MONOCYSTIS.: 



>TQI 



the free stage, two individuals may unite in the curious 

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

 different 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 



FIG. 44. Life history ol Monocystis. After Btitschli. 



x. 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 (sfi.c., pseudonavicellae) within a cyst. 



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



secretes a membrane and becomes a spore-case. The 

 nucleus divides up, and eight elongated spores are formed 

 round a residual core. The spore-case now takes its typical 

 shape and is known as a pseudonavicella. The spores are 

 considerably larger than those of Gregarina. Eventually, in 

 the alimentary canal of another earthworm the cyst bursts, 

 the spore-cases are extruded, the spores emerge from their 

 firm chitinoid cases. The young spore (sporozoite) 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. Intracellular parasitism and copious food 

 naturally act as checks to activity, and the adult is sluggish. 

 The allies of Monocystis occur chiefly in "Worms," 

 Tunicates, and Arthropods ; none are known in Vertebrates 



