100 (99) Cirri of midventral axis in single rows or isolated groups, but not in a zigzag 

 series 



101 



101 (100) Practically all cirri arranged in parallel rows 102 



101 (100) Cirri of midventral surface arranged in groups. Marginal cirrus rows some- 

 times present 103 



102 (101) Cell oval, with large buccal cavity. Rows of cirri curved, paralleling 

 edges of buccal cavity. Undulating membrane well developed. Fam- 

 ily PSILOTRICHIDAE. 



102 (101) Cell more elongate, cirri in parallel rows that extend well beyond the buccal 

 cavity. Family UROSTYLIDAE. 



103 (101) At least 2 rows of cirri present, each with more than 5 (usually 20 or more) 

 cirri in addition to groups of cirri. Family OXYTRICHIDAE. 



103 (101) No such marginal rows present, or at least 

 1 row reduced to fewer than 5 cirri; remain- 

 ing ventral cirri in groups. Family EUPLO- 

 TIDAE. 



37 



