THE GREGARINES AND COCCIDIA 355 



It follows, from the course of evolution sketched in the foregoing paragraphs, 

 that in both gregarinea and coccidia the cyst is to be regarded as a secondary 

 acquisition. In the ancestral form there were simply scattered zygotes 

 from which the spore with its contained sporozoites arose ; the spore may, in 

 fact, be regarded as representing the primary form of the encysted parasite, 

 comparable to an encysted zygote of the Flagellata. It is indeed obvious 

 that the cyst of gregarines and coccidia respectively are quite different 

 things. In gregarines the cyst is formed round the two associated gameto- 

 cytes it is a " copularium," asLegerhas termed it ; in coccidia the cyst is 

 a protective membrane formed round the zygote, immediately after fertiliza- 

 tion. In the genus Legerdla among coccidia, however, the cyst is the sole 

 protective membrane formed to enclose the sporozoites, no sporocysts being 

 produced, a condition which is of interest, since it leads on to that found 

 in the Haemosporidia. 



In both coccidia and gregarines secondary departures from the primary 

 type of the life- cycle occur. In coccidia the gametocytes of certain forms 

 (Adeleidce) have acquired the habit of association prior to gamete-formation ; 

 this has not led, however, to a development in the direction of isogamy, as 

 in gregarines, but merely to a reduction in the number of male gametes formed. 

 In some gregarines, on the other hand, notably in those forms of " ccelomic " 

 habitat, or parasitic in the heemoccele, the sporonts in the later stages of growth 

 are inert and motionless ; this condition has led to neogamous association 

 of young sporonts while still motile and capable of coming together proprio 

 motu. 



Here mention must be made of the remarkable form Schaudinnella de- 

 scribed by Nusbaum (624), parasitic in the gut of an oligochaete worm. The 

 full-grown trophozoites of Schaudinnella are gregarine-like, and may be either 

 free in the lumen of the gut or attached to the epithelium by an epimerite ; 

 the body is non-septate. Temporary associations (syzygies) may be formed 

 which have nothing to do with sexual conjugation, since the associates part 

 again and produce gametes separately and independently. The full-grown 

 sporonts are distinguishable as male and female forms. The female sporonts 

 divide up into eight or ten spherical cells, the macrogametes. The male 

 sporonts divide up into a great number of minute spindle-shaped elements, 

 the microgametes. Copulation takes place between a microgamete and a 

 macrogamete. The zygote may become encysted and cast out with the 

 faeces, or may penetrate into the wall of the intestine. In the first manner 

 infection of new hosts is brought about ; in the second, multiplication of the 

 parasite in the same host. The zygotes in the wall of the intestine grow in 

 size, and divide each into a number of sporozoites. 



Some doubt may be felt as to whether the life-history of Schaudinnella has 

 been interpreted correctly throughout ; it is unusual for endogenous mult, 

 plication to be preceded by sexual processes, and the development requires 

 further examination. If, however, the account of the gamete-formation be 

 correct, Schaudinnella is a form which in this respect stands very near to the 

 hypothetical ancestral f o e rm of gregarines and coccidia. 



There can be no doubt that the gregarines and coccidia are closely allied 

 in every respect, and that the two groups are distinguished by points of 

 difference which can be referred quite simply to adaptation to slightly different 

 habits in their parasitic life. 



Bibliography. For references see p. 494. 



