284 SOME MINUTE ANIMAL PARASITES 



puscles, while at the edge of the structure there are 

 numbers of immature spores. 



Though the Sarcosporidia have been known for 

 many years, their life-history is still incompletely 

 understood. The means whereby their hosts became 

 infected has not been fully shown in any one case. 

 More is known about the means of distribution of 

 Sarcocystis muris than of any other Sarcosporidian. 

 The first fact established was that healthy mice, by 

 eating the flesh of diseased ones, could become in- 

 fected experimentally after a long incubation period. 

 Also, guinea-pigs similarly harboured the parasite 

 after their food had been contaminated with spores 

 from the muscles of mice. Cannibalism, then, may 

 be responsible for the spread of S. muris among mice, 

 at any rate to some extent. Mice fed on S. tenella from 

 the sheep also became infected with this parasite. 

 Again, if the Sarcosporidian was forced to change its 

 host for example, from mouse to guinea-pig the 

 parasite underwent considerable change in its appear- 

 ance, so that, had not the circumstances been known, 

 it would probably have been considered to be a new 

 species. 



An advance was made when Negre fed mice with 

 food contaminated with excrement from their in- 

 fected fellows, and found that they became infected 

 also. Even if the faeces were kept sixty days, they 

 still retained the power of infection. This suggests 

 a means whereby human infection has occurred. 

 Accidental and undetected contamination of food by 

 mice may have caused the ingestion of the spores by 

 persons using the food, and so brought about their 



