294 THE PROTOZOA 



which appears homogeneous under low magnification, is, in reality, 

 caused by a vast number of minute light centres. These are closely 

 grouped together in the centre of the light area, but are easily distin- 

 guished at the edges (Fig. ^50). Radziszewski ('80) found that a 

 number of different substances which are present in living organisms 

 have this power of phosphorescence when in an alkaline medium. 



Fig. 150. Phosphorescence in Noctiluca miliaris Sur. [QUATKEFAGES.] 

 A portion of the body is represented with numerous scintillating dots. 



Amongst them are fat, lecithin, cholestearin, certain oils, grape sugar, 

 etc. Watas6 ('98) regarded the protrusion or secretion of such sub- 

 stances as due to protoplasmic contraction induced by any external 

 stimulus, such as a blow or shock of any kind. 



D. IRRITABILITY 



When, by reason of an external stimulus, any normal movement of 

 a protozodn is interrupted, or changed to another type of motion 

 (which may be definite or indefinite in regard to the source of the 

 stimulus), the change is brought about by protoplasmic reaction, 

 which is expressed by the general term irritability. 



The effect of heat upon the activities of Protozoa was early recog- 

 nized by the students of the group. Spallanzani, for example, noted 

 that increased warmth brought about increased activity in the con- 

 tractile vacuole, as well as in movements of the animal. Subsequent 

 observers have established the fact that movement is possible only 

 within certain temperatures, and, further, that this limit in either 

 direction varies with the organism considered. Between such limits, 

 the amoeboid movements of Rhizopoda and the vibratile movements 

 of Flagellidia and Ciliata increase to a certain optimum, which again 

 varies with the organism, but after this optimum is passed, movement 

 gradually decreases until, at the point of maximum temperature, heat 

 rigor sets in ; and this temperature, if passed, results in death. On 

 the other hand, with cooling, the movements become slower and 

 slower, until a minimum temperature is reached and all motion ceases, 



