DO 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 it the question 
