THE EVIDENCE OF THE ORGANS OF VISION 37 
to be joined by filaments to the cells in the anterior wall of the eye, 
a conception fatal to the action of such knobs as lenses. 
The discovery that the vertebrate possesses, in addition to the 
lateral eyes, a pair of median eyes, which are most conspicuous in 
the lowest living vertebrate, together with the fact that such eyes 
are built up on the same plan as the median eyes of living crus- 
taceans or arachnids, not only with respect to the eye itself but also 
to its nerve and optic ganglion, constitutes a fact of the very greatest 
importance for any theory of the origin of vertebrates ; especially in 
view of the further fact, that similar eyes in the same position are 
found not only in all the members of the Palwostraca, but also in all 
those ancient forms (classed as fishes) which lived at that time. At 
one and the same moment it proves the utter impossibility of 
reversing dorsal and ventral surfaces, points in the very strongest 
manner to the origin of the vertebrate from some member or other 
of the paleostracan group, and insists that the advocates of the 
origin of vertebrates from the Hemichordata, etc., should give an 
explanation of the presence of these two median eyes of a more con- 
vineing character than that given here. 
THE LATERAL EYES. 
Turning now to the consideration of the lateral eyes, we see that 
these eyes in the arachnids often possess an inverted retina, in the 
crustaceans always an upright retina. In the arachnids they possess 
a simple retina, while in the crustaceans their retina is compound ; 
so that in the latter case the so-called optic nerve is in reality a 
tract of fibres connecting together the brain-region with a variable 
number of optic ganglia, which have been left at the periphery in 
close contact with the retinal cells, when the brain sunk away from 
the superficial epithelial covering. 
There is, then, this difference between the lateral eyes of crus- 
taceans and arachnids, that the retina of the former is compound, but 
never inverted, while that of the latter may be inverted, but is 
always simple. 
The retina of the lateral eyes of the vertebrate resembles both of 
these, for it is compound, as in the crustacean, and inverted as in 
the arachnid. 
It must always be borne in mind that in the paleostracan epoch 
