282 ZOOLOGY 



insect bites a health}' person, the latter becomes inoculated 

 with the disease. Thus biting insects are disseminators of 

 disease. 



Protozoa were unknown to man until the latter half of the 

 seventeenth century, when a Dutch naturalist named Lcou- 

 wenhoek, by means of the newly invented compound micro- 

 scope, studied and described several kinds which he had found 

 in stanthng water and called animalcula or water insects. 

 Later, these creatures were called Infusoria because they de- 

 velop in infusions of organic matter in water. As the micro- 

 scope became perfected, progress was made in the study of 

 these organisms, but less than a century ago several eminent 

 zoologists maintained that the Infusoria possessed digestive, 

 nervous, circulatory, and reproductive organs. The proper 

 structure of Protozoa has been generally recognized only 

 within the last sixty years. 



The ideas formerly entertained concerning the origin of 

 Protozoa were as erroneous as those relating to their internal 

 structure. These erroneous ideas were a tradition from a time 

 when even scientific men held that many of the larger animals, 

 such as eels, bees, and flies, were generated without parents. 

 This was the theorj^ of " spontaneous generation." In time 

 this theory became much more restricted. It was found that 

 the maggots in putrid meat are not generated " spontaneously" 

 out of the meat, but are derived from flies' eggs, and, in their 

 turn, develop into fertile flies. But the idea that " Infusoria " 

 arc formed out of the organic material of infusions continued 

 to be held until much more recently, — until Pasteur, '^ Tyndall, 

 and others ])roved that fluids heated to a sufficiently high tem- 

 perature for a long enough time, and then, while hot, sealed 



' Fig. 269. 



