pelagic, [291];

genital organs, [274]

Opisthotrema, [73]

Opistoma, [50]

Orbigny, D', on Polyzoa, [519], [520]

Orifice, of zooecium, [466], [469], [470], [524];

secondary, [522], [524]

Örley, on classification of Nematodes, [137]

Orthonectidae, [13], [92], [94] f.

Otocyst (and Otolith), of Turbellaria, [26];