298 SOME MINUTE ANIMAL PARASITES 



may be said to have inherited the acquired character 

 of lack of blepharoplasts, and to be able to perpetuate 

 the same, though the excitant cause of the peculiarity, 

 the action of some drug, e.g., pyronin, may have been 

 removed. A natural parallel occurs in Trypanosoma 

 equinum, of " mal de caderas" in horses, in which 

 parasite the blepharoplast is either absent or very 

 minute. 



The degree of virulence of parasitic Protozoa 

 towards their hosts varies. If a strain of deadly 

 parasites remains deadly continuously, its very vio- 

 lence may possibly pave the way for its own destruc- 

 tion. The lack of hosts may be sufficient to cause it 

 to disappear. Alternatively, as with the more virulent 

 form of sleeping sickness, parasites are maintained 

 in reservoirs such as wild animals that tolerate the 

 trypanosomes, and are none the worse for their 

 presence. Some trypanosomes when inoculated into 

 a succession of rats become weaker in their virulence 

 as time goes on, but a passage to a guinea-pig and 

 then again to rats has restored their lethal power. 

 New problems -are thus ever being presented to the 

 parasitologist. 



Among higher animals a certain amount of light 

 seems essential to their well-being, but the natural 

 condition of most parasitic organisms seems to be 

 one of darkness. The body cavity in which a 

 Gregarine lives, the caecal tissue harbouring coccidia, 

 the blood containing trypanosomes, the gut of an 

 insect with its parasitic flagellates, all are relatively 

 opaque and dark. Bright light is an annoyance to 

 many parasites; and though continuous exposure 



