ANIMAL PARASITISM 



441 



and is usually transmitted by kissing, and the two intestinal species 

 E. coli (nonpathogenic) and E. histolytica. The latter species breaks 

 down the cells of the intestinal lining by means of enzymes which it 

 secretes, and then ingests the broken cells in the same way that the 

 common free-living amoebae take in their food. The disease caused 

 by E. histolytica is kno^vn as ''amoebic dysentery." Infection occurs 

 as the result of eating food or drinking water which has been con- 

 taminated by the feces of infected people, as in the case of the 

 Chicago hotels where contamination of drinking water by water 

 siphoned up from the toilet drains into the water pipes caused a 

 serious outbreak in 1933. 



Fig. 186. — Endamoeba coli. A, stained vegetative amoeba; Bj cyst with eight 

 nuclei ; n., nucleus, showing coarse peripheral chromatin granules, chromatin 

 granules in "clear zone" between periphery and karyosome which is eccentric in 

 position; chr.b., remnant of chromatoid body. Numerous food vacuoles in vegeta- 

 tive form. (Reprinted by permission from Introduction to Human Parasitology by 

 Chandler, John Wiley and Sons, Inc., after Dobell. ) 



The malaria parasites, of which there are three species infecting 

 man (Plasmodium vivax, P. falciparum, and P. malariae, each caus- 

 ing a different form of malaria), are Protozoa belonging to the class 

 Sporozoa, and are very highly modified for parasitic life. The adapta- 

 tions for parasitism and for transmission from host to host involve 

 a very complex life cycle. The two main phases of the life cycle are 

 the vegetative or schizont stage (merozoites) and the sexually repro- 

 ductive or sporont stage. The biology of this parasite has been dis- 

 cussed in the earlier chapter on Protozoa under class Sporozoa and 

 Economic Relations of Protozoa. 



