LIGHT AND PIGMENTATION 



95 



chromatophorotropiiis antagonizing and supplementing each other, 

 neither species- nor genus -specific, secreted in various jjarts of the 

 neuro-secretory system ^ (Fig. 08). In some cases the process is more 

 complex and adaptation to the background is achieved by the produc- 

 tion of different hormones when the dorsal or the ventral aspect of the 

 retina is stimulated (the Isopod, Ligia oceana — Smith, 1938). 



In CYCLOSTOMES and the more primitive fishes such as the 

 SELACHIANS - liomiones derived from the pituitary seem to be the 

 only active agents in the control of the chromatophores, a pigment- 



Ligia 



Sun ^^ 



Background 



Fig. 69. — The Controlling Mechanism of the JMelanophores of a 

 Fish (the Eel, Asouilla). 



The direct light from the suti strikes the ventral portion of the retina, VR, 

 while the light reflected from the background strikes the dorsal portion, DR. 

 From the retina nerve paths lead to the central nervous system, CNS, and 

 thence directly to the chromatophores by adrenergic nerve fibres, AN, and 

 cholinergic nerve fibres, CN. A second relay is made to the pituitary body. 

 Pit, from which hormones are flistributed through the lilood vessels, BV. 

 CM is a chromatophore with its melanin concentrated ; DM with its melanin 

 dispersed (after Parker). 



dispersing agent (intermedin) causing darkening and a pigment- 

 concentrating factor causing blanching ; the colour of the fish is 

 determined by the ratio of these two antagonistic substances in the 

 blood, this ratio being in turn controlled by visual stimuli through the 

 differential effects of dorsal and central stimulation of the retina 

 (Zondek and Krohn, 1932 ; Lundstrom and Bard, 1932 ; Young, 



1 The shrimps, Crago and Pala'inon, the fiddler crab of the Pacific coast of N. 

 America, Uca, the lobster, Homarus, etc. (Hanstrom, 1933-35 ; Brown and his colleagues, 

 1933-52 ; Webb et ah. 1951). 



^ Cyclostomes, such as the lamprey, Lampeira — Young (1935) ; Selachians, such 

 as the skate, liaja — Parker (1937), and the dog-fish, Scyllium — Young (1933). 



