68 
J. BEARD. 
the optic vesicle, as it grows towards the surface of the body, 
must catch the light, and this surface is obliged to remain as 
retina. If the phylogeny of the unpaired eye were the same as 
that of the paired eyes, the retina would be of the so-called 
Vertebrate type. 
Biitsclili (No. 5, p 178), in dealing with the problems pre- 
sented by the eye of Pecten, sees the solution in the nature of 
the lens. It seems to me that here, as in the Vertebrate eye, it is 
the form of the retina — a closed cup — which gives rise to the 
cellular lens and the inverted retina. 
The appearances that one meets in Pecten are carried still 
further in the eye of Onchidium. I have, through Professor 
Howes’ and Dr. Gunther’s kindness, been able to study this 
peculiar eye, though, as there were not many eyes on the two 
specimens at my disposal, I could not follow the development. 
In spite of Patten’s off-hand criticism in his paper on “Eyes 
of Molluscs and Arthropods,” Semper was right in his state- 
ments that the eye is pierced by the optic nerve, and that thus 
an eye of the so-called Vertebrate type is formed. 
An interesting point in my specimens is that the nerve is 
double, and enters the optic cup at two points. This, I think, 
throws light on the way in which the Onichidium eye has 
developed from an eye of the Pecten type. The nerve-fibres 
must originally, as in Pecten, have gone round the front 
wall of the cup to their destination, and their piercing the 
hinder wall is only a shorter way of getting to their destina- 
tion. 
After all, I think the development does show that the parietal 
eye is a slightly later development than the paired eyes, but I 
still hold to the view that the organ has developed in connec- 
tion with the paired eyes. For this conclusion the two sorts of 
end elements, rods, and cones, described by me in Petromyzon, 
are of importance, as is also the fact that fibres have been traced 
from the thalami optici to the epiphysis. 
Most of us now accept the view of Balfour, Carriere (No. 6), 
and others, that the eyes were once structures opening dorsally 
on the surface of the unclosed neural plate, somewhat in the 
