INTRODUCTION. 11 



other the Mastigophora and their derivatives (Telosporidia, 

 Infusoria) by speciaHzation of the flagellar apparatus, 

 combined with the acquisition of a cortex and loss of amoeboid 

 movement. Hartmann (1923-5) and Fantham (1936) have 

 also discussed the philogeny of the Sporozoa. 



Taking the Telosporidia (^Sporozoa sensu stricto) first, 

 we are struck by the obvious points of difference between the 

 Gregarinida and the Coccidiomorpha. In the Grbgarinida 

 two sporonts become enveloped in a common cj^st before they 

 give rise to gametes, and the union of gametes takes place 

 within the cyst. The cyst contains many zygotes, and each 

 zygote gives rise to a single spore. In the Coccidiomorpha, 

 on the other hand, the gametocytes are more or less widely 

 separated from one another when producing the gametes ; the 

 female gametocyte remains undivided to form a single macro - 

 gamete, which is very much larger than the microgametes ; 

 and the zygote undergoes a process of division, giving rise to 

 a large number of sporoblasts and sporozoites. The common 

 ancestral type may be assumed to have been one in which 

 the trophozoites that grew into gametocytes were separated 

 from one another and produced their gametes separately, as 

 in CocciDiA ; but each gametocyte produced a number of 

 gametes which were more or less ahke, as in the Gregarinida. 



As the Gregarinida came to acquire an intercellular trophic 

 phase, the sporonts became free and motile, and it was thus 

 possible for the gametocytes to associate and encyst together, 

 producing their gametes in close proximity. There would 

 be neither any difficulty for the male gametes to find the 

 female, nor any need for increased speciaHzation of the gametes. 

 They would lose even the slight degree of speciaHzation in- 

 herited from the ancestral form, with the result that they 

 would be similar and would be produced in equal numbers by 

 the two gametocytes. 



The CocciDiA, on the other hand, retained an intracellular 

 habitat for their trophozoites, and the gametocytes were 

 widely separated when producing the gametes. The gametes 

 thus had to seek each other, and this led to greater speciaHza- 

 tion. The male gametes became very small and very motile, 

 and were produced in large numbers ; the female gametocyte 

 no longer divided into a number of gametes, but became a single 

 macrogamete. After fertilization the suppressed divisions 

 of the female gametocyte would take place in the zygote, 

 leading to the production of a number of sporoblasts, spores, 

 and sporozoites. The spore may be regarded as comparable 

 to the encysted zygote of the Flagellata. In the suborder 

 Adelbidea, however, the gametocytes acquired the habit of 

 association prior to gamete formation, but this led merely 

 to a reduction in the number of male gametes produced. 



