REACTION TO STIMULI— INFLUENCE OF ENVIRONMENT 129 



possible that the gliding movements performed by the small gregarine-like 

 merozoites or sporozoites may be explained in a similar manner. 



REACTION TO STIMULI. — The actual direction of progression is the 

 direct result of external stimuli acting on the organism. Practically all 

 Protozoa react to stimuli, whether mechanical, chemical, thermal, electric, 

 or photic. The response to such stimuli has been chiefly studied in the 

 case of ciliates, in which it has been frequently found that the region of 

 the cytostome is the most sensitive part of the body. It is evident that 

 for any Protozoon there is an optimum condition of the medium in which 

 it lives, and if, during progression, it reaches an environment which is 

 less favourable to its existence than that which it has just left, there will 

 be a stimulation of the sensitive area of the body. This stimulation will 

 result in an altered action of the organs of locomotion, with a consequent 

 withdrawal from the unfavourable stimulus. The movements of ciliates 

 when subject to adverse stimuli are very precise, and have been the 

 subject of extensive investigations. The attraction and repulsion are 

 known as positive and negative taxis respectively. Generally speaking, 

 positive taxis indicates a movement towards and a negative taxis a 

 movement from any particular environment. 



INFLUENCE OF ENVIRONMENT.— The actual condition of the environ- 

 ment in which a Protozoon finds itself is a very important factor in its 

 development. As already remarked, for each there is an optimum 

 condition which suits it best. Departures from this are followed by 

 conditions of depression resulting in degeneration or even death. Lack 

 of food or excess of it, leading to starvation or overfeeding, also brings 

 about degenerative changes which are seen in alterations in the structure 

 of the nuclei, which frequently become enlarged. In certain cases the 

 nuclei break up entirely, leading to the final death of the organism. To 

 a certain extent Protozoa can be gradually adapted to changes in environ- 

 ment, provided these are not brought about too suddenly. It is possible 

 by gradually raising the temperature of cultures to obtain a race of 

 organisms which can live at a temperature which would have quickly 

 killed if applied suddenly. Provided that degeneration has not proceeded 

 too far, recovery is possible if the conditions are improved. Regeneration 

 of the degenerate parts takes place. Similarly, Protozoa which have been 

 mutilated or deprived of portions of their bodies are able to regenerate 

 themselves, provided the nucleus remains intact. 



The majority of Protozoa are able to protect themselves against adverse 

 conditions by the process of encystment. The tough resistant capsule 

 which is secreted shuts them off from their environment, so that they are 

 able to survive unharmed till conditions favourable to a free-living 



I. 9 



