DOESAL EYES OF ONCIIIDICM. 



371 



course none can be formed ; hence vertebrate animals are blind 

 in that particular spot, whicb is in fact known scientifically 

 as 'the blind spot.' Such' a spot is absent in the eyes of the 

 second category ; the optic nerve extends over the outside of the 

 eyes, and the rods and cones situated within cover the whole 

 inner surface of the retina uniformly. 



So far as concerns the eyes placed on its head the genus 

 Onchidium in no way differs from the allied groups. But the 

 greater number of species in this genus are further distinguished 

 by having other eyes situated on the shell-less but coriaceous 



Fig. 99. — Section of the dorsal eye of Onchidium verrvculafttm, letters as in fig. 93. 



back of the animal. These dorsal eyes (see fig. 99) are extremely 

 interesting, for, simple as they are in structure, they are iden- 

 tical in type with those of the vertebrata. A comparison of 

 the two sections here given of the eye of a vertebrate animal 

 and of one of these dorsal eyes will sufiice to exhibit the 

 resemblance ; in both there is the ' blind spot,' because the 

 optic nerve must pierce the external layer of the retina ; and in 

 both the layer of rods and cones forms the outer layer of the 

 retina. This is the only example hitherto known of an eye i 

 so constructed in an invertebrate animal. 



