Ocelli, [70]

Œsophagus of Ammocœtes, [405]

Œsop"agus of Arthropod, compared to tube of infundibulum, [4], [244], [440]

Olfactory apparatus, evidence of the, [218]

Olfa"tory organs of the Scorpion group, [220]

Olfa"tory tube of Ammocœtes, [219], [225], [244], [317]

Oligochæta, [421], [478]

Operculum of Eurypterus, [191], [212], [291]

Operc"lum of Limulus, [189], [202], [235], [295]

Operc"lum of Phrynus, [191]