Ophlitaspongia, [225]

Ophrydium, [138], [158]

Ophryocystis, [97]

Oplorhiza (Campanulariidae, [280]), [277]

Oractis, [377]

Oral apparatus, of Ciliata, its atrophy and regeneration during conjugation, [151]

Oral blood-ring, of Asterias rubens, [450];

of Ophiothrix fragilis, [488];

of Holothuria nigra, [567]

Oral cleft or groove of Noctiluca, [132] f.