THE group of animal parasites to which the malaria-causing organism belongs is relatively unimportant when compared with the bacteria, a group of plant parasites, including the causes of most zymotic diseases in man—typhoid, cholera, diphtheria, scarlet fever, tuberculosis and the like, as well as beneficial forms which aid man in various ways. Nevertheless, it is a group of considerable economic importance, about which little is known outside of scientific circles. The name Sporozoa suggests, to the average reader, no disquieting apprehension of physical pain or of financial loss, yet this class of primitive, unicellular animals includes, besides the malaria-causing blood parasites, forms which, like the silkworm parasite (Glugea bombycis), have cost communities untold millions of dollars. In connection with the losses due to one of these silkworm epidemics, Huxley writes in 18.0:
Analogous epidemics, which may be traced to Sporozoa, are liable to break out at any time among other animals having commercial value. Thus 'Texas fever,' a cattle disease due to a sporozoan blood parasite (Piroplasma bigeminum), occasions great loss to cattle breeders. Muscle parasites, belonging to the same class, cause trichinosis-like diseases in hogs, cows, cats, dogs and other domestic animals; while in fish they occasion great loss to fish-culturists through epidemics. Other parasites in the same class are the causes of disease in horses, sheep, goats, etc.
The Sporozoa are comparatively harmless to man personally, but, unlike some bacteria, they are never beneficial in any sense. Invariably parasites, the diseases which they induce are confined mainly to the lower animals, but so widely are they distributed that no type of animals is free from them altogether. One significant feature about the