheimer (p. 446), 

 but is not confirmed by other observers. It can at least be asserted 

 that in the more highly organized Ciliata a stimulus may lead to- 

 sudden movements in which different sets of contractile structures 

 take a concerted part. 



The reactions of Protozoa to stimuli have been the subject of a 

 great deal of experimental research by many investigators, amongst 

 whom Verworn, Loeb, Jennings (165), and Putter (199), deserve 

 special mention. The results of these investigations can only be 

 summarized briefly here. The various reactions are classified in the 

 first instance, according to the nature of the stimulus, by the use of 

 a terminology in which each principal category is denoted by a 

 word terminating in taxis, or in adjectival form tactic. Thus we 

 can distinguish (a) Chemotaxis, or reactions to chemical stimuli ; 

 (5) Phototaxis, or reactions to light ; (c) Thermotaxis, or reactions 

 to heat or cold ; (d) Barotaxis, or reactions to mechanical stimuli ; 

 and (e) Galvanotaxis, or reactions to electrical stimuli. A given 

 Protozoon may be quite unaffected by a particular stimulus ; or, on 

 the other hand, it may be affected by it in such a way that it tends 

 to move towards the source of the stimulus (positive taxis) or away 

 from it (negative taxis). The result depends, in many cases, on 

 the intensity of the stimulus applied ; thus, a Euglena will move 

 towards a moderate light (positive phototaxis), but away from a 

 too intense illumination (negative phototaxis). In each case an 

 optimum condition exists, in which the positive taxis reaches its 

 maximum. 



In such experiments the Ciliata are the objects of choice, on 

 account of the definite polarity of their movements as compared 

 with forms less highly organized, such as amoeba. In the Ciliata 

 a negative taxis results in an " avoiding reaction " (Schreck- 

 bewegung), in which the animal shrinks back with reversal of the 

 ciliary movements, " turning towards a structurally-defined side, 

 followed by a movement forward " (Jennings). Repeated experi- 

 ments have shown that the forms taken by the avoiding movements 

 do not depend on the nature of the stimulus, but on the organization 

 of the animal itself, and are always the same for a given species. 

 An Oxytricha, for example, turns always to the right, whatever the 

 direction from which the stimulus comes. The movement is deter- 

 mined automatically by the structure of the body. " The same 

 symptom can be called forth by the most diverse stimuli " (Piitter, 

 199). 



The various taxes may now be considered briefly : 



(a) Chemotaxis and Effects of Environment. This category in- 

 cludes reactions to liquids or gases diffused in the water ; reactions 



