294 SOME MINUTE ANIMAL PARASITES 



merozoites set free, and the parasite is in a position 

 to infect the mollusc. The chances of the successful 

 transference of the Aggregata from crab to mollusc 

 and vice versa, are fairly good, though the final 

 difficulty of transmission is that special crabs allow 

 of the development of Aggregata from particular mol- 

 luscs only, as a rule. It is likely that the enormous 

 numbers of merozoites in the crab and of spores in 

 the mollusc are direct adaptations to meet the 

 difficulty of transference. 



The effect of change of host on the life of a parasite 

 may not be very marked. The parasite itself shows 

 great adaptability to new conditions, and it is able to 

 cope with chemical differences of the medium around 

 it, differences of temperature and of pressure. The 

 threefold adaptation necessary is carried out by some 

 parasites without much change of form or manifest 

 alteration. Trypanosoma gambiense passes from the 

 blood of man into the gut of the tsetse fly Glossina 

 palpalis, and T. rhodesiense similarly into the digestive 

 tract of G. morsitans, whence both make their way to 

 the salivary glands. There is a great change from 

 the fluid environment of the living human blood to 

 the chemically altered and partly digested blood in 

 the stomach of the tsetse, and again to the salivary 

 glands of the fly. Accompanying this change, there 

 is a fall in temperature and also a difference in the 

 pressure exerted on the trypanosomes. But in the 

 intestine of the flies no markedly essential change of 

 form of the parasite is observed, the trypanosome 

 appearance being maintained, though changes in 

 size occur. However, a temporary crithidial stage 



