458 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Molluscs the optic bulb is moved by muscles, and this 

 is the kind of mechanism which obtains in the Verte- 

 brata, where four rectal and two oblique muscles are, 

 as in man, almost always developed. 



Influence of light and darkness on the 

 development of the eye. While our knowledge 

 of such terrestrial forms as the earthworm, the 

 probeus, or the mole, would lead us to think that a 

 diminution of light is constantly associated with a 

 degenerate condition or loss of the eye, it is very 

 remarkable that among aquatic forms we find species 

 which support, and others that unmistakably con- 

 tradict this hypothesis. Among Crustacea we have, 

 for example, Ethusa granulata, which in shallow 

 water has eyes of the ordinary character, but, when 

 taken from depths of 110 to 370 fathoms, is found 

 to have the stalked eye replaced by a calcareous 

 knob ; it is a case to which the words of Darwin 

 are altogether applicable : " The stand for the tele- 

 scope is there, though the telescope with its glasses 

 has been lost." Specimens of the same species, 

 taken from a greater depth (500 to 700 fathoms), 

 showed that the eye-stalk had undergone a change 

 of function, and had become converted into a 

 pointed rostrum, which probably serves as an organ 

 of touch. On the other hand, species of Munida 

 that were dredged at the same time as the Ethusa 

 were found to have exceedingly large eyes. What is 

 true of Crustacea is true also of fishes ; at moderate 

 depths the eyes are generally large, at greater depths 

 they may be very large or very small. Where the 

 eyes are small the fish has its tactile organs very 

 largely developed. Fishes or Crustaceans, however, 

 taken from very great depths (e.g. 1,900 fathoms), 

 have been found to have both the eyes rudimentary 

 and the special organs of touch absent. There is no 

 reason to suppose that the so-called eye-like organs of 



