SENSORY ORGANS AND RECEPTION 331 



Coastal waters vary greatly in their absorption characteristics, depend- 

 ing on turbidity, but transparency is frequently greatest in the green 

 region of the spectrum. The greatest transparency of clear ocean water is 

 at a wave-length of 480 m/v, i.e. in the blue region of the spectrum (vide 

 Chapter 1). Absorption in both coastal and deep waters increases greatly 

 above 550 m//. It is within this general range of 480-550 m/n that the rhodop- 

 sins or visual purples offish and invertebrates are most sensitive. It follows 

 then that visual pigments having absorption maxima in this range will 

 possess maximal efficiency in a marine environment. Furthermore there 

 is now evidence accumulating that luminescent light, of animal origin, 

 may be of considerable significance for vision in the ocean depths where 

 there is little residual daylight, and it is of interest that the emission 

 spectra of some species (only a few have been studied) lie in this general 

 range (about 490-510 m/u). The majority of species studied hitherto have 

 been from inshore waters, and further photochemical studies of pelagic 

 species will prove of great interest. The X max of the visual pigments of 

 Loligo and Euphausia, it will be noted, lie in the blue, below 500 m/u, 

 in correlation with the greater transparency of clear ocean water to shorter 

 wave-lengths. 



Electrical Activity of the Eye 



Two kinds of electrical activity can be recorded from the neighbourhood 

 of the eye, namely retinal potentials and optic-nerve potentials. When 

 electrodes are connected to front and back of the eye, a potential differ- 

 ence between the two regions is detected. Illumination of the eye produces 

 a potential change known as the retinal action potential or electro- 

 retinogram. This is in the direction of increased negativity of the free distal 

 ends of the photoreceptor cells. In the vertebrate eye, with its inverted 

 retina, there is an initial negative/positive response at the onset of light 

 (a and b waves), and the potential change is completed with a positive 

 off-effect at cessation of illumination. The components of the electro- 

 retinogram (ERG) refer to changes taking place in the visual cells. 



Electroretinograms have been obtained from the eyes of various in- 

 vertebrates. The response to light is often a simple negative wave succeeded 

 by a sustained negative potential at a lower plateau level throughout the 

 duration of illumination (Limulus, Loligo, Asterias, etc.) (Figs. 8.20, 8.21). 

 The ERG recorded from the compound eye of Ligia begins with a nega- 

 tive on-effect, quickly followed by an early positive deflexion and rapid 

 return to base line during illumination, and ends with a positive off-effect 

 (53, 63, 125, 140, 165, 166). 



Photic stimulation of the eye produces a train of action potentials in 

 the optic nerve. This is well illustrated in records from single optic nerve 

 fibres of Limulus (Fig. 8.22). When stimulated with a long light exposure, 

 the optic fibre begins to discharge after a short latent period, initially at 

 a high frequency, soon followed by a rather steady discharge at a lower 

 frequency; with light off, the discharge ceases. This is an on-effect, the 



