170 COMPARATIVE ANATOMY 



ensure the perpetuation of the species, for however numerous 

 the gregarines contained in a single earthworm, and however 

 numerous the spores produced by them, the chances of any 

 given spore surviving to be swallowed by another host of 

 the right species are, to say the least of it, precarious. Thus, 

 though it is usual to describe the life cycle of Monocystis as 

 monogenetic i.e. as consisting of a single cell generation a 

 more careful analysis shows that it is in fact digenetic. There 

 is clearly an alternation of a sexual generation the gameto- 

 cytes with a generation that multiplies itself asexually the 

 zygote or sporoplasm. It is the offspring of this second 

 generation that grows up into the sexual form. There are, 

 however, some gregarines in which the zygote does not 

 multiply within the sporocyst, but simply becomes a single 

 sporozoite. Such forms are truly monogenetic. But attention 

 must be directed to the fact that the sporozoite of Monocystis, 

 when once it has found a lodgment in the locality appropriate 

 to it, leads a purely vegetative existence and grows up into the 

 gametocyte without multiplying itself asexually and causing 

 re-infection of other tissue cells of its host. Since only a 

 limited number of cells are attacked at one time Monocystis 

 and its allies are harmless parasites and do not cause the 

 death of or even seriously inconvenience the animals they 

 infest. The various members of the order Gregarinida, of 

 which Monocystis is an example, are parasitic in all kinds of 

 invertebrate animals, but not in vertebrates. They are mostly 

 innocuous, and with the rarest exceptions the whole life cycle 

 of any given species is completed within a single host. The 

 resistant spores produced by them are adapted for dissemina- 

 tion and the casual infection of new hosts. The spores 

 either pass out of the host's body by natural channels, and are 

 scattered about to be swallowed accidentally by another 

 animal of the same species, or if swallowed by an animal of 

 a different species they pass unaltered through its digestive 

 tract and do not undergo any developmental changes within 

 it. In this case the second animal is only an agent in dis- 

 semination, is not itself infected, and does not serve as a 

 carrier of infection. But many other Sporozoa, particularly 

 those infesting vertebrate animals, have a most pernicious 

 effect upon their hosts. The trophozoites in such cases 

 multiply very rapidly by division and their numerous offspring 



