300 THE PROTOZOA 



tozoa, by Haeckel ('70) on Rhizopoda and Radiolaria, and by Verworn 

 ('89), who demonstrated on a number of Rhizopoda that stimuli of 

 different grades of intensity cause different degrees of reaction, and 

 that a stimulus applied at the end of a pseudopodium causes only 

 a local irritation expressed by contraction at that point, and he 

 expressed the belief that a regular graduated scale of irritability 

 could be established for this group. Many forms show almost no 

 reaction to slight stimuli, others a distinct contraction. 



Among the Mastigophora and Infusoria, not only is the body more 

 easily affected by mechanical stimuli, but transmission of the stimulus 

 is far more rapid than in the Sarcodina. Impact of a ciliate with any 

 object, possibly an organism much smaller than itself, causes a vig- 

 orous reaction expressed by sudden change of motion (Flagellidia, 

 hypotrichous and holotrichous Ciliata), by contraction of myonemes 

 (Stentor, Vorticellidae, etc.), or by body contractions (Euglena, 

 Spirostomum, etc.), all of which may be expressed by Jennings's term 

 motor response. 



A general result of mechanical stimulation is a motor response fol- 

 lowed by the tendency to turn away from the source, and the general 

 reaction, whether positive or negative, since it deals with the question 

 of pressure in some form or other, is called barotaxis (Verworn). 



In all forms of barotaxis, e.g. in thigmotaxis ', the reactions may be 

 either positive or negative. Negative thigmotaxis, the ordinary reac- 

 tion to mechanical stimuli, such as pricking or crushing a local area, is 

 expressed by movement away from the source of irritation. Positive 

 thigmotaxis is expressed by movement toward the source of the 

 stimulus. The latter is commonly seen in the attachment of pseudo- 

 podia of Amceba and other fresh-water Rhizopoda to solid bodies, and 

 it affords an explanation of the ingulfing of solid bodies (e.g. food par- 

 ticles). Dewitz ('86) found positive thigmotaxis on the part of sper- 

 matozoa of the cockroach, a discovery of the greatest interest in view, 

 again, of the question of fertilization. 



The reactions to galvanic currents are, as a rule, quite definite, and 

 as in iron filings over a magnet, there is usually a definite polar 

 arrangement of the single-celled organisms, a reaction expressed by 

 the term galvanotaxis. Pearl ('oo), examining the reactions to gal- 

 vanic stimuli in several ciliates and in the flagellate Chilomonas para- 

 moecium in the light of Jennings's theory of motor responses, came to 

 the conclusion that the usual reaction is distinctly affected by the 

 current so that a "forced movement" in many cases is superimposed 

 upon the normal motor response. The forced movement in ciliates, 

 he states, is due to the action of certain cilia which are constrained 

 by the current to lie in certain definite positions, those on the 

 cathode surface pointing toward the anterior end, those on the anode 



