51 (50) Undulating membrane long, extending in a spiral along the periphery 

 of the shallow buccal cavity 



52 



51 (50) Undulating membrane short, linear, inconspicuous, in an oblique .• ^ '\ 



vestibulum near posterior end 53 " "T 



w 



52 (51) Undulating membrane closely paralleled by ribbon of cilia, 

 2-3 cilia wide. Cytostome at or near posterior pole. On 

 gills of bivalve molluscs or endozoic in echinoderms. Family 

 HEMISPEIRIDAE in part (also see couplet 105). 



52 {51) Buccal cilia do not closely parallel undulating membrane; 

 some arranged as transverse membranelles in forms of V 

 or Y near cytostome. Commensal on hydroids and mus- 

 sels. PLEURONEMATIDAE, in part, (Pleurocoptes, 

 Peniculistoma) (also see couplet 42). 



53 (51) Ciliary rows in vestibulum contiguous with somatic rows 

 to right of cytostome. Endozoic in mussels. Family CON- 

 CHOPHTHIRIDAE (also see couplet 22). 





53 (51) Ciliary rows in vestibulum contiguous with somatic cilium 

 rows to left of cytostome. On gills of bivalve molluscs. 

 Family THIGMOPHRYIDAE (also see couplet 22). 



22 



