SENSORY ORGANS AND RECEPTION 313 



acuity. In contrast, many rods feed via secondary neurones into a single 

 fibre. The latter arrangement, which results in a high degree of summation 

 of the more sensitive photoreceptors, enhances sensitivity at the expense 

 of visual acuity. 



Probably no other organ in the vertebrate body shows such fine adapta- 

 tions to the functional needs of the animal as does the eye. Only a few 

 factors relating to vision in marine fishes can be considered, and for fuller 

 treatment the reader is referred to Walls (146). 



The eye in most elasmobranchs and teleosts possesses the normal 

 vertebrate structure. Some deep-water selachians and teleosts have degen- 

 erate eyes and are blind, e.g. Benthobatis (ray) and Ditropichthys (teleost). 

 The majority of bathypelagic and bathybenthic fishes, however, have 

 functional eyes and this is probably due to the widespread occurrence of 

 luminescence in deep-water animals (Chapter 13). The eyes of deep-sea 

 fishes are believed to be the most sensitive in existence and contain enor- 

 mous numbers of rods per unit area of retinal surface (103). 



Pure rod retinas are found in most elasmobranchs and deep-sea teleosts. 

 This factor, combined with a high degree of summation, makes for in- 

 creased visual sensitivity. The former animals are largely nocturnal in 

 habits, and the latter live in dimly-lit or dark waters below the photo- 

 synthetic zone. A few selachians are known to possess cones, e.g. 

 Mustelus, which is diurnal, and Myliobatfs, which is pelagic in habits. 

 Retinae with rods and cones are characteristic of teleosts from well-lighted 

 waters. 



The eyes of mesopelagic fishes are often relatively large and have wide 

 pupils and large lenses, factors related to the dim light of the regions which 

 they inhabit. Some species — e.g. the hatchet-fish Argyropelecus — have 

 tubular (so called telescopic) eyes, in which the lens is enlarged relative to 

 the size of the eye. The retina in the equatorial region appears thin and 

 degenerate, and the functional retinal surface is confined to the fundus. 

 Because of these optical features a small, bright image is thrown on the 

 retina. 



In lower vertebrates accommodation is usually accomplished by dis- 

 placement of the lens. The lens at rest is adjusted for near vision (myopia) 

 or distant vision (hypermetropia), according to the animal. In a myopic 

 eye, movement of the lens backwards adjusts for distant objects by ad- 

 vancing the image and bringing it into focus on the retina. A converse 

 process takes place in hypermetropic eyes capable of accommodation. 



Teleosts are myopic and the eyes at rest are set for near vision. Attached 

 to the lens ligament is a small retractor lentis muscle (campanula Halleri), 

 which is capable of displacing the lens backwards and accommodating 

 to some extent for distant objects. Nervous control is mediated by the 

 oculomotor nerve. In the tubular eyes of deep-sea teleosts little or no 

 lens movement is possible. Such eyes possess an accessory retina lying on 

 the cylindrical walls near the lens, whereas the main retina lies at the 

 back of the fundus. The accessory retina takes care of distance vision, and 



