372 THE INFLUENCE OF LIVING SUEROUNDIKGS. 



It is evident that these eyes must be of the greatest im- 

 portance in the life of the animal possessing them. In the 

 first place it is simply inconceivable that well-developed eyes, 

 capable of fulfilling their functions, should be useless. If eyes 

 occurred in other univalves in the same place as in Onchidium, 

 we should naturally think at once that the dorsal eyes of 

 Onchidium were degenerate eyes; but they are found exclu- 

 sively in this genus, whence we may infer with considerable 

 confidence that they must have originated in it. But suppos- 

 ing they were, nevertheless, rudimentary eyes inherited from 

 extinct ancestors '^^ of this family, they must in some way 

 prove themselves to be such ; we should expect to find some 

 part absent — the lens, or the rods and cones, or the pigment of 

 the retina. All these parts, however — recognised as being 

 essential to the normal use of the eye — are present in the dorsal 

 eyes of the Onchidium, and not in one species only, but in above 

 twenty forms that I myself have examined. Finally, all these 

 twenty difierent varieties of dorsal eyes represent an unbroken 

 series, from those of very low development up to the highest, 

 and they all exhibit the essential parts of a seeing eye, varying 

 in arrangement, it is true, but quite normal in structure. This 

 irrefutably proves that these eyes have originated independently 

 in the family of Onchidiadse, and that they are no doubt of 

 great importance in the life of the mollusc. 



During many years of travel in tropical regions these eyes 

 were perfectly unknown to me ; but on other grounds I had 

 devoted much attention to the mode of life of the Onchidia. 

 They live exclusively on the seashore or in brackish marshes ; 

 ■they creep along close to the edge of the water, hiding in clefts 

 of the rocks or under large stones. Together with them, in the 

 same spots, live numerous specimens of two genera of fishes, 

 Periophthalmus and the nearly allied Bohophthalnius ; these 

 skip along the strand with long leaps, evidently seeking then- 

 food, which, besides insects, consists principally of this very 

 genus of mollusca. This, as it seems to me, afibrds a way of 

 accounting — ^though only hypothetically, it is true — for the 

 development of these dorsal eyes. The Onchidia are terribly 

 slow creatures, perfectly incapable of escaping or of withdrawing 



