SOME PROBLEMS IN THE PHYSIOLOGY OF THE PROTOZOA 289 



has but an historic interest, has recently been rather feebly advocated 

 by Greeff ('91) and Penard ('90), who adopted it without the supposed 

 justification which the older naturalists had in comparing the contractile 

 vacuole of Protozoa with the water vascular system of the flatworms. 

 Until Leydig ('57) demonstrated the excretory function of the water 

 vascular system, it was supposed that the flatworms obtained oxygen 

 from a stream of water taken in by the vascular system from the out- 

 side, in a manner analogous to the air supply from the tracheae of 

 insects, and Schmidt ('67), following Dujardin, and followed by Bal- 

 biani ('60, '61) and Maupas ('79), attempted to explain respiration in 

 Protozoa in the same manner. At the present time, while the prob- 

 ability is very strong that the contractile vacuole expels water in which 

 the oxygen has been replaced by carbon dioxid, there have been singu- 

 larly few actual observations to confirm it. Certes's ('85) experiments 

 show that some alteration takes place in the water after its entrance 

 into the protoplasm. This was demonstrated by placing Infusoria in 

 water colored by dissolved aniline dyes ; the water of the contractile 

 vacuole remained clear and colorless, although the surrounding 

 medium was intensely colored. The only direct observations of the 

 presence of carbon dioxid was made by Brandt ('81) upon Amceba. 

 Placing these organisms in a medium colored by dilute haematoxylin, 

 he found that the water of the vacuole became yellow and then red, 

 thus showing the characteristic reaction of haematoxylin in the 

 presence of an acid. The observation is not conclusive, however, 

 for the presence of uric acid might also give this reaction. 



Numerous attempts have been made to describe the series of events 

 which lead up to, and cause, the contraction of the vacuole. Being 

 entirely hypothetical, they may be dismissed with a brief mention. 

 Schwalbe, Rossbach, Engelmann, Maupas, and many others explained 

 the bursting as due to the contractility of the protoplasm. Schwalbe 

 attempted to trace the impulse or stimulus of contraction to the 

 products of destructive metabolism, which become stored up in 

 the vacuole so that the latter, when full, presses upon the pro- 

 toplasm and causes it to contract. Rossbach ('72) more obscurely 

 attempted to trace the stimulus to the chemical change which 

 takes place at the moment of oxidation. Each oxidation forms an 

 oxidation product which, as soon as formed, incites the stimulus. 

 Zenker ('66) was more clear in describing the process as due to the 

 attraction of protoplasm for oxygen-holding water, and repulsion for 

 water without oxygen, the result being that when the oxygen is re- 

 moved from the imbibed water, the latter is expelled from the body. 

 Rhumbler ('98) gave a similar interpretation and adduced experi- 

 ments with inorganic fluids simulating the contractile vacuole. 

 Butschli ('83) regarded the systole as due to a simple physical 



