GENERAL SKETCH 6 1 



indefinitely, but that periods of division recur at intervals. This sug- 

 gestion was confirmed by Butschli ('76) and by Engelmann ('76), who 

 demonstrated in connection with a number of Ciliata that after a 

 certain number of divisions the resulting individuals become reduced 

 in size and show other evidences of degeneration. Butschli regarded 

 this as evidence of old age, and he observed that the normal size and 

 the general vitality of the reduced organisms are restored by conju- 

 gation ; and he was one of the first to demonstrate that the function 

 of conjugation is not for purposes of reproduction, but for the renewal 

 of vitality as expressed in his term Verjungung (rejuvenation). 

 Maupas ('88), finally, has confirmed the latter conception of conjuga- 

 tion, and in a series of brilliantly planned and carefully executed 

 experiments has shown that Protozoa, contrary to Weismann's a priori 

 assumption, may die of old age unless they be reinvigorated by 

 conjugation. " Senescence," says Maupas, " appears to be a very 

 general phenomenon, at least in the animal kingdom. ... It is 

 inherent in the organism and comes from internal causes which act 

 independently of the surrounding conditions. ... Its deleterious 

 action is offset and annulled by sexual rejuvenescence or conjugation." 1 



4. Irritability. 



Unlike the Metazoa, where the phenomena which are characterized 

 as manifestations of consciousness are expressed through special or- 

 gans of the nervous system, the Protozoa in the simplest forms have 

 nothing, so far as we know at present, but the undifferentiated proto- 

 plasm which at the same time must be the seat of all functions. 



The sensory phenomena are, however, very little known. All Pro- 

 tozoa are irritable, reacting in certain definite ways, although in dif- 

 ferent degrees, toward various external stimuli. All are sensitive to 

 electrical, mechanical, thermal, and chemical irritations, and many 

 to light, while few or none are affected by acoustic vibrations. The 

 reactions to these stimuli are usually expressed in motion of some 

 sort, which may be either indefinite or definite, in the latter case, 

 as a rule, either positive, i.e. toward the source of irritation, or nega- 

 tive, i.e. away from it. In many Protozoa, particularly in the lower 

 forms, there seems to be no portion of the cell more sensitive than 

 others ; in the higher forms, however, there is a greater or less degree 

 of sensory localization. Here, as a rule, the ectoplasm reacts ener- 

 getically, and, like the cuticle of Metazoa, becomes a general sensory 

 organ. The appendages frequently serve as special sensory organs 

 of touch, as in the aboral cirri of the hypotrichous ciliates, while spe- 

 cial organoids are frequently present in the form of " eye-spots," etc. 



i ('88), p. 272. 



