PARASITISM AND COLOUR 299 



to strong light results commonly in a certain amount 

 of toleration for it, yet the shelter of debris is usually 

 sought by parasitic Protozoa, which burrow into it 

 in an endeavour to reach the desired darkness. Green 

 light is less harmful to many flagellates than is white 

 light, and they live longer therein. Violet light, on 

 the contrary, stimulates them to intense activity, 

 resulting in rapid death from exhaustion. The action 

 of red light varies considerably, in some cases ac- 

 celerating movements, in others slowing them, while 

 blue light seems neutral. 



The colour of Protozoa in relation to parasitism 

 is a subject concerning which very little is known. 

 Many of the Gregarines and allied parasites are 

 frequently opaque and densely granular. In our 

 investigations of Gregarines from the Scolopendra, 

 it was easy to pick out the dense white, opaque 

 parasites from the gut without using any lens. 

 Living Trypanosomes and Haemogregarines often 

 show a faint bluish tint, and it has been suggested 

 that this is possibly due to a dilute pigment of an 

 unknown nature. Green coloration, except among 

 the Algal flagellates, is comparatively uncommon, 

 since the absence of light often precludes its formation. 

 Interesting pigment-grains occur in some of the 

 Myxosporidia in the gall-bladders of fish, the grains 

 varying in colour from green to brown. Sometimes 

 they form clusters, at others they lie singly in the 

 cytoplasm. It is probable that they consist of 

 altered bile pigment. No obvious value seems to be 

 attached to colour production among the parasitic 

 Protozoa. 



