25 (24) Anterior end with holdfast apparatus of skeletal 

 elements and areas of thigmotactic cilia 



26 



25 (24) Holdfast apparatus, if present, without skeletal elements 27 



26 (25) One or two hooks in holdfast apparatus under thig-motactic area. Cell about 

 21/2 times longer than wide. Endocommensal in turbellarians. 

 Family HAPTOPHRYIDAE. 





26 (25) No hooks in holdfast apparatus. Cell length more than 2i/o times width, 

 docommensal in gut of polychaetes. Family HOPLITOPHRYIDAE. 



En- 



27 (25) Macronuclei scattered. Cell pyriform. 

 Endozoic in polychaetes. Cyrtocm-yum 

 halosydnae, Family OPALINOPSIDAE. 



27 (25) Macronucleus single, central. Cell variously shaped 28 



28 (27) Anterior suctorial tentacle present, variously shaped. Ectocommensal on gills 

 of bivalve molluscs. Macronucleus variably shaped. Family ANCISTRO- 

 COMIDAE in part (also see couplets 98 and 110). 



14 



