206 THE PROTOZOA 



Amoeba Umax, for example, was very resistant to the rays, while other Protozoa 

 were very soon injured by them. In all cases long exposure to the rays was 

 fatal. The first effect of the rays was generally to quicken the movements; 

 the next was an injurious action. The rays appear to act more particularly 

 upon the nucleus in the first instance, with subsequent gradual deleterious 

 effects upon the cytoplasm. 



In experiments on the effect of Rontgen-rays on Paramecium and Volvox 

 (Joseph and Prowazek, 169), these forms wore found to exhibit a negative 

 taxis, collecting in ten to fifteen minutes in a part not exposed to the rays. 

 Exposure of Paramecium to the rays caused the pulse of the contractile 

 vacuoles to become slower to a marked degree as a rule, but individual 

 variations were observed in this reaction, the effect being inconsiderable in 

 some cases ; and the animals gradually regain the normal pulse. Intro, vitam 

 staining of the nucleus of Paramecium exposed to the rays gave a result similar 

 to that obtained by staining Paramecia fatigued by being shaken evenly and 

 continuously for two hours. Long- continued action of the rays killed the 

 organisms. 



(c) Thermotaxis end Effect* of Temperature. For a given species 

 of the Protozoa there is an optimum temperature at which its vital 

 activity is at its highest pitch, and above which the activity is 

 diminished until ii reaches a point at which the vitality is impaired 

 and the animal is finally killed. A temperature, however, at which 

 the animal succumbs sooner or later may at first have a quickening 

 effect upon the "ital functions. Thus, many experiments have 

 shown that a rise of temperature increases greatly the rapidity and 

 frequency of the pulsations of the contractile vacuoles ; and in the 

 case of Glaucoma colpidium Degen (154) found that, although the 

 animal was killed by a temperature above 30 C., the maximum 

 frequency of the pulsations was produced temporarily by a tempera- 

 ture of 34 C., above which the frequency was rapidly diminished 

 (compare also Khainsky, 170-5). 



The optimum temperature may, however, be different at different 

 stages in the life-cycle, as in parasitic Protozoa which infest a warm- 

 blooded and a cold-blooded host alternately ; in such cases a change 

 of temperature may perhaps be a factor in bringing about develop- 

 mental changes. In free-living Protozoa the phases of the life-cycle 

 are often related to seasonal changes, and are probably induced 

 largely by conditions of temperature. 



Experimentally it has been shown that Protozoa tend to move towards 

 regions of more favourable temperature, and away from those less favourable. 

 Khainsky (170'5) found that rise of temperature produced a quickening of 

 the digestive processes in Paramecium, very marked at 24 C. or above. At 

 30 C. and above Paramecium takes up scarcely any more food ; the contents 

 of the food-vacuoles, which continue to be formed, then consist almost entirely 

 of water. 



The effects of temperature on the development in cultures are very marked. 

 Popoff studied the growth of Frontonia leucas in cultures kept at 14 C. and 

 25 C. respectively ; at the lower temperature the animals divide once in 

 about eighty or ninety hours, in the warmer culture once in about seventeen 

 hours ; in the cold both the nucleus and the body grow to a size absolutely 

 larger than in the warmth, but in the former case the nucleus is about -<*j, 

 in the latter about -J 4 , the bulk of the whole body (Hertwig, 92). In the case 



