35 (5^) Zone of membranelles longitudinal. Ciliary rows to right of zone of mem- 

 branelles uniformly spaced. Family SPIROSTOMATIDAE. 



35 (5-4) Zone of membranelles spiraling 90°-180° to the right, paralleled anteriorly by 

 4-5 closely set rows of cilia. Family METOPIDAE. 



36 (29) Cytostome in anterior two-thirds of cell, usually in central one-third of body. 



Free-living or endocommensal 37 



36 (29) Cytostome in posterior one-fifth of cell. Endocommensal in various invertebrates. . . 49 



37 (36) One to three peniculi along left side of buccal cavity 

 or winding across floor of buccal cavity. Undulating 

 membrane inconspicuous. Cell densely ciliated, often 

 over 150 /urn long, with sharply delimited buccal cavity. 



38 



37 (36) No peniculi present. Undulating membrane present 

 along right edge of buccal cavity. Remainder of buc- 

 cal cilia variously arranged. Cell of various sizes. . . . 



40 



17 



