9o PROTOZOA. 



vacuole is absent, but the significance of this is not quite 

 obvious. 



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 "pseudonavicellae," enclosed in a firm sheath, and 

 each spore case seems to contain several, usually eight, 

 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 more 



FIG. 19. Life history of Monocystis. (After BUTSCHLI.) 



1. Gregarine lies within a sperm-mother-cell of earthworm. 



2. Conjugation of two Gregarines within a cyst. 



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



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



active than the adult ; indeed, in some Gregarines, it is for 

 a brief period flagellate, then amoeboid, then like 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 



~\7"krf pkV^ro frf^c 



Vertebrates. 



Fourth Type PARAMCECIUM. 



Paramcecium. A type of Infusorians, especially of 

 those which are uniformly covered with short cilia 

 (Holotricha). 



Description. Specimens of Param&tium may be readily 

 and abundantly obtained, by leaving fragments of hay to 



