vii. 14 EYE OF TELEOSTS 213 



is the wavelength that penetrates farthest into the water. In order to 

 extract the maximum of information at high as well as low intensities 

 it is necessary to adjust the sensitivity, and hence the signal/noise 

 ratio. For this purpose teleosts have developed retinas with distinct 

 rods, cones, and twin-cones and in some there is a fovea composed of 

 numerous thin cones (e.g. in Blennius). The pupil usually varies little 

 in diameter, and adjustment of sensitivity is by migration of pigment 



scleral cartilage 



epichorioKJal lymph space 



choriad 

 'glond 



Fig. 130. Diagrammatic vertical section of a typical teleostean eye. Not all the structures 

 here shown are found in all species. (From Walls, The Vertebrate Eye.) 



between the receptors and contraction of a 'myoid' segment of the 

 latter. In bright light the pigment expands, the cones contract for- 

 ward, towards the light and the rods contract back, beneath the pig- 

 ment. These photo-mechanical changes thus serve the same end as 

 changes of pupil diameter in other vertebrates. 



The photochemical change in the rods of marine fishes is the same 

 as that of land vertebrates, namely the breakdown of the rose- 

 coloured 'visual purple' (rhodopsin) first to the yellow retinene and 

 then to colourless vitamin A v In freshwater fishes there is a different 

 pigment porphyropsin, or visual violet, which breaks down to vita- 

 min A 2 . Intermediates between these may be found. 



In all fishes there is a very large, dense, spherical lens, to which is 

 attached a retractor muscle (campanula Halleri) inserted on to a fal- 

 ciform ligament, which occupies the persistent choroidal fissure in the 

 retina (Fig. 130). The eye is usually said to be myopic at rest and to be 

 accommodated for distant vision by pulling the lens nearer to the 



