104 MICROBES AND TOXINS 



surface of the bodies of fishes can no longer live even in the 

 same water when it becomes detached and floats free. The 

 infusoria from the paunch of ruminants or the caecum of 

 horses cannot live outside the bodies of these animals except 

 at body temperature, 37C. : they are thus examples of 

 semi-parasitism. The parasites of mammalian blood are 

 habituated or even confined to life at definite temperatures ; 

 hence the effects of climate and season. 



The same parasites attach themselves to one host only and 

 their presence becomes a specific character of the latter. The 

 parasite of malaria is peculiar to man and among the mosquitoes 

 it can only inhabit those of the genus Anopheles. But one 

 trypanosome can live in several species of host, one species 

 often serving as a sort of reservoir for others (it is probable, for 

 example, that cattle form a reservoir for the Trypanosoma 

 gambiense, which causes sleeping sickness in man). 



Lamblia intestinalis is a parasite of the small intestine : 

 gregarines are only found in the large intestine, in the 

 peritoneum, and in the genital organs of their hosts (inverte- 

 brates) : coccidia inhabit the epithelial cells : the haemosporidia 

 of malaria only the red corpuscles of the blood : while the 

 sarcosporidia only occupy the muscle cells. But parasites 

 exist which infect all the organs of the host, e.g., Myxobolus 

 pfeifferi in the barbel disease. 



Chemiotactic phenomena, positive or negative, are observed 

 among protozoa as among bacteria, and it is by an action of 

 this sort or by a choice of soil (which closely resembles it) 

 that the affinity of the sporozoites of the malarial haemo- 

 sporidia is explained for the salivary glands of the mosquito 

 which inoculates man. The hgemosporidia sucked from the 

 blood of the patient gain the stomach of the mosquito and 

 there enter upon the sexual cycle, a cycle which cannot go on 

 in any other surroundings : this specific action is no doubt due 

 to certain physical and chemical conditions which are only 

 realised there and of which the following fact may give some 

 idea : the appearance of sexual forms in human blood is 

 favoured by adding a little distilled water to the blood of 



