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The last class, the Sporozoa, or the spore-forming animals, while small in the number of known species, 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.
They may dry up or freeze or lie for a long time in the ground or water until the time comes when they are introduced into another host. The class Sporozoa is divided into five small groups or orders. These are dependent on two hosts for their existence, the sexual generation usually occuring in an insect or other invertebrate and the asexual generation in some vertebrate.
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.
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.
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