354 THE PROTOZOA 



sporocyst, and contains, in different species, from three to twenty-four 

 eporozoites. The various species of Agqregata appear to be specific to par- 

 ticular hosts, whether crabs or cephalopoda. 



If the Aggregatidoe, are coccidia-, they differ from other coccidia in having 

 an alternation of hosts, and in the absence of an ob'cyst formed round the 

 zygote. If, on the other hand, they are gregarines, they differ from all other 

 known gregarines (with the exception of the doubtful form Schaudmnella, 

 see p. 355, infra), not only in the alternation of hosts, but also in the fact 

 that the gametocytes remain separate and produce gametes without previous 

 association. If the view put forward by Moroff is the true one, they are to 

 be regarded rather as forms derived from the ancestral form of gregarines and 

 coccidia (see below), before the habit of association of gametocytes, so charac- 

 teristic of gregarines, had been acquired. 



Comparison of the Life-Cycles of Coccidia and Gregarines. It is seen that a 

 typical coccidian, such as Coccidium schubergi, differs from a typical gregarine 

 mainly in the following points : (1) The trophozoites are intracellular ; (2) the 

 gametocytes ar* more or less widely separated from one another at the time 

 they produce gametes ; (3) the female gametocyte does not divide into a 

 number of gametes, but remains undivided to form a single macrogamete, 

 disproportionately large as compared with the male gametes ; (4) the zygote 

 undergoes a process of division, with the result that all the spores produced 

 within the cyst are the offspring of a single zygote, while in gregarines the 

 cyst contains many zygotes and each zygote gives rise to a single spore. 



When, however, the coccidia are considered as a whole, it is seen at once 

 that the first two points do not furnish absolute distinctions ; in Selenococ- 

 cidium the trophozoites are motile and extracellular, and in Addeidce the game- 

 tocytes associate together. There remains only the sporogony which stands 

 out as the distinctive feature of each group. It is by no means difficult to 

 understand, however, the manner in whicn the two types of sporogony, 

 different as they may appear, could have arisen from a common source. 



The common ancestral form, from which the two groups arose by divergent 

 evolution and adaptation to different modes of parasitism, may be supposed 

 to have been a parasitic organism in which the trophozoites that grew into 

 gametocytes were separated from one another, as in coccidia, and consequently, 

 when full-grown, produced their gametes separately ; and each gametocyte 

 produced a number of gametes which differed only slightly from one another, 

 as in gregarines. 



From such a form the coccidia arose by the acquisition of an intracellular 

 habitat on the part of the trophozoites, whereby the gametocytes remained 

 more or less widely separated when they produced gametes. As a result of 

 this condition the gametes have to seek each other out, and may easily miss 

 one another ; consequently there was a tendency to greater specialization of 

 the gametes. The male gametes became very small and very motile, and were 

 produced in large numbers. The female gametocyte, on the other hand, no 

 longer divided up into a number of gametes, but became a single large macro - 

 gamete. As soon, however, as fertilization is effected, the suppressed divisions 

 of the female gametocyte take place in the zygote, which divides into the 

 sporoblasts produced formerly by the division of the gametocyte. 



The gregarine-type, on the other hand, arose from the ancestral form by 

 the trophozoites which grow into sporonts being free and motile in the later 

 stages of their growth ; consequently, gametocytes of different sexes were able 

 to come together and produce their gametes in close proximity, and finally 

 to associate intimately and produce their gametes within a common cyst. 

 In such a condition it was impossible that the gametes should miss one 

 another ; consequently there was no tendency to increased specialization of 

 the gametes, but, on the contrary, a tendency for the gametes to lose even 

 the slight degree of specialization inherited from the ancestral form, with the 

 result that a more or less perfect isogamy was developed ; and instead of the 

 microgametes being produced in excess, the numbers of each kind of gamete 

 produced are approximately equal. 



