76)i STRUCTURE OP THE DORSAL PAPILLA OF ONCHIDIUM, 



9. Nothing is to be added to Semper's statements regai-ding the- 

 nervi optici. 



10. It is remarkable that the Onchidium Damelii never retracts 

 her tubercles or feelei's, however near the forceps or scissors 

 approach them until they are actually iia contact. This might lead 

 one to assume that this animal is farsighted. 



11. The concave lenses on the upper ends of the facets, below 

 the large spherical main lens, appear as a secondary arrangement 

 produced for the purpose of counteracting the bad effect of an oval 

 or spherical lens in air. 



The lens was originally, probably, adapted for seeing in water, 

 and therefore had such a great curvature and short focus. When 

 the Onchidium took to living on land this lens was too strong — for 

 use in air — and then the little concave cells might have been 

 produced to counteract the excessive power of the main lens. 



