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possesses a large number of remarkable eyes on the mantle edge. 
They are remarkable not only for their complexity, which is prob- 
ably only approached by the Cephalopoda amongst the Mollusca, 
but for the large number present. The Eye consists of a vesicle, 
the wall of which is formed of the connective tissue of the thickened 
mantle edge. This tissue is reduced in thickness and is more trans- 
parent in front of the lens. Covering it at this place is the ecto- 
derm of the mantle which is free from pigment and forms a cornea. 
Below the cornea and the underlying connective tissue is a cellular 
lens composed of rather peculiar, cells. Across the optic vesicle is 
a septum which acts as the distal boundary of the retina and lies in 
contact with it. 
Now the retina is highly characteristic. It comprises two sepa- 
rate and distinct layers of sense cells, and the optic nerve bifurcates 
before reaching the optic vesicle in order to innervate these two 
sensory strata. One series of cells — the distal of the two — is not 
unlike a layer of ciliated epithelial cells with the cilia-like processes 
directed towards the lens. The other stratum is thicker and con- 
sists of rod cells or retinopliorae, bearing rods. These cells are in- 
verted so that the rods are turned away from the lens. The nerve 
fibres reach the retinopliorae by the periphery of the retina. The nerve 
fibres reach the distal layer of sense cells by perforating the septum. 
By no stretch of the imagination can the structure or the de- 
velopment of this eye be said to resemble the human eye, except 
that both, eyes have an inversion of sensory elements in the retina. 
Bergson, however, assumes (probably from an ancient loose bio- 
logical description) that the eye of Pecten and the human eye are 
closely alike in structure . 1 Taking the view that the vertebrates and 
the molluscs separated long before the appearance of a visual organ 
so complex he asks “Whence, then, the structural analogy'?” 
The same author points out that an explanation of the evolution 
of either of these eyes by the selection of small variations, or large 
mutations involving many simultaneous small changes, is sur- 
rounded with difficulties. The organ will be of no use and will not 
give selection any hold unless it functions. It will, moreover, be of 
no use if the retina develop without the other parts of the eye. If 
then small variations are responsible how could they have arisen in 
every part of the organ at the same time and in such a way that the 
eye would, from the beginning, be able to perform its work'? If 
large mutations have resulted in the evolution of the eye, then what 
factor lias governed the development so that all parts of the sense 
organ, having changed, yet remain so co-ordinated that the function 
of sight is still observed 1 ? Let us grant the possibility, suggests 
Bergson, of such a state of affairs taking place in one or other of 
the cases referred to, out of myriads of failures — is it conceivable 
that such a process could have occurred twice in unrelated organ- 
isms if no special organising factor were present? 
Bergson. Creative Evolution. (Eng. Trails.). London, 1913. 
