318 



THE BIOLOGY OF MARINE ANIMALS 



contracts in direct response to illumination and is not under nervous 

 control, but the movement is much more marked in nocturnal species 

 (Fig. 8.1 1). The dilator muscle is under control of the oculomotor nerve, 

 stimulation of which causes the pupil to open. 



Pupillary responses occur in some benthic teleosts (eel, star-gazer, 

 angler fish, flat fishes), but are the exception rather than the rule, since the 

 iris shows little movement in the majority of teleosts. The pupil of the 

 eel {Anguilla) is capable of wide changes in diameter, but control is mainly 

 by direct response of the sphincter muscle to incident light. The pupil 

 of the isolated eye constricts when illuminated and re-expands in darkness. 

 The well-developed pupillary reaction of other teleosts (Uranoscopus, 





Minutes 



W 



Fig. 8.11. Movement of the Dorsal Margin of the Pupil in the Isolated Eye 

 of Scyliorhinus stellaris, in Response to Illumination (from Young (168).) 



Lophius) is reflexly controlled by antagonistic nerves. Sphincter muscles are 

 supplied by the sympathetic system, stimulation of which causes constric- 

 tion of the pupil. Dilator fibres originate in the ciliary ganglion, and stimu- 

 lation of the oculomotor nerve produces dilatation of the pupil (16, 167, 

 168, 169). 



Retinal Photomechanical Changes. In elasmobranchs {Mustelus, 

 Galeus, Raja, etc.), the chorioid contains a guanin layer (tapetum) which 

 can be exposed or occluded by migratory chorioidal pigmented cells 

 (Fig. 8.12). The guanin-containing cells extend obliquely towards the 

 retina and overlap each other like tiles. Internally, there is a layer of 

 pigmented cells having processes which can project over the guanin cells. 

 In bright light the migratory pigment cells expand and cover the guanin 

 cells with pigmented processes. Consequently, the guanin cells are screened 



