THE president's ADDRESS. 9 



very probable, however, that the spirochaetes found infe.^ting 

 certain bivalve molluscs, such as Spirochaeta halbianii of the 

 oyster, and aS'. anodontae of the pond-mussel, swim out of one 

 mollusc and into another. Active migration of this kind, how- 

 ever, is very rare amongst entozoic parasites, if it occurs at all. 

 In the first place the conditions of life within a living body, in 

 the midst of organic fluids, are so different from those in the open 

 water, whether salt or fresh, that it is hardly to be expected that 

 a delicate unicellular organism adapted to the one mode of life 

 could stand the sudden change to the other. In the second place, 

 it is clear that active migration of parasitic Protozoa could only 

 be effected when the host is an aquatic animal, and not w^hen it 

 is a terrestrial organism. The only instances of active migration 

 of entozoic parasites known with certainty are those in which the 

 parasite can penetrate a mucous membrane, and is thus able to pass 

 from one host to another when two such surfaces are in contact. 

 In this Avay the trypanosome of dourine in horses {T. equvperdum) 

 passes from one host to another during coitus, and the trans- 

 mission of the parasite of syphilis is another instance. 



Speaking generally, and excluding for the moment those cases 

 in which the transmission is brought about by means of an 

 intermediary host, the propagative phases of Protozoan parasites 

 take the form of inactive, resting stages in which the body of the 

 parasite is protected against adv^erse external conditions by a tough 

 protective membrane. In the form of resistant cysts or spores the 

 parasites in a dormant state brave the rigours of the external 

 world, and are able to withstand the heat and drought of summer, 

 the cold and frost of winter. Like seeds they offer an inert 

 resistance to the elements and are disseminated passively, and 

 like seeds they germinate when they reach a suitable soil, but 

 not till then. 



Let us consider now the manner in which the first step in the 

 propagation is effected, that is to say, how the propagative phases 

 leave the body of the host. 



Many, perhaps the majority of Protozoan parasites occupy 

 positions in the body of the host whence the propagative phases 

 can pass without difiiculty to the exterior. This is the case when 



