690 Comparative Animal Physiology 



The American species of Arhacia, Arhacia pinctnlata, appears to show 

 no color changes comparable to those just described. ^-^ 



Cephalopods. Man\' cephalopods show remarkably rapid color changes as 

 a result of the activity of their peculiar type of chromatophores. These changes 

 mav result from manv different types of stimuli, but light is one of the most 

 important. 



The chromatophores are primarily controlled by the nervous system. Cut- 

 ting a nerve innervating a particular region of the body results in an immedi- 

 ate cessation of all color changes in that region. It was shown long ago that 

 after a mantle connective is cut ^^ there is a paralysis of the chromatophores 

 of the corresponding half of the body and a consequent blanched coloration. 

 Since the connectives between mantle ganglion and the chromatophores are 

 still intact it is obvious that the ganglion contributes little to the control by 

 itself, and that the normal control over the responses resides in higher cen- 

 ters of the nervous system. The cerebral ganglion of the brain appears to 

 possess an inhibitory center for the chromatophore system;'^'*' ^^^ '"* after its 

 destruction or inactivation there is tonic expansion of the chromatophores. 

 The inhibitory center is believed to operate through control of a color cen- 

 ter^^'' located in the central ganglia, which in turn operates through motor 

 centers found in the subesophageal ganglia. The motor centers each control 

 the chromatophores of the corresponding halves of the body. 



The eyes are the chief sense organs influencing the central nervous cen- 

 ters. Bilateral blinding does not eliminate changes of color, but the changes 

 which then occur are in no sense adaptive changes. If only one eye is 

 blinded, the responses of the chromatophores on the corresponding side of 

 the body are diminished. 



Another significant source of influence on the color-control centers are the 

 suckers on the arms. If these are all extirpated there is a considerable loss 

 of tone in the chromatophores and hence skin lightening occurs.^^^ Removal 

 of both eyes and suckers, however, does not entirely eliminate the chromato- 

 phore responses. After such an operation vigorous stimuli will still result in 

 color changes, probably as a result of stimulation of tactile organs, organs 

 of equilibrium, etc. 



The chromatophores of the side of the animal that is lowermost, when 

 the animal is in contact with the substrate, always are more contracted than 

 those of the remainder of the body.^^^ This is not a direct influence of il- 

 lumination as one might first suspect, for it cannot be reversed by illumina- 

 tion from below instead of above. Rather, it appears to be part of a postural 

 response involving stimulation of tactile receptors reflexly through the cen- 

 tral nervous System, resulting in the kxralized chromatophore contraction. 



The substances tyramine and betaine are known to be present in the blood 

 of cephalopods. The former, like adrenalin, increases the tonus of the motor 

 centers, resulting in a darker coloration. ^^^ Betaine, on the other hand, 

 like pilocarpine or acetylcholine, appears to decrease the chromatophore 

 tone by stimulation of the inhibitory center. If one transfuses blood from a 

 characteristically darker species, such as Eledone or Octopus macropus, into 

 a lighter O. vulgaris, the latter darkens.*^^ Interconnection of the circula- 

 tory system of the two will yield comparable results. Tyramine is known to 

 be more concentrated in the blood of these darker species than in that of 



