BO CRAYFISHES HEAR AND SEE \ 125 



the apparent wide difference between it and the verte- 

 brate eye gives place to a fundamental resemblance. The 

 rods and cones of the retina of the vertebrate eye are 

 extraordinarily similar in their form and their relations 

 to the fibres of the optic nerve, to the visual rods of the 

 arthropod eye. And the morphological discrepancy? 

 which is at first so striking, and which arises from the 

 fact that the free ends of the visual rods are turned 

 towards the light, while those of the rods and cones 

 of the vertebrate eye are turned from it, becomes a confir- 

 mation of the parallel between the two when the develop- 

 ment of the vertebrate eye is taken into account For it 

 is demonstrable that the deep surface of the retina in 

 which the rods and cones lie, is really a part of the outer 

 surface of the body turned inwards, in the course of the 

 singular developmental changes which give rise to the 

 brain and the eye of vertebrate animals. 



Thus the crayfish has, at any rate, two of the higher 

 sense organs, the ear and the eye, which we possess our- 

 selves; and it may seem a superfluous, not to say a 

 frivolous, question, if any one should ask whether it can 

 hear and see. 



But, in truth, the inquiry, if properly limited, is a very 

 pertinent one. That the crayfish is led by the use of its 

 eyes and ears to approach some objects and avoid others, 

 is beyond all doubt ; and, in this sense, most indubit- 

 ably it can both hear and see. But if the question 



