§ 3.22 EFFECTORS WITH MOVABLE PIGMENT GRANULES 73 



If this were the whole story, cephalopod chromatophores would 

 deserve no place in the present context; but overall changes of 

 colour can also be produced by chemicals in the blood. Tyramine 

 from the posterior salivary glands is normally used as poison for 

 paralysing the prey, but is also associated with pigment dispersion, 

 so that animals usually become much darker in colour when the 

 salivary glands are active ; betaine in the blood is associated with 

 pigment concentration. Moreover, of the three Mediterranean 

 octopuses, Eledone moschata and Octopus macropus are normally 

 well-coloured species, but Octopus vulgaris is pale and habitually 

 has less tyramine in its blood. Transfusion of blood from either of 

 the first two into the last of these species results in darkening. 

 Denervated chromatophores, on the other hand, are completely 

 insensitive to these chemicals (Bacq and Ghiretti, 1951). Tyramine 

 and betaine can therefore be better compared to "para-activators" 

 (§ 1.2) than to true hormones, in that they seem to act through the 

 nervous system and not directly upon the effectors. Their mode of 

 action is still uncertain; Sereni (1930) postulated that they might 

 act directly upon the inhibitory and excitatory centres in the brain, 

 but this has not been fully established. 



3.22 pigmentary effectors with movable 

 pigment granules 



Although varying in form and situation, these effectors all 

 contain pigment granules which move to and fro within them, 

 dispersing widely to give a large coloured area, or concentrating 

 into a limited space to give only a small spot. Dispersal here gives 

 the same effect as contraction of the muscles around a cephalopod 

 chromatophore. 



The most plausible explanation of how this granule movement 

 is brought about is that given by Marsland (1944) for the branched 

 chromatophores of fish; but it seems probable that the same 

 principle underlies all cases. The pigment granules are attached to 

 a partially gelated system of long protein molecules in the cyto- 

 plasm of the cell, and as the cytoplasm gelates fully the molecules 

 contract, drawing the granules ''as on a string bobbing through 

 the current" towards the centre of the cell, while squeezing the 



