314 THE BIOLOGY OF MARINE ANIMALS 



the main retina perceives near objects. The eyes of selachians are hyperme- 

 tropic at rest but are capable of some accommodation for near vision. 

 The ciliary body bears a small protractor lentis muscle, which is so oriented 

 that it moves the lens outwards towards the cornea on contracting, and 

 so focuses for near objects (16, 40, 146). 



Regulation of Light falling upon the Retina 



Devices exist in various animals for controlling the intensity of light 

 reaching the retinal cells. This may be achieved at the pupillary entrance 

 by movable lids or iris; or by movement of retinal and chorioidal pigments. 

 Migration of pigments in the eye, as will be noted later, has other physio- 

 logical consequences. 



Cephalopods. The iris of the cephalopod eye is pigmented and highly 

 muscular, and by contraction and dilation it regulates the amount of 

 light entering through the pupil {Sepia, Octopus). Strong illumination 

 causes the pupil to close, darkness produces opening. The two pupils 

 react to light independently of each other. The pupillary reaction to illumina- 

 tion is a reflex whose centre lies in the suboesophageal ganglion. Afferent and 

 efferent pathways pass through the optic peduncle, section of which pro- 

 duces maximal dilation. The reflex is susceptible to control by higher centres 

 as revealed by the fact that when an octopus is excited the pupil becomes 

 dilated. An additional pathway, in the superior ophthalmic nerve, inhibits 

 closure of the pupil (153). 



The retinal sensory cells contain a dark pigment making excursions in 

 light and darkness (Loligo, Sepia, Eledone). After the animal has been in 

 the dark for 24-48 hours, the pigment becomes densely concentrated at 

 the base of each retinal cell. On exposing the animal to light, some of the 

 pigment remains in a basal position while the remainder becomes scattered 

 through the retinal cell and accumulates also at its distal end. Thus, there 

 is a movement of retinal pigment proximally in darkness, and distally in 

 the light (118). 



Crustacea. The position of crustacean eye-pigments differs in the light- 

 and dark-adapted eye, and shows a pattern characteristic of each condition. 

 In the light-adapted eye of Palaemonetes, for instance, the distal retinal 

 pigment envelops the ommatidium and extends inwards as far as the basal 

 retinal pigment, which is dispersed outwards under illumination. The extent 

 of migration depends on the level of incident illumination. In the dark- 

 adapted eye the distal pigment migrates peripherally, while the basal 

 pigment moves inwards and assumes a position below the basement 

 membrane. In the light-adapted eye, therefore, each ommatidium is 

 enclosed in a light-absorbing sleeve of retinal pigment, the separate 

 ommatidia are screened from each other and the retinular cells are stimu- 

 lated only by light entering that ommatidium (appositional eye). But in 

 the dark-adapted state, as the result of pigment dispersion away from the 

 centre of the ommatidium, the separate elements attain optical continuity 

 and may become exposed to light rays passing through neighbouring 



