208 THE PROTOZOA 



(e) {jrdlvanotaxis and Effects of Electrical Stimuli. Protozoa 

 placed in an electric field that is to say, in a drop of water between 

 the two poles of a battery under a cover-glass on a slide are 

 affected to a marked degree, but with opposite results in different 

 species. Opalina places itself parallel to the direction of the current, 

 with its anterior end towards the anode. With a current of 

 moderate intensity it swims towards the anode ; but with a stronger 

 current the speed at which the animal moves is diminished, and 

 with still more increased strength of current it is carried passively 

 towards the kathode, with its hinder end forward, as the result of 

 kataphoric action (Wallengren, 215). Chilomonas behaves in a 

 similar manner. Paramecium and Colpidium, on the other hand, 

 move towards the kathode. Spirostomum with a moderate 

 current also moves towards the kathode, but with stronger currents 

 it first contracts its myonemes spasmodically, and then takes up 

 a position transverse to the direction of the current, and remains 

 still. 



According to Wallengren (215), the apparently different galvano- 

 tactic phenomena exhibited by different ciliates admits of a uniform 

 explanation, by a combination of two effects. In the first place, 

 in the half of the body turned towards the kathode the expansion- 

 phase of the ciliary movement is stimulated ; in the anodic half of the 

 body, the contraction-phase is stimulated. In the second place, the 

 turning movements of the ciliates are determined mechanically 

 (compare the " avoiding reactions " mentioned above), and may be 

 effected either by the expansion or by the contraction of certain 

 cilia. Consequently, if the turning movements are effectel by 

 beats of expansion, the animal places itself automatically in a posi- 

 tion in which it moves towards the anode ; if beats of contraction 

 are effective in the turning movement, it moves towards the 

 kathode. According to Statkewitsch (209), the galvanotactic re- 

 action is one which overcomes chemo tactic stimuli, and leads the 

 animals irresistibly into toxic media in which they are killed. 



6. Degeneration and Regeneration. The fact that under certain 

 conditions Protozoa undergo a process of physiological degenera- 

 tion, which may end in death, has been observed frequently by all 

 those who have kept cultures of Protozoa under observation for 

 a long time. It has been pointed out in a previous chapter (p. 135) 

 that the life-cycles of Protozoa exhibit depression-periods (Calkins) 

 which are characterized chiefly by cessation of feeding, metabolism, 

 growth, and reproduction, together with increase in the size of the 

 nucleus, and tendency to deposition of grains of fat or other sub- 

 stances in the protoplasm, giving the body a characteristic dark- 

 grey appearance. Such periods recur regularly and apparently 

 normally in the life-cycles both of Protozoan and Metazoan cells 



