COLOUR CHANGES 511 



always more contracted than on the upper surface, and experimental 

 analysis has revealed that this is a postural response involving tactile 

 stimulation of the lower surface, and reflex action through the central 

 nervous system. 



Sensory impulses from these peripheral fields impinge upon the brain 

 where there is a hierarchy of three chromatic centres, inhibitory and general 

 colour centres in the supra-oesophageal ganglia, and chromatophore motor 

 centres in the suboesophageal ganglia. The supra-oesophageal chromato- 

 phore centres are symmetrically disposed, and each may act on either side 

 of the body. These higher centres receive information from optic, tactile 

 and static sense organs. The suboesophageal chromatophore centres — ■ 

 located in four lobes — contain the cell bodies of the chromatophore effer- 

 ent nerves, which form the final common pathway to these structures. One 



# 



(b) (c) 



Fig. 12.3. Behaviour of a Dark Chromatophore in the Shrimp 

 Palaemonetes vulgaris on Different Backgrounds 



(a) Completely expanded chromatophore on a black background ; (b) same chromato- 

 phore on a white background for 30 min; (c) completely contracted chromatophore 

 after shrimp has been on a white background for 2 hours. (Drawn from photographs of 

 Perkins, 1928.) 



pair of lobes supplies the chromatophores of the visceral mass ; the other, 

 those of the head and arms. Each suboesophageal chromatophore centre 

 innervates only the chromatophores of its own side of the body, and the 

 different centres cannot substitute for one another. 



From the motor centres fibres proceed in the several nerve trunks to the 

 periphery of the body. The control of the complex colour changes of these 

 animals through differential activities of several kinds of chromatophores 

 — red, yellow, brown and black — must depend upon several categories of 

 nerve fibres supplying the various kinds of peripheral colour cells, as well 

 as considerable refinement of central control. By means of these discrete 

 nerve fibres, temporal and spatial regulation of colour alterations and 

 neutral shading is accomplished (7, 54). 



Colour changes in cephalopods are very rapid, and often take the form 

 of waves passing over the body. Measurements by a photo-electric method 

 have shown that the change from complete contraction to expansion of a 



