THE SPOROZOA 163 



and there develops into the minute trophozoite which is formed 

 later within the sperm-morula. With this stage the life-cycle has 

 come round again to the point at which the description of it was 

 commenced. 



A few points with regard to the life-cycle require brief further dis- 

 cussion. It has been suggested that the spores may sometimes germinate 

 in the host in which they are formed, and so increase the numbers of the 

 parasite within it. But the improbability of this occurring is very 

 great, as was pointed out by Butschli, in view of the relatively small 

 number of parasites in the trophic stage which are met with, as com- 

 pared with the vast number of spores. Thus in a given earthworm there 

 will be found in the sperm -sacs perhaps a dozen trophozoites and as 

 many ripe cysts. Each of the latter contains, however, at a low esti- 

 mate about fifty spores, and each spore eight sporozoites. A single cyst 

 contains, therefore, about four hundred individuals, more or less, and if 

 it were a frequent occurrence for the spores to germinate in the same host, 

 the number of trophozoites in each earthworm might be expected to be 

 vastly greater than is usually the case. 



Another question which may be raised is whether the Monocystis has 

 any method of multiplication during the trophic phase, that is to say, in 

 the period from sporozoite to sporont. It has sometimes been stated that 

 the trophozoites multiply by division during the earlier stages of growth. 

 From what is known of other Sporozoa, there is nothing inherently 

 improbable in this view, but it has not been proved satisfactorily 

 that such multiplication can take place in Monocystis, and the above- 

 mentioned paucity of the trophozoites is an argument against its 

 occurrence. 



With regard to the passage of the spores to the exterior, precise 

 information is lacking as to how this is effected. In Sporozoa generally 

 we find one of two conditions. In some cases the spores are produced 

 in a position where they can leave the body by natural channels, as in 

 the numerous instances of sporozoan parasites lodged in the digestive 

 tract, when the cysts and spores are cast out with the faeces. In other 

 cases the spores cannot pass out by natural channels, and are set free 

 either by provoking suppuration or other organic disturbance, or by 

 the death and break-up of the host. In the case of the Monocystis of 

 the earthworm, the spores could only be discharged from the body, in the 

 ordinary course of events, by passing out of the sperm-sac with the sperm 

 at copulation. They would then be transferred to the spermathecae or 

 receptacula semiuis of another worm, and would pass ultimately into" the 

 cocoon in which the eggs are laid ; but there is no record of their occur- 

 rence in either of these situations. It seems more probable that spores 

 are set free by the dissolution of their host. Very possibly birds or 

 some other of the numerous creatures which prey upon worms are the 

 agents by which the dissemination is effected. If a bird swallowed an 

 earthworm containing spores of Monocystis, from which very few worms 

 are free, the spores would probably pass unaltered thorough the bird's 

 digestive tract. Uninjured spores of Gregarines have been observed by 



