SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 301 



surface toward the posterior end. 1 In some forms, e.g. Pardmaecium 

 and the majority of Ciliata, and different species of Amoeba, mak- 

 ing the current results in active and direct movements toward the 

 kathodic electrode ; in others, e.g. the flagellate Polytoma, the reverse 

 reaction takes place, the organisms swimming vigorously toward the 

 anode, and still a third reaction may be observed in Spirostomum 

 ambiguum, which turns at right angles to the direction of the current 

 and remains so (Verworn, '89). 



From the 'various experiments which have been made upon Proto- 

 zoa, it appears that more or less similar motor reactions follow 

 different kinds of stimuli, and that the organism reacts similarly to 

 the same stimulus. With each type of stimulus there is, in most 

 cases, an optimum of intensity below which a positive reaction is 

 observed, but which, if exceeded, is followed by a negative reaction, 

 while over-stimulation results in no response or in death. Further- 

 more, the experiments show that each type of organism behaves in 

 its own way, a given stimulus violently affecting some species, while 

 others are unaffected, although different individuals of the same 

 species always react similarly. The directive effects of certain 

 stimuli through operation in certain lines or On certain sides giving 

 rise to unequal stimulation of the protoplasm, induce reactions which 

 again are invariable. The thermotactic response of Amoeba, or the 

 phototactic reactions of Euglena, can be predicted with almost as 

 much certainty as the reaction of mercury to heat. In some 

 instances, notably in the more complicated forms, it appears that the 

 organism as a whole is endowed with a set of motor responses which 

 might be identified as instinctive. Stimulation at one point induces 

 not a local response, as in Rhizopoda, but a reaction of the entire 

 organism, which is poorly explained by the assumption of a machine- 

 like organization of the cell, or by the statement that these responses 

 are merely the expression of chemical and physical forces. 



E. General Considerations 



All of the normal functions of the protozoan cell show themselves 

 second in complexity only to the analogous processes which take 

 place in the Metazoa, and, as with the latter, many of them still 

 remain unexplained. One point, nevertheless, has been very thor- 

 oughly demonstrated, vis. the importance of the nucleus in the main- 

 tenance of many of these functions. Brandt ('77) was the first to 

 observe that non-nucleated fragments of Actinosphcerium will die, 

 while nucleated parts will live, and his observations were made the 



1 Loc. cit., p. 123. 



