li'l;i] on Blood-Parasites 745 



of some blood-sucking iuvurtebrate, such as a fly, mosquito or tick, 

 which transfers the parasite to another animal whilst feeding from 

 him. It was thought formerly that blood-parasites would be a 

 restricted order, but the work of recent years has shown that they 

 have an enormous distribution both geographically and as regards 

 their hosts. For instance, during the last five years I have had the 

 opportunity of examining all the animals (in the large sense of the 

 word) which have died in the Zoological Gardens. I have examined 

 the l)lood of over 8000 animals, coming from all parts of the 

 world, and I have found parasites in the blood of 587 of them, 

 that is in about 7 per cent, and in 295 species of animals I have 

 found them for the first time. I mention this just to give you some 

 numerical idea of their occurrence and distribution. 



It will be better to take first those parasites which live in the 

 plasma, and then those that live in the corpuscles, rather than to 

 attempt to take them in their at present rather uncertain biological 

 order : and I will begin at the bottom, biologically speaking, that is 

 with the bacteria which are plants. These only require mention, 

 since they do not live in the blood as parasites proper, but only as 

 accidental parasites — -that is, parasitism is not necessary to their life- 

 cycle ; they get into the blood in the later, or in certain, stages of 

 certain diseases. 



An example is the blood of a Senegal turtle dove which died in 

 twenty-six hours from fowl cholera. This bacillus was discovered 

 by Pasteur, and is interesting, as it was his work upon it which led 

 to his discovery of the attenuation of a virus, and of its trans- 

 formation thereby into a protective vaccine. 



The first parasites proper I shall mention are the Spirochetes. 

 These have at present rather an insecure position in our idea of 

 Nature ; they were formerly classed close to the bacteria, but now 

 they are placed tentatively among animals, and they are not yet quite 

 sure of their place. But they, nevertheless, although insecure of 

 their place in the books, produce grave diseases, such as relapsing 

 fever, tick fever of man, the spirochetoses of horses, oxen and birds, 

 syphilis, and yaw^s. They, with the exception of the last two, are 

 carried by, and developed in, ticks and bugs ; and in tick fever the 

 parasite is also found in the nymph form of the tick, and this is one 

 of the rare instances of heredity of a parasite. 



The spirochete of relapsing fever in man was discovered by 

 Obermeier in 1868, and he died from inoculating himself with the 

 blood of a patient with the disease. He was one of the first scientific 

 martyrs ; he established our knowledge of the cause of this disease 

 at the expense of his own hfe. 



We will now take a long jump to the Filarise. These are 

 nematode worms, the embryo forms of which live in the blood ; the 

 parent forms, being too large to get through the capillaries, Hve 

 in many other parts of the body. The larval form lives in the 

 body of some invertebrate — in a few known cases in a mosquito, or 



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