Sporozoa differ greatly as regards the effects which they produce upon their hosts.
The recognition of this fact is expressed, at the present day, by the division of the Sporozoa into several well-defined orders, which are grouped in two main divisions, each containing more or less closely related forms. One of these groups consists of the Gregarines, Coccidia and Haer_aosporidia (qq.v.).
The nomenclature assigned to these two principal divisions of the Sporozoa by different writers has varied according to the particular character on which they have primarily based the arrangement.
In other words, under the heading Sporozoa, as at present used, are included two entirely independent series of Protozoan parasites; the general resemblances which these exhibit are due to convergence brought about by their specialized mode of life.