MORPHOLOGY AND TAXONOMY OF THE SPOROZOA 537 



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others move steadily in one direction, a characteristic mode of pro- 

 gression which has given rise to the term gregariniform movement. 

 Motile forms are limited to the free types in the digestive tract 

 or body cavity. Quiescent forms are usually attached to some epi- 

 thelial cell by the epimerite. 



The life history varies from a relatively simple and uncomplicated 

 progression from sporozoite to sporozoite to a complex alternation 

 of generations involving different hosts. 

 The simpler histories are found in the 

 Eugregarmida such as Monoeystis species 

 or in Lankesteria ascidiae (Fig. 213). The 

 latter is a parasite of the digestive tract 

 of the ascidian Ciona intestinalis which 

 becomes infected by eating contaminated 

 food. The sporozoites are liberated from 

 the sporocysts and enter epithelial cells 

 where they develop into gamonts. The 

 adult forms are free in the lumen of the 

 gut and are characterized by the possession 

 of a peculiar pseudopodium-like knob which 

 is regarded as a tactile organ. Two of these 

 adults which show evidence of sexual differ- 

 ences (Fig. 214) come together in "pseudo- 

 conjugation." A delicate membrane — 

 gametocyst — is formed and within this 

 membrane each of the individuals forms 

 a large number of gametes. From the 

 great nucleus a smaller nucleus is formed 

 and this divides repeatedly, its products 

 passing to the periphery where small buds, 

 each containing a nucleus, are pinched off 

 as gametes. A gamete from one individual 

 meets and fuses with a gamete from the 

 other. A fertilization membrane is formed 

 which becomes the capsule of the sporo- 

 blast. The synkaryon divides three times 

 and eight daughter nuclei are formed which 

 become the nuclei of eight sporozoites. In 

 each sporocyst, therefore, there is a possi- 

 bility of as many zygotes and sporoblasts 

 as there are gametes formed by one of the original gregarines. The 

 parasites are passed out of the intestine with the feces and further 

 development is inhibited until the sporoblasts are eaten by another 

 host. 



A more complex, but still simple, life history involves a change 

 of hosts. The genus Porospora appears to be represented by several 



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Fie;. 214. — Nina gracilis 

 in pseudo-c onjug a t i o n, 

 above male, below female 

 roll. Lipoids (gray) and fats 

 (Mack) are more abundant 

 in the female than in the 

 male. X 500. (After Joyet- 

 Lavergne, Arch. d'Anatomie 

 Microscopique, courtesy of 

 Masson el < !ie. I 



