508 THE BIOLOGY OF MARINE ANIMALS 



has a functional role as a light receptor in chromatic activity (smolts of 

 Oncorhynchus nerka). When the pineal is destroyed and the fish illuminated, 

 they become intermediate in shade between normal and blinded specimens. 

 The melanophore response thus depends on sensory information from the 

 eyes and pineal organ (8, 33, 54, 56, 60, 63, 77). 



In colour responses mediated by the eyes it has been found that the 

 determining factor is the ratio of reflected to incident light — that is, light 

 reflected from the background and that reaching the eye from above. When 

 this ratio is small, as on a black background, the animal darkens, and when 

 this ratio is large, as on a white background, the animal blanches. These 

 responses take place within a wide range of intensity of total illumination. 

 To explain these results it has been assumed that the eye or retina is 

 structurally and functionally polarized. Thus in Fundulus heteroclitus, 

 various regions of the retina have been tested by rotating the eyes, and by 

 the use of screens shielding half the eye. When light was admitted to the 

 ventral region of the retina, but was excluded from the dorsal region, the 

 animal darkened; and when light was admitted to the dorsal region, but 

 excluded from the ventral region, the fish blanched. The dprsal region of 

 the eye, consequently, is stimulated by light proceeding from below, as 

 on a white background, and causes the fish to blanch. Light coming pre- 

 dominantly from above stimulates the ventral region of the eye, and results 

 in darkening (14, 54). 



Colour Responses in Echinoids and Annelids. The echinoids Centro- 

 stephanus longispimts and Arbacia lixula (=pustulosd) darken in the light, 

 and blanch in darkness, changes which are due to chromatophoral move- 

 ments. When illuminated, Arbacia becomes black whether the background 

 is light or dark, and turns brown after being in the dark for a few hours. 

 Centrostephamis is dark purple in the light, and changes to grey in the dark. 

 The chromatophores respond directly to illumination, and have maximal 

 spectral sensitivity at about 470 m// (Diadema setosum) (11a, lib). Another 

 species, Arbacia punctulata, does not show alterations in colour (38). 



Among polychaetes there are conspicuous melanophores in the larvae 

 of Polydora and Poecilochaetus, which expand in light and become punc- 

 tate in darkness (Fig. 11.7) (46, 75). The polychaete Platynereis dumerili 

 contains conspicuous yellow and violet chromatophores, which by dis- 

 persion and contraction alter the colour of the worm. The chromatophores 

 become punctate in darkness, or in the absence of the eyes, and expanded 

 when the intact animal is illuminated. By their expansion they serve as a 

 shield to protect the animal against excessive illumination, rather than to 

 adapt it to a particular background (14, 54). It is in cephalopods, Crustacea 

 and fishes, however, that colour changes are most marked, and these three 

 groups will be treated separately. 



Colour Responses of Cephalopods. The chromatophores of cephalopods 

 are localized in the dermis, and are more abundant on the upper surface 

 than below. Their structure is remarkably complex, for each one consists 

 of a central spherical sac surrounded by a system of radially arranged 



