THE GENERAL PHYSIOLOGY OF THE PROTOZOA 205 



(6) Phototaxis and Effects of Light and Other Rays. Many Pro- 

 tozoa appear quite indifferent to light at least of ordinary intensity ; 

 others show a very decided reaction, as already mentioned, either 

 negative or positive. Thus many amoebae, Pdomyxa, etc., are 

 negatively phototactic, and pass at once into a condition of rest 

 and inactivity when exposed to light. According to Mast (176), a 

 sudden increase in the intensity of the illumination inhibits move- 

 ment in Amoeba proteits ; but if the illumination remains constant,, 

 movement begins again in a few moments. 

 If the illumination is very gradually in- 

 creased, it produces no response. In 

 strong light Amoeba proteus orientates 

 itself, producing pseudopodia only on the 

 less illuminated side. 



Many flagellates, on the other hand, 

 especially the holophytic forms such as 

 Dinoflagellates, Phytornastigina, Eugle- 

 noids, etc., show the opposite reaction, 

 moving towards the light or becoming 

 active when exposed to it, and passing 

 into a resting state in the dark. The 

 positive phototaxis of the holophytic FIG. 84. 

 Protozoa has an obvious bionomical sig- 

 nificance, since the holophytic nutrition 

 can only proceed in the presence of light. 



A, Anterior end of 

 Euglena viridis. ft., Flagel- 

 lum ; ces., oesophagus ; bl., 

 thickening (blepharoplast ?) 

 on one of the two roots of the 

 flagellum ; st., stigma ; rh, 

 the two roots of the flagellum 

 passing through the reservoir 

 (R) of the contractile vacuoles, 

 two to be attached to its 

 opposite side. B, Stigma in 

 surface view, highly magni- 

 fied, showing the pigment- 

 grains imbedded in a proto- 

 plasmic basis. After Wager 

 (213). 



In the majority of holophytic flagellates the 

 phototactic reaction is associated with the 

 possession of a special organ, the stigma or 

 "eye-spot." The stigma of Euglena consists 

 of a protoplasmic ground-substance forming a 

 fine network, in which is embedded pigment 

 in the form of drop-like bodies. The pigment 

 granules are brightly refractile, with a distinct 

 outline, and form a single layer. In some cases 

 the granules are spherical and all of the same 



size ; in others they are more irregular in form and of different sizes. The 

 pigment appears to be a derivative of chlorophyll. The stigma is in close 

 contact with a well-marked thickening on one of the two branches into 

 which the flagellum bifurcates at its base. Wager (213) suggests that this 

 thickening (blepharoplast ?) is a specialized sensitive organ which is stimulated 

 by the light-absorbing pigment-spot, the stigma, and that in this way the 

 reaction of Euglena to light is determined. Euglena swims towards a moderate 

 light, but away from strong sunlight. If kept in bright sunlight it comes to 

 rest, rounds itself off, and ultimately becomes encysted. 



The blue and violet parts of the spectrum exert the strongest stimulus 

 on flagellates. In the case of Amoeba profess. Mast (176) found the blue rays 

 nearly as efficient as white light in causing reactions, but violet, green, yellow, 

 and red, to be but slightly active. Paramecium and some other Protozoa are 

 stated to react only to the ultra-violet rays. 



The effect of radium-rays upon various Protozoa has been investigated 

 by Zuelzer (221). Some species are more affected by them than others; 



