SPOROZOA, GREGARINIDA 527 



animals to complete their development, there occur naked sporo- 

 zoites instead of spores. 



The infection of a new host begins with the entrance of mature 

 spores through mouth, or with the introduction of the sporozoites 

 by blood-sucking invertebrates directly into the blood stream. The 

 sporozoites enter specific host cells and there grow at the expense of 

 the latter. In the Coccidia and the Haemosporidia, the trophozoite 

 continues its intracellular existence, but in the Gregarinida it leaves 

 the host cell and grows in an organ cavity. Except Eugregarinina, 

 the vegetative form undergoes schizogony and produces a large 

 number of daughter individuals which invade new host cells, thus 

 spreading the infection within the host body. The trophozoites fi- 

 nally develop into gametocytes. In the Coccidia and the Haemospo- 

 ridia, anisogametes are, as a rule, produced. Each macrogametocyte 

 develops into a single macrogamete and each microgametocyte, 

 into several microgametes. Fusion of the gametes in pairs results in 

 formation of a large number of zygotes, each of which develops either 

 into one to many spores or into a number of naked sporozoites. In 

 the Gregarinida, two fully mature trophozoites (or gametocytes) 

 encyst together and the nucleus in each multiplies repeatedly to 

 form numerous gametes, which fuse in pairs with those produced in 

 the other individual within the common envelope. The zygotes de- 

 velop into spores, each containing variable number of sporozoites. 

 When these spores enter a new host, the changes outlined above are 

 repeated. The Telosporidia are parasitic in vertebrates and higher 

 invertebrates. 



Three orders are distinguished in this subclass: 



Mature trophozoite extracellular, large; zygote not motile; sporozoites 

 enveloped Order 1 Gregarinida 



Mature trophozoite intracellular, small 



Zygote not motile; sporozoites enveloped. . .Order 2 Coccidia (p. 570) 

 Zygote motile; sporozoites naked. . . .Order 3 Haemosphoridia (p. 599) 



Order 1 Gregarinida Lankester 



The gregarines are chiefly coelozoic parasites in invertebrates, 

 especially arthropods and annelids. They obtain their nourishment 

 from the host organ-cavity through osmosis. The vast majority of 

 gregarines do not undergo schizogony and an increase in number is 

 carried on solely by sporogony. In a small group, however, schizog- 

 ony takes place and this is used as the basis for grouping these 

 protozoans into two suborders as follows: 



No schizogony Suborder 1 Eugregarinina (p. 528) 



Schizogony occurs Suborder 2 Schizogregarinina (p. 560) 



