MONOCYSTIS 



123 



Description. — Two species (M. agilis and M. magna) 

 infest the male reproductive organs of the earthworm. The 

 full-grown adults are visible to the naked eye — flattened 

 worm-like cells ; the shape alters during the sluggish move- 

 ments. Peripherally there is a porous cuticle, a clear cortical 

 zone, and a network of myoneme fibrils. In a more fluid 

 medullary substance, the large nucleus floats. There are 

 numerous granules of protein, carbohydrate, and other sub- 

 stances. In one species there is an anterior projection like 

 the cap of Gregarina, otherwise unrepresented in Monocystis. 

 As in Gregarina, and many other parasitic forms, a con- 

 tractile vacuole is absent. 



Life-history. — The young form of M. agilis is parasitic 



sp.c 



Fig. 55. — Life-history of Monocystis. — After BUtschli. 



1. Young Gregarine lying within a sperm mother cell of earthworm. 



2. Association of two Gregarines within a cyst, ready to form gametes. 



3. Numerous spore-cases (sp.c, pseudonavicellae) within a cyst. 



4. A spore-case with eight spores (sp.) and a residual core {rb.). 



* 



within one of the sperm mother cells of the earthworm. It 

 grows, and becomes free from the cell as a trophozoite. In 

 the free stage, two individuals may unite in a curious 

 end-to-end manner observed also in Gregarina. Quite 

 diflFerent is the association of two individuals (gametocytes) 

 inside a common cyst. After a process of " reduction " the 

 nucleus of each divides repeatedly, and the daughter nuclei 

 migrate to the surface of the cell, where each is surrounded 

 by a little mass of protoplasm. Each of the gametocytes 

 thus gives rise to a number of gametes ; there remains a 

 mass of residual protoplasm. The wall between the two 

 gametocytes now breaks down and the gametes conjugate in 

 pairs, forming zygotes . In each pair of conjugating gametes 

 one is probably derived from each gametocyte. Each zygote 

 secretes a membrane and becomes a spore-case. The 



