72 THE FAUNA OF THE DEEP SEA 



its eye, so that it can throw no light upon the pro- 

 blems we are now discussing. 



Among the deep-sea gasteropods we find the 

 same irregularity in the possession of eyes that we 

 have just described among fishes. Thus a species of 

 Pleurotoma, dredged by the ' Porcupine,' in 2,090 

 fathoms, has a pair of well-developed eyes on short 

 footstalks, but Pleurotoma nivalis, obtained by the 

 'Talisman/ is blind. Again a species of Fusus, ob- 

 tained by the ' Porcupine,' in 1,207 fathoms, is pro- 

 vided with well-developed eyes, but Fusus abyssorum, 

 obtained by the ' Talisman,' is blind. Among the 

 Lamellibranchs there are very few genera that possess 

 well-marked eyes. The genus Pecten is one of those 

 that in shallow waters possess numerous highly com- 

 plicated visual organs situated on the edge of the 

 mantle. In the deep-sea species, Pecten fragilis, these 

 eyes are wanting, but we have not sufficient evidence 

 at present to enable us to assert that all the deep- 

 sea species of this genus are blind. 



Among the Crustacea there is a very general 

 tendency to lose the eyes at a depth of a few hundred 

 fathoms of water. 



In Ethusa granulata, for example, the eyes dis- 

 appear at 500 fathoms and the eye-stalks become 



