THE SIMPLE EYE 



147 



double backwards to reach the o])tic panghon (Figs. 119-121). In such 

 cases the proximal cells of the vesicle usually contain an absorbing pig- 

 ment, and the recejitive ends of the visual cells approximate closely to 

 them, thus reducing the vesicle to a slit-like potential cavity. An 

 arrangement which might at first sight seem anomalous thus acquires a 

 distinct biological value. Moreover, in many species a reflecting crys- 

 talline layer, or tapetum, is found next to the receptive ends of the visual 



i l'l-l-l-l-M-h i ' 



Fig. 



118. — The Verted Retina of 

 THE Vesicular Eye. 



Fir.. 



119. — The Inverted Retina of 

 the Vesicular Eye. 



Fig. 120. — The Arrangement of the 

 Visual Cells in the Verted 

 Retina. 



Fio. 121. — The Simplest Arrange- 

 ment OF THE Visual Cells in the 

 Inverted Retina. 



In each case light falls upon the visual cells from above (modified from 



Buxton, li)12). 



cells which reflects the incident light backwards so that it traverses the 

 sensory cells a second time thus doubling the intensity for stimulation 

 and incidentally giving the eye a metallic sheen. This arrangement is 

 therefore characteristic of animals to which vision in dim illuminations 

 is important . 



An inverted retina of this type is typical of Vertebrates but is rare 

 among Invertebrates, being seen in a few Molluscs and Arachnids. 



Among MOLLUSCS it is found in four species — in its simplest form in 

 the pulmonate, Onchidmm. and in the cockle, Cardium. and in its most 

 elaborate form in two bivalves, the scallop, Pecten, and Spondylus. 

 In the jDulmonate mollusc, Onchidium, the visual cells of a simple vesi- 

 cular eye are inverted and the optic flbres, issuing from their distal ends, 

 pierce the posterior pole of the vesicle in a bundle exactly as does the 

 optic nerve of Vertebrates (Fig. 122) (Semper, 1883). This peculiar eye 

 is also unique in that the " vitreous "' filling the optic cavity is made up 

 of a small number of enormous cells. In Cardium the arrangement of 

 the visual cells is somewhat similar but that of the optic nerve fibres 



Cardium 



