296 SOME MINUTE ANIMAL PARASITES 



The adaptation of a parasite to different but nearly 

 allied hosts, without change of form occurring, is 

 used in the study of blood parasites such as Spiro- 

 chaetes and Trypanosomes. Thus it is known that 

 animals such as mice, rats, and guinea-pigs can 

 harbour flagellates or Spirochaetes in their blood 

 without causing change in the organism concerned. 

 The possibilities of the cure of human complaints by 

 the action of drugs can thus be tested, and already 

 much alleviation of human suffering has resulted from 

 such researches. At the same time, the parasit- 

 ologist cannot but remember the enormous powers 

 of adaptation shown by Spirochaetes capable of living 

 in the blood of man, monkeys, rats, mice, and guinea- 

 pigs, and finally in the gut, body cavity and Mal- 

 pighian tubules of the agent of transference a tick or 

 a louse. Due regard must be paid to morphological 

 variation, in order to avoid errors of interpretation. 



Change of host can occur quite naturally among 

 the Spirochaetes of molluscs. The oyster and the 

 edible Tapes " clovisses " live together in the 

 Mediterranean, .and Spirochaetes from the one can 

 pass into and live in the crystalline style of the other. 

 The style of the oyster is far less firm than that of 

 the Tapes t and as a result, Spiroch&ta balbianii adapts 

 itself in the Tapes, and divides more frequently, so 

 that somewhat thinner forms are produced than is 

 usual in the organism in the oyster. Morphological 

 variation is thus one result of difference of host. 



All parasites are far from being so complaisant. 

 Trypanosoma lewisi lives in the blood of rats. When 

 this organism is transferred to a snake ? it seems 



