VII 



MOLLUSGA—THE SENSOSY ORGANS 



171 



eyes. In the Tetrabranchiate Nautilus, as we have seen, the cup-shaped 

 eye persists throughout life. 



These lower stages (i.e. the cup-shaped and vesicular stages) of the 

 eye are passed through ontogenetically. First a cup-like depression is 

 formed (primary optic pit), then this becomes constricted to form a 

 vesicle (primary optic vesicle), the inner wall of which becomes the 

 retina, while the outer (which corresponds with the inner cornea of 

 the vesicular eye) becomes the inner corpus epitheliale. This em- 

 bryonic optic vesicle then becomes further complicated ; the integu- 

 ment over it (the outer cornea of the vesicular eye) rises in the form 



Fig. 147.— Development of the eye of tlie dlbrancMate Cephalopoda. 1, Body epithelium, 

 which becomes the outer corpus epitheliale ; 2, inner wall of the optic depression, which becomes 

 the retina ; 3, outer wall of the optic vesicle, which becomes the inner corpus epitheliale ; 4, fold 

 which forms the iris ; 5, fold which forms the secondary cornea ; 6, portion of the lens formed by 

 the outer corpus epitheliale ; 7, portion of the hame formed by the inner corpus epitheliale ; S, rod 

 layer of the retina. 



of a circular rampart, and then grows forward towards the axis of 

 the eye like a diaphragm, which forms the iris, the aperture left in 

 the same being the pupil. The integument which spreads out over 

 the circular base of the iris is in close contact with the inner corpus 

 epitheliale, and becomes the outer corpus epitheliale. 



The inner corpus epitheliale forms towards the cavity of the primary 

 vesicle an almost hemispherical lens, the outer corpus epitheliale form- 

 ing a similar lens outwards towards the pupil. The two hemispheres 

 lie in such a way as to form something like a complete sphere ; its 



