SENS ATI ox. 89 



gous to that already noticed with reference to the organs of 

 sight. Thus, among the lower Mollusca the organs of hearing 

 consist of a pair of small vesicles attached to auditory 

 nerves, and filled with fluid in which an otolith is suspended. 

 In the Cephalopoda, however, while the same general plan of 

 structure is adhered to, we find an approximation to the 

 auditory apparatus of a fish ; for the vesicle or sac is now 

 embedded in the cartilage of the head, is of larger size, and 

 in general analogous to the organ of hearing of the Verte- 

 brata. That at all events the majority of the Mollusca are 

 able to smell, is proved by the readiness with which they find 

 food, and the octopus is said to show a strong aversion to 

 certain odours (Marshall). In the Cephalopoda the olfactory 

 organs are probably two small cavities near the back of the 

 eye, and in the other Mollusca they are surmised to be situated 

 in the small tentacles near the mouth. Touch is provided 

 for both by these and by the larger tentacles (as well as by 

 the general soft exterior) ; but in the Cephalopoda by the long, 

 snake-like arms, which I think must be regarded as giving 

 these animals a greater power of receiving tactile impressions 

 than is enjoyed by any other marine animal. 



Among Fish sight is well developed. A trout will dis- ) "? 

 tinguish a worm suspended in muddy water ; a salmon can 

 avoid obstacles when swimming with immense velocity ; and 

 a Chelmon rostratus can take unerring aim with its little 

 water projectile at a fly. The blind fish, which live habitually 

 in the dark, have lost their eyes merely from disuse ; but in 

 this connection it must be noted that we meet with a curious 

 biological puzzle in the case of many of the deep sea fishes 

 dredged by the Challenger. For although living at depths to 

 which no light can be supposed to penetrate, some of these 

 fish have large eyes. It may be suggested that the use of 

 these eyes is that of seeing the many self-luminous forms of 

 life which, as the Challenger dredgings also show^, inhabit the 

 deep sea. But if this is suggested, the question immediately 

 arises as to why these forms have become luminous; for if 

 thus rendered conspicuous to the fish, their luminosity must so 

 far be a disadvantage to them. In the case of the lumi- 

 nous animals which themselves have eyes, we may suppose 

 that this disadvantage is more than compensated for by the 

 advantage of enabling the sexes to find each other; but this 

 explanation does not apply to the blind forms. 



