48 (45) Cell length variable, commonly over 70 fim. Anterior end tapered 

 to blunt point. Right dorsolateral surface typically concave. Mem- 

 branelles massive; cytostome anterior to equator. Family PHIL- 

 ASTERIDAE. 



48 (45) Cell usually under 50 yu.m in length. Anterior end truncated, con- 

 spicuously naked. Right dorsolateral surface typically convex. 

 Membranelles inconspicuous. Cytostome often subequatorial. 

 Family URONEMATIDAE. 



49 (36) Anterior end with nonciliated V-shaped sucker. Endozoic in oligo- 

 chaetes. Family HYSTEROCINETIDAE. 



49 (36) No such anterior nonciliated sucker. 



50 



50 (49) Cell flattened, with distinct caudal projection. Endozoic in sea urchins. Big- ^ 

 garia (see Annotated Systematic List). 



50 (49) Cell oval or circular in cross section. Posterior end rounded 51 



21 



