MONOCYSTIS. 



projection which resembles the cap of Gregarina, otherwise 

 unrepresented in Monocyslis: As in Gregarina, and many 

 other parasitic forms, a contractile vacuole is absent. 



Life history. — The young form is parasitic within one of 

 the reproductive cells of the earthworm. It grows, and 

 becomes free from the cell. In the free stage, two indi- 

 viduals may unite in the curious end - to - end manner 

 observed also in Gregarina. Encystation occurs, involving 

 either a single individual or two together. Within the 

 rounded cyst, orderly nuclear division results in the forma- 

 tion of spore-forming masses. These form elliptical spore- 

 cases, or "pseudonavicelbe," enclosed in a firm sheath, and 

 each spore-case seems to contain several, usually eight, 



Fig. 36. — Life history of Monocystis. — After Biitschli. 



i. Gregarine lying within a sperm mother cell of earthworm. 



2. Conjugation of two Gregarines within a cyst. 



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



4. A spore-case with eight spores (sp.) ami a residual core (rd.) 



spores, lying around a residual core. The spores are con- 

 siderably larger than those of Gregarina. Eventually the 

 cyst bursts, the spore-cases are extruded, the spores emerge 

 from their firm chitinoid cases. The young spore is like a 

 bent spindle (falciform), and seems next door to being 

 flagellate. It bores into a mother sperm cell, and from this 

 it afterwards passes as an adult into the cavity of the 

 seminal vesicles. In some allied Sporozoa the young form 

 is first flagellate, and then amoeboid, before it becomes the 

 sluggish adult. Intracellular parasitism and copious food 

 naturally act as checks to activity. 



The species of Monocystis occur chiefly in " Worms " and 

 Tunicates ; none are known in Arthropods, Molluscs, or 

 Vertebrates. 



