230 PRACTICAL ZOOLOGY 



trolling these germ carriers have been described in Chapters 

 VIII, IX, X, and XIII. The control of the other diseases is 

 largely a matter of care and cleanliness. 



Protozoan Parasites of Domestic Animals. — The most im- 

 portant disease of domestic animals caused by a protozoon in this 

 country is the Texas fever of cattle, described in Chapter XIII 

 (p. 121). Besides this parasite there are a number of different 

 species of trypanosomes, similar to that which causes sleeping 

 sickness in man, that produce diseases in cattle, horses, and 

 other domestic animals in various parts of the world. 



The silkworm disease, pebrine, which appeared in the south 

 of France at about the middle of the nineteenth century is 

 'especially interesting because of the fact that Pasteur's studies 

 revealed its cause and devised methods for its control, thus pav- 

 ing the way for the control of infectious diseases in man. Pas- 

 teur advised the silkworm raisers to destroy all diseased cater- 

 pillars and eggs and to raise silkworms only from eggs that were 

 free from parasites. This advice was followed and resulted in 

 freeing the silk industry in France from the disease. The para- 

 site is known as Nosema bombycis. 



Protozoa in Drinking Water. — Our drinking water comes 

 from three main sources. Rain water is pure except for the 

 organisms that the rain drops gather in the air. Ground water, 

 which comes from springs, wells, and infiltration basins, is usually 

 free from Protozoa. Surface water from streams, lakes, ponds, 

 and reservoirs contains more Protozoa than either rain water or 

 ground water, and standing water is, as a rule, more crowded 

 with them than running water. 



The organisms that float passively in the water without seek- 

 ing the shore or bottom constitute a group known as plankton. 

 An examination of the waters of a typical river (the Illinois 

 River) revealed 528 different species of plankton. These were 

 mostly plants (algae), Protozoa, wheel animalcules, and Ento- 

 mostraca (see p. 144). One hundred and eighty-five of the 

 528 species recorded were Protozoa, and every cubic meter of 



