LIGHT AND PIGMENTATION 



93 



dominated by the nervous mechanism as in Gephalopods such as the 

 octopus or the more highly developed Teleosteans such as the cat- 

 fish. The hormones {chroniatophorotropins), of which there may be 

 more than one with mutually antagonistic reactions, are elaborated in 

 Crustaceans in the neuro -secretory cells formed in the eye-stalks and 

 elsewhere in the central nervous system ^ ; in Vertebrates the pituitary- 

 hypothalamic complex ^ is the primary source although the adrenals 

 may provide an antagonistic element. The nerve supply may be 

 simple with one tj^^je of fibre which is pigment -concentrating as in the 

 dogfish or the chameleon, or it may be dual comprised of two opposing 

 types of fibre, one resembling adrenalin with a pigment-concentrating 

 (symjDathetic) action being antagonized by a second resembling acetyl- 

 choline with a pigment-dispersing (parasympathetic) effect. 



Among ANNELIDS, such as some polychsete worms (Hempelmann, 

 1939) and leeches (Wells, 1932 ; Janzen, 1932 ; Smith, 1942), most of 

 which become pale in darkness and dark when illuminated, the evidence 

 suggests that the control is primarily nervous, correlated most effec- 

 tively through the ocelli at the anterior end but operated less efficiently 

 by widely distributed photoreceptors through segmental reflexes. 

 Thus if a leech is decapitated or stimulated faradically, a pale animal 

 kept in darkness will become pigmented, but if the nerve-cord is 

 truncated the change passes only to the level of trans- section ; while 

 decapitated animals show the same responses as normal animals but 

 respond more sluggishly (Smith, 1942). Among these animals there 

 is no evidence of a response to the background. 



In CEPHALOPODS there is a slowly acting hormonal control, 

 probably mediated by substances of the nature of tyramine and 

 betaine ; the former has an adrenalin -like action increasing the tonus 

 of motor centres and producing a dark coloration, while the latter, like 

 acetylcholine, decreases the tone of the chromatophores and lightens the 

 animal. The injection of these substances produces the same colour 

 changes as also does the transference of blood from a dark to a light 

 animal (Sereni, 1928-30). This simple and fundamental chemical 

 action, however, is dominated by nervous activity ; the stimulus is 

 received primarily through the eyes, control being maintained through 

 centres in the cerebral and suboesophageal ganglia, the isolation of 

 which by nerve section stops all colour change in the area affected, and 

 the response is effected probably by a double innervation, both 

 excitatory and inhibitory, to the muscles of the chromatophores 

 (Phisahxi 1892-94 ; Sereni, 1927-28 ; Bozler, 1928-29). 



Among INSECTS the control is both hormonal and nervous and the 

 part played by the eyes varies. In this respect the stick-insect, 

 Dixippus, has been investigated most extensively (Giersberg, 1928). 



' p. .').52. - p. 556. 



Leech 



]!ephalopod 

 (Octopus) 



