Chap, xii.] EAR OF MEDUSA. 459 



some fishes (e.g. Scopelus) have any optic function. 

 They appear, from the accounts given by those who 

 have seen such fishes alive, to be phosphorescent 



organs. 



The ear. Definite auditory organs are wanting 

 in various lower forms, which are, so far as we can 

 tell, without any sense of hearing ; an intermediate 

 condition, between that of absolute incapacity to hear 

 and the possession of this sense, is, perhaps, presented 

 to us by the earless earthworm, on which the vibra- 

 tions of air which are heard by man are absolutely 

 without effect, though the worms are very sensitive to 

 vibrations conveyed along solid objects ; in the ex- 

 periments made by Darwin, " the vibrations, before 

 reaching their bodies, had to pass from the sounding 

 board of the piano, through the saucer, the bottom of 

 the pot and the damp, not very compact earth on 

 which they lay with their tails in their burrows ; " 

 though the connection was so slight, yet the result of 

 striking a note on the piano was the immediate with- 

 drawal of the worms into their burrows. 



Definite auditory organs of a low degree of or- 

 ganisation are to be found in some Medusae, but it 

 is a remarkable fact that, so far as we know at pre- 

 sent, distinct auditory and optic organs are never 

 developed in one and the same species. A very simple 

 condition is found in such a jelly-fish as Euchilota 

 or Tiaropsis, where the lower surface of the velum 

 is, at various points, indented by open-mouthed pits ; 

 the sides and base of this pit are formed by the 

 epithelial cells of the velum, and of these some on 

 the inner face become provided with projecting audi- 

 tory hairs, and so become special sense cells, while 

 others develop within their contents the calcareous 

 concretions which form the otolith ; the sense cells are 

 continuous by their bases with the lower nerve ring. 

 The epithelial cells on the outer surface of the pit 



