22 (21) Vestibular ciliary rows contiguous with somatic ciliary rows to right of 

 cytostome. Endozoic in bivalves and snails. Family CONCHOPHTHIR- 

 IDAE (also see couplet 53) . 





^:.^:^-:':^_ 



'^■,'',',',',111! , 



22 {21) Vestibular ciliary rows contiguous with somatic ciliary rows to left of 

 cytostome. Ectocommensal on gills of bivalve molluscs. Family THIG- 

 MOPHRYIDAE (also see couplet 53). 



23 {21) Cell oval. Vestibulum one-third body length from anterior end and rel- 

 atively small. Macronucleus oval. Endocommensal in bivalves. Gullmarel- 

 la faurei, order Trichostomatida (see Annotated Systematic List). 



23 {21) Cell vermiform. Vestibulum large, triangular, near anterior end. Macronucleus 

 elongate. Free-living. Conchostoma longissirmim, order Trichostomatida (see An- 

 notated Systematic List). 



24 {3) Cell vermiform, attached by apex to renal epithelium of cephalopods. Fam- 

 ily OPALINOPSIDAE. 



24 {3) Not as above 25 



13 



