No. 2.] FRESH-WATER PROTOZOA. 63 



Stalk retractile; all zooids contracting together. , . . 



Zoothamnium* 

 Stalk not retractile; peristomial disk broad. . . .Epistylis* 

 Stalk not retractile ; peristomial disk long-stalked . . 



Opercidaria*^ 



7. Stalk retractile Vorticella* 



Stalk not retractile; without an operculum. .Rhabdostyla* 

 Stalk not retractile ; with operculum Pyxidium^ 



8. Lorica gelatinous Ophrydium 



Lorica chitinous 9 



9. Cylindrical to ovate, unstalked or with a short 



cylindrical stalk Cothurnia 



Ovate, cup-shaped or flattened, with an ear-shaped 

 projection bearing the mouth ; a short, thick 



ringed stalk Cothurniopsis 



Flattened, with the whole of one side attached to the 

 support ; animal attached to the lorica by a stalk- 

 like outgrowth Vaginicola 



Flattened, with posterior flattened end attached to 

 the support; with the peristome process attached 

 to the edge of the mouth Lagenophrys 



Description of Genera. 

 Trichodina Ehrbg. 



Solitary, short, cylindrical or barrel-form, with a pos- 

 terior row of cilia, over which lies a ring-like fold run- 

 ning around the body. The circular flat base with which 

 the animal attaches itself forms a sucker. It is covered 

 by a chitinous membrane whose peripheral zone is ra- 

 dially striated. Adoral zone extends spirally around the 

 flattened anterior end. Nucleus ribbon-shaped and one 

 contractile vacuole. Animals all parasitic. 

 P. pediculus Ehrbg., Fig. 301. On Hydra. 

 Vorticella Linn. 



Body bell-formed, with a more or less evident, out- 

 wardly extending peristome ; cuticle often ringed. With 

 a contractile stalk, frequently longer than the body, by 

 which the animal is attached. Adoral zone describing 

 more than a circle. Nucleus simple, ribbon or horse-shoe 



