Onuphin, [290]

Onuphis, [290], [318], [319];

O. conchylega, tube, [287]

Onychochaeta, [388];

chaeta, [351]

Onyx, [131]

Ootype, in Polystomum, [59].

Operculum, of Serpulidae, [261], [276], [339];

of Spirorbis, as brood-pouch, [261], [276];

of Cheilostomata, [466], [477], [481], [482], [522], [524]