110 
ANNIVERSARY ADDRESS 
ences in the eyes of the Vertebrata of which we can only guess 
the meaning, and under any circumstance we must admit that the 
anatomical basis of the perception of colour remains, like that of 
consciousness, hidden from our understanding. 
The Nascent Eye. 
We will now proceed to consider the nascent eye, by which I 
mean those structures which may be taken as the forerunner of the 
eye. Pigment-cells are more sensitive to light than unpigmented 
cells, and they may transmit the excitation caused by the chemical 
changes produced by the waves of light to the adjacent nervous 
substance. If this be the case, we may take the spots of colour of 
the Infusoria when affected by light to be capable of producing 
some response in these creatures. Ehrenberg describes these eye¬ 
like pigment-spots as veritable optic organs, but this view is un¬ 
tenable, for no nervous system exists in the Infusoria. A further 
advance towards vision is seen in the eye-spots of the Yermes. 
These consist of pigment-cells in connection with nerves, and they 
are sometimes provided with refractive bodies which may perform 
the function of lenses. Such organs are found in many animals, 
in a greater or less degree of development, and especially in the 
Coelenterata, in which nerve-structure and sight are generally 
accepted as first occurring. It must, however, be borne in mind 
that the incidence of light on a nerve-ending surrounded by pig¬ 
ment, and even accompanied by refractive bodies, may only produce 
the sensation of heat. All nerves of special sense have special 
endings. The special endings of the optic nerve are either rod-like 
or conical bodies, called rods or cones, and they are essential to 
convert the external movement of the waves of ether into a stimulus, 
which, after travelling along the nerve to the central organ, is by 
it perceived as light.* 
The Simple Eye. 
The expression simple eyes is used in contradistinction to com¬ 
pound eyes, which are generally found in the Crustacea and In- 
secta, and simple eyes are widely distributed in the Invertebrata. 
The optical principle of these is the same as that of the eyes of 
Yertebrata. The latter, however, are more perfected, and we will 
consider them hereafter. The sensation of light may in many of 
the simple eyes be limited to mere opalescence without any percep¬ 
tion of the form of external objects. Yet even such light would 
* I have purposely limited my observations under this heading, and refer the 
general reader to Sir John Lubbock’s recent publication ‘ The Senses of Animals.’ 
See also ‘ Die Sehorgane der Thiere ’ by Carriere, Munich, 1885, and ‘ Comparative 
Biology,’ by Balfour, vol. ii, p. 387. 
