INDEX 



821 



Orientation, to visual pattern, 73 



visual, 667 

 Ornithorhynchus. See Platypus. 

 Orthokinesis, 34 

 Orthoptera, 218, 220 

 eyes of, 223 



See also Blattella, Loc^tsta, Di.vippus, 

 etc. 

 Orycteropiis. See Aard-vark. 

 Oryctolagus. See Rabbit. 

 Os opticus of birds, 403 

 Osseous plaques (scleral), in chondro- 

 steans, 317 

 in Latimeria, 315 

 Ossicle of Gemminger, in birds, 403 

 Ossicles, scleral, accommodation and, 649 

 in birds, 403 

 chelonians, 369 

 lacertilians, 356 

 Sphenodon, 380 

 teleosts, 294 

 occurrence of, 450 

 Osteichthyes, 234, 235, 278 

 eyes of,' 291 



See olso Teleosteans, etc. 

 Osteolcemus tetraspis, iris, PL VIII 

 Ostracods, 207 

 eyes of, 209 



luminous organs in, 740, 746 

 See also Cypris, etc. 

 Ostrich, African. See St ruth io. 

 American. See Rhea. 

 eye of, 399 

 Otariidae. See Sea-lions. 

 Otter, 445 



accommodation in, 654, 655 

 optic axis, 688 

 shrew. See Shrew, otter-, 

 sphincter muscle, 468 

 Otus bakkamcena, iris, PI. XI 

 Owls, ciliary muscle, 407 

 colour vision in, 630 

 eyes of, 399, 605 

 immobility of, 695 

 shape of, 402 

 fovea, 417, 418 

 iris, 407 

 lens, 409 

 pecten, 411 

 threshold to light, 674 



absolute, 617 

 visibility to infra-red, 620, 630 



Owls, visual acuity of, 662 



field, binocular, 682, 683, 684 

 uniocular, 670, 672 

 See also Bubo, Otus, Strix. 

 Ox. See Cattle. 

 Oxybelis, iris, 371 

 Oysters, 196 



light-shadow reflex, 574 

 pearl. See Avicula. 



Pachydactylus niaculatus, eyelids lacking 



in, 367 

 Palcemon, colour changes in, 91 



rhythmic, 20 

 Palcemonetes, colour changes in, 94 



retinal pigment, migration of, 555 

 Palfeocortex, 543 

 Palseognatha?. See Ratitse. 

 Pallial eyes, in bivalves, 201 

 Pan. See Chimpanzee. 

 Panda {Ailurus), 445 

 pupils, 471 

 giant (Ailuropoda), nictitating mem- 

 brane, 493 

 Pandalus, visual pigments in, 121 

 Pangolin, 442, 445 



Pantodon, visvxal field, binocular, 680 

 Pantopoda. See Pycnogonida. 

 Papio. See Baboon. 

 Paracentrotus Hindus, sensitivity of, 184 

 Paralichthys albiguttus, colour changes in, 



92 

 Paramoecium, 179, 180 

 geotaxis in, 45 

 klinokinesis in, 36 



modified by electric shock, 571 

 bursar is, reproductive cycle in, 16 

 Paraj^ineal organ. See Parietal organ. 

 Parasites, external, degenerate eyes in, 

 734 

 internal, degenerate eyes in, 733 

 Paraventricular nucleus, 557 

 Parazoa, 181 



See also Leucosolenia. 

 Parietal eye, of lamprey, 713, 716, 717 

 lizard, 715, 716, 718 

 Sphenodon, 715, 716 

 origin of, 242 

 foramen, in Sphenodon, 715 

 organ, 538, 713 



