THE GENERAL PHYSIOLOGY OF THE PROTOZOA 207 



of Actinosphcerium, the experiments of Smith (207), Mackinnon (174), and 

 Boissevain (151), show that increased temperature hastens on encystment, 

 and causes fewer and larger cysts to be formed in which the nuclei are larger 

 but poorer in chromatin ; while at lower temperatures the encystment is 

 retarded, and finally inhibited altogether, and the cysts produced are smaller 

 and more numerous, with nuclei smaller than the normal but rich in chromatin. 



(d) Barotaxis and Effects of Mechanical Stimuli. This category 

 includes Geotaxis, or reactions to gravity ; Thigmotaxis, or reactions 

 to the mechanical contacts of hard surfaces ; and Rheotaxis, or 

 reactions to the pressure of currents in the surrounding medium. 

 The influence of gravity is seen in the manner in which many 

 Protozoa, when placed in a vessel, seek of their own accord the 

 bottom in some cases, the surface-film in others. The plasmodia of 

 Mycetozoa exhibit often a well-marked rheotaxis, and move in the 

 opposite direction to currents of water. It has been suggested that 

 a similar rheotaxis may explain the passage of blood-parasites from 

 the invertebrate to the vertebrate host during the act of blood- 

 sucking ; but it is probable that such migrations are purely passive, 

 so far as the parasites are concerned. 



Contact-stimuli acting from one side often have a marked effect 

 on the movements of Protozoa. An amoeba tends to adhere to, and 

 spread itself over, a firm surface with which it comes in contact. 

 The movements of Ciliata often cease when they come in contact 

 with a firm substance, and the animal remains still ; Putter (197) 

 has shown that the contact-stimulus may be sufficient to prevent 

 a Paramecium from reacting to thermal or electric stimuli, which 

 would otherwise produce a marked effect upon its movements. 



Under effects of mechanical stimuli must be included those brought about 

 by changes in the tonicity of the surrounding medium. Such effects have 

 already been discussed above as regards their action on the contractile 

 vacuoles. For the remarkable experiments of Verworn on the change in 

 body-form and in the nature of the pseudopodia exhibited by amoebae under 

 the action of different media, see p. 217, infra. Free-living Protozoa are 

 probably seldom if ever subject to such changes, though they might well 

 occur in the environment of marine forms living near the upper limit of the 

 tide-marks, in rock-pools, or other places where the tonicity of the medium 

 might be lowered temporarily by influx of fresh water, as the result of rain 

 or other natural causes. On the other hand, parasitic forms, and especially 

 those which pass from one host to the other, may be subject to rapid changes 

 of tonicity in their environment. In this connection special interest attaches 

 to the experiments of Robertson (503) on fish-trypanosomes ; it was found 

 that in undiluted blood or in blood diluted with isotonic solutions the 

 trypanosomes underwent no change in vitro, but that when the blood was 

 diluted with water the trypanosomes multiplied by division, and went through 

 changes similar to the first stages of the natural development in the leech. 

 It was concluded, therefore, that the principal stimulus which initiates the 

 developmental changes in the organism was a lowering of the osmotic tension, 

 with consequent absorption of water by the protoplasm. Neumann (677) 

 also found that the " exflagellation " of the Profcosowa-parasite of birds was 

 greatly furthered by addition to the blood of not more than one-fifth of its 

 volume of water. 



