41 (40) Cilia to right of undulating membrane appear as transverse rows. 

 Contractile vacuole midventral and central. Family LOXOCEPHALI- 

 DAE. 



41 (iO) No transverse rows apparent at level of buccal cavity, 

 posterior. Family TETRAHYMENIDAE. 



Contractile vacuole 



42 (40) Undulating membrane long, saillike. Other buccal cilia variously posi- 

 tioned but not immediately adjacent to undulating membrane. Family 

 PL5:UR0NEMATIDAE in part (also see couplet 52). 



42 (40) Undulating membrane short, linear, or C-shaped, not extending beyond cyto- 

 stome. Other buccal cilia closely adjacent to undulating membrane 



43 



43 (i2) Buccal cilia appearing as 2 dense parallel longitudinal 



series, neither more than 2 ciliary rows wide 44 



43 (42) Buccal cilia appearing as a single right-hand undulating mem- 

 brane and 2 to several membranelles usually 2 cilia or more wide. 



if 



im 



lily 



o 



45 



19 



