SPOROZOA, a group of protozoa, ranked as a class or more recently as a phylum, which is composed exclusively of parasitic species. They are characterized by complete absence of structures common in free-living protozoa, viz. (1) locomotor processes, pseudopodia, flagella and cilia, (2) openings for taking in solid food or ejecting waste, so that nutrition is purely osmotic, and (3) contractile vacuoles. The life cycle always includes a period in which a large number of spores is produced, and usually two such periods alternate; in one spores are produced asexually by schizogony and in the other by a so-called sexual process (sporogony). These two occur in different organs if not in different hosts so that alternation of genera tions is regularly associated with alternation of hosts, The development is often extraor dinarily complex and no decision as to position or relationships can be reached until the life history is worked out. As this has been done in but few cases the classification and descrip tion of the group is subject to constant and marked changes. The sexual phase which may involve isogametes or heterogametes is re garded as more significant than the asexual in deciding relationships.
The class Sporozoa, as originally conceived by Leuckart (1879), embraced not only the Gregarinida, Coccidioidea, and Hmmosporidia which later Schaudinn grouped together as Telosporidia, i.e., those forms producing spores at the close of a stage in the life cycle; but also the Neosporidia which produce spores con tinuously. The latter which have arnceboid spores and one or more polar capsules, are now grouped in a separate class, the Cnidosporidia, by many. They include the orders (ha, Microsporidia, Actinomyxidia, Sarcospori dia and Haplosporidia.
The Gregarinida are chiefly arthropod parasites. They begin the asexual stage as intracellular parasites in the intestinal wall and desert the cells to become adult in the in testine of the host. They are elongate and the cell is often divided into two regions by a cross partition. The gametes are similar or , dis similar.
The Coccidioidea parasitize epithelial cells of vertebrates particularly in the alimentary canal and its glands. In form they are spherical or nearly so and without differentiated ecto- and endoplasm. The host cell shrinks as the para site grows until it is reduced to an empty cyst membrane within which the parasite breaks up into a number of crescentic spores surrounding a central residual miss. By rupture of the membrane these nierozoites are scattered to infect new cells. At times some stimulus, per haps excessive infection, leads to the produc tion of micro- and macrogametes which fuse in pairs to form sporoblasts and these encysting became sporocysts in which are produced sporo zoites. The latter are the agents by which new hosts are infected and their production takes place usually outside the host. The Coc cidioidea cause serious epidemics in rabbits, hares and other animals; they occur in many hosts, even including man.
The Hmmosporidia are clearly coccidia which have been adapted for life in the blood. The asexual stage is amceboid in the red blood corpuscles where merozoites are produced that infect other corpuscles, repeating the process until gametes are formed that await a change of host to find opportunity for further develop ment. This is given by withdrawal of the blood into the stomach of some biting insect where the gametes complete their development and fuse. The sporoblasts and in them the sporozoites are formed in the wall of the in sect's stomach and when the sporozoites are set free they penetrate the salivary gland and proboscis by which in the act of biting they are introduced into a new host. (For further details and illustration see article PLASMODIUM). The most famous members of this group are the organisms producing malarial fevers in man of which there are at least three distinct types. Other species occur in birds and re lated genera in Amphibia, fishes and reptiles. The mosquito (q.v.) is the inoculating agent for malarial fevers, and since it shelters the sexual generation must be considered the primary host whereas man is the secondary. A leech is believed to play a similar role for certain species parasitic in fish. Probably the most widely known disease among domestic animals which is caused by a member of this group is Texas fever in cattle, transmitted by the cattle ticks. See RINDERPEST.
The Myxosporidia are practically exclusively fish parasites. Certain forms are common in the gills, others in the skin, where they form conspicuous masses. Less well-known forms inhabit the gall bladder or form ulcers in the muscles and are the cause of serious diseases. The Myxosporidia possess polar capsules which resemble much the nettling cells of Ccelenterata.
The Microsporidia are not well known despite the fact that they are of conspicuous economic importance since one species is the cause of Nosema, a serious disease of the honey bee and another produces pebrine of the silkworm; the latter was the subject of early important studies by Louis Pasteur.
The Sarcosporidia are found parasitic in the muscle cells of vertebrates and are suf ficiently large to be conspicuous to the naked eye. They were originally designated Miescher's or Rainey's corpuscles and occur occasionally in man. A serious Australian disease of sheep known as is caused by one of these forms; it is also found in North America. By dissecting Sarcosporidia out of host tissue and subjecting them to chemical study a protozoan toxin named Sarcosporidin has been isolated; this is the first discovered in this group.
Calkins, G. N.,