IV 



MONOCYSTIS 



285 



earthworm is cut open, certain white, lobed organs 

 known as sperm-sacs are very apparent a short distance 

 from the anterior end of the animal (compare Chapter 

 VI., p. 347). In these, the sperms of the worm undergo 

 development, and amongst and within the sperm- 

 mother-cells Monocystis lives and passes through the 



D 



FIG. 74. Monocystis agilis. (x about 250.) 



A, trophozoites in different stages of contraction. B, encysted gametocytes. 

 C, division of gametocytes into gametes. D (semi-diagrammatic), conjugation 

 of gametes to form zygotes. E, cyst enclosing ripe spores formed from the 

 zygotes. F, single spore, drawn to a larger scale, showing the (8) sporozoites 

 in its interior. G, group of developing sperm-cells of the earthworm, enclosing 

 a sporozoite in the centre. H, young trophozoites still surrounded with the 

 tails of the degenerated sperms, nu, nuclei. 



various stages of its life-history. The two commonest 

 species are known as M. agilis and M. magna. 



The encysted, stage is the most easily recognised, and a 

 number of spherical cysts, varying between about \ and 



