Bacteria and Protozoa 



23 



ties, only about three hundred kinds being known, 

 is extremely important. A number of diseases in 

 man and other animals are due to the presence of 

 these Sporozoans, for they are all parasitic. Few 

 if any animals are exempt from their attacks. 

 They even attack other minute Protozoa. One 

 hundred and fifty-seven species have been recorded 

 as attacking insects, one hundred species attack 

 birds, fifty-two reptiles, eighty crustaceans, twenty- 

 two fish, and so through the list. Ten have been 

 recorded as attacking man. In some instances 

 the parasite is always present in the host and some 

 hosts may harbor several different species of 

 Sporozoa. 



Very little work had been done on this group of 

 parasites prior to 1900. Since that time most of 

 the species that we now know have been discov- 

 ered, and within the last few years the life-histories 

 of many of these have been worked out quite com- 

 pletely. No other group of animals is being studied 

 more to-day by both the physicians and biologists. 



The Sporozoa vary greatly in appearance, 

 organization and life-history. They are so very 

 plastic that they can adapt themselves readily to 

 their various hosts, hence we have a great variety 

 of forms. But they all agree in certain characters; 

 all take their food and oxygen and carry on ex- 



