312 



THE BIOLOGY OF MARINE ANIMALS 



cephalopods the eye has an external cornea, closed over in some forms, 

 a lens, iris-folds and retina (Fig. 8.7). Enclosing the eye is a cartilaginous 

 sclera perforated by fine holes through which pass the optic nerve fibres. 

 The visual cells of the retina bear long rods and contain retinal pigment. 



Numerous small muscles are attached to the exterior of the eyeball and 

 can move the eye to some extent in all directions. In some forms, e.g. 

 Sepia and Octopus, there is an external eye-fold which closes the eye. There 

 are many peculiarities in the eyes of pelagic cephalopods, the significance of 

 which is poorly understood. One of these, the telescopic eye, found in 

 Toxeuma for example, is an adaptation for weak illumination. 



A mechanism exists for accommodation in the eye of the cuttlefish 

 {Sepia). In accommodating for distant vision the ciliary muscles contract 



Fig. 8.7. Camera Eyes 



{Left) Sepia. (Right) a teleost. L, lens; G, optic ganglion; O, optic nerve; El, eyelid; 

 Re, retinal cells; GL, ganglion layers. (After Plate (121).) 



and draw the lens towards the retina. It has been found that stimulation 

 of the sclerotic muscles causes the lens to be displaced forwards, and this 

 displacement may be a means of accommodating for very near vision (141). 



Vertebrate Eyes. These are all built on the same fundamental pattern 

 and, except when degenerate, are provided with a lens capable of throwing 

 an image upon the photosensitive retina (Fig. 8.7). The latter is inverted 

 and from front to rear shows the following layers: a feltwork of optic 

 nerve fibres; two layers of ganglion cells (tertiary and secondary), separ- 

 ated by a synaptic layer; a basal layer of rods and cones, the photo- 

 receptors proper. 



The rods and cones, by virtue of their inherent sensitivities or nervous 

 connexions, are adapted for photoreception over different intensities of 

 illumination. Rods are highly sensitive visual cells, functioning at low in- 

 tensities, whereas the cones operate in bright light. Corresponding to these 

 differences in function are differences in photosensitive substances (p. 

 329). Neural connexions in the retina are very complex. Cones are con- 

 nected to one or a few nerve fibres, arrangements which promote visual 



