22 PROTOZOA 



then are repelled instead of being attracted. The most active 

 part of the spectrum in determining reactions of movement are 

 the violet and blue rays of wave-length between 40 /i/lO and 

 49 /U./10, while the warmer and less refractive half of the spectrum 

 is inert save in so far as it determines changes in the tempera- 

 ture of the medium. 



(v.) The movements of many Protozoa are rendered sluggish 

 by cold, and active by a rise of Temperature up to what we 

 may term the " optimum " ; the species becomes sluggish again 

 as the temperature continues to rise to a certain point when 

 the movements are arrested, and the being is said to be in 

 a state of " heat-rigor." Most Protozoa, again, tend to move in 

 an unequally heated medium to the position nearest to their 

 respective optimum temperature. This is called " thermotaxy." 

 The temperature to which Amoeba is thermotactic is recorded as 

 35° C. (95° P.); that oi Paramecium is 28° C. (82° P.). 



(vi.) Most active Protozoa tend to take up a definite position 

 in respect to a current of Electricity passing through the 

 medium, and in the majority of cases, including most Ciliates, 

 Amoeba, and Trachelomonas, they orient their long diameters 

 in the direction of the lines of force and swim along these to 

 assemble behind the cathode. The phenomenon is called 

 " galvanotaxy," and this particular form is " negative." Opalina 

 (Fig. 41, p. 123), however, and most Plagellates are "positively 

 galvanotactic," and move towards the anode. H. H. Dale ^ has 

 shown that the phenomenon may be possibly in reality a case 

 of chemiotaxy, for the direction of motion varies with the 

 nature and concentration of the medium. It would thus be a 

 reaction to the " ion " liberated in contact with the one or other 

 extremity of the being. Induction shocks, as we have seen, if 

 slight, arrest the movements of Protozoa, or if a little stronger 

 determine movements of contraction ; if of sufficient intensity 

 they kill them. No observation seems to have been made on 

 the behaviour of Protista in an electric field. A magnetic field 

 of the highest intensity appears to be indifferent to all Protista. 



(vii.) We have already referred to the effect of dissolved 

 Chemical Substances present in the water. If the substance 

 is in itself not harmful, and the effect varies with the concentra- 

 tion, we term the reaction one of " tonotaxy," which combines 



1 " Galvanotaxis and Chemotaxis, " Journ. of Physiol, vol. x.\:vi. 1900-1901, p. 291. 



