90 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 curioVis 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 "pseudonavicelte," 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 BiJTSCHLi.) 



t. Gregarine lies within a sperm-moLher-cell of earthworm. 



2. Conjugation of two Gregarines within a cyst. 



3. Numerous spore-cases (pseudonavicella;) within a cyst. 



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



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

 a brief period flagellate, then amceboid, 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 

 Vertebrates. 



Fourth Type — P.-\R;\MaECiUM. 



Paramecium. — A type of Infusorians, especially of 

 those which are uniformly covered with short cilia 

 (Holotricha). 



Description. — Specimens of Paramaxium may be readily 

 and abundantly obtained, by leaving fragments of hay to 



