ANATOMY AND LIFE HISTORY OP MOLLUSCA. 169 



•character of an insect's eye. Yet the small included eyes are not 

 like facets : they seem to be formed of little rings of pigment in 

 the substance of the eye-capsule. The two smaller eyes on the 

 outer ring of pigment and below the cornea, are similar structures 

 to the organs which have been described in the preceding paragraph. 



This variety of structure, if surprising, is no novelty, for in the 

 case of Onchidium, a shell-less Mollusk, there are hundreds of eyes 

 on the back of many sizes and varieties. They were discovered 

 by Dr. Karl Semper, who describes them and indeed the whole 

 •animal with a minuteness worthy of the high reputation of so 

 great a naturalist and observer.* He states that he has found 

 twenty different forms of eyes amongst the animals that he has 

 examined, with one peculiarity about them that is worthy of 

 special mention. The dorsal eyes are contained in little warty 

 excrescences on the back, which give an appearance to the skin 

 very much like the wrinkled leathery covering of a toad. The 

 organs are of various sizes, disposed with the utmost irregularity, 

 in fact like the eyes that I have been describing in many species. 

 In addition to these dorsal eyes there are also two small tentacular 

 eyes. These, however, belong to a different type, to appreciate 

 which some little explanation is necessary. 



There are two types of eyes found amongst the Vertebrates 

 and Invertebrates. Amongst Mollusca the optic nerve becomes 

 gradually merged in a layer of tissue called the retina, where the 

 fibre-end forms a layer of rods and cones, called the columnar 

 layer. InVertebrata the optic nerve penetrates the outer membrane 

 of the eye and spreads within it, but the ends of the nerve are 

 turned away from the lens and have their free ends directed 

 outwards. In the eyes on the tentacles of snails the rods are in 

 a contrary position ; that is the final spreading out of the nerve- 

 fibres is towards the lens. Amongst Vertebrates the layer of 

 rods and cones is pierced by the optic nerve, and in that particular 

 spot there are no rods. Hence there is no vision, and it is known 

 scientifically as the blind spot. There is no such spot usually 

 amongst Invertebrates, at least there was none known until 

 Dr. Karl Semper made his discoveries. The optic nerve was 

 thought to extend over the outside of the eyes of Invertebrates, 

 so that the columnar layer of rods and cones covered the whole 

 inner surface of the retina without interruption. Dr. Karl Semper, 

 however, while investigating the newly discovered eyes in 

 Onchidium, found that the dorsal ones had the rods turned away 

 from the lens, as in Vertebrata. These very interesting and 

 surprising visual organs have a rather simple structure ; but the 



*Beisen im Archipel der Philippinen, Wiesbaden, 1877. Uber Sehorgane 

 vom Tjpus der Wirbeltbieraugen auf dem Eiicken von Schnecken. 



