COLOUR CHANGES 



509 



muscle fibres (Fig. 12.1). The sac is a pigment cell with an elastic wall, and 

 it enlarges by contraction of the adherent muscle fibres, and shrinks to a 

 minute sphere due to the elasticity of its wall when the muscle fibres relax 

 and elongate. 



Chromatophore pigments are black or brown, red, orange and yellow 

 in colour. They belong to the recently characterized group of ommo- 

 chromes, which are phenoxazone pigments found previously in insects 

 and Crustacea. In Sepia the chromatophores lie in three layers, which 

 can be distinguished by the colour of their pigments. In the outer layer the 

 chromatophores contain a bright yellow pigment, in the middle layer an 

 orange-red pigment and in the inner layer a brown or blackish pigment. 

 In addition, there are immobile iridophores which contain iridescent 



(a > (b) 



Fig. 12.1. Chromatophores with Attached Muscle Fibres 



in the Skin of Loligo 



In (a) the muscle fibres are relaxed and the pigment cell contracted and punctate; 

 in (b) the muscle fibres are contracted, and the pigment cell expanded. (From Bozler, 

 1928.) 



guanine particles, and which form sheets in the skin beneath the other 

 chromatophores and on many internal organs (35, 48). 



The radial muscle fibres are supplied by nerves which control their con- 

 tractions. Stimulation of chromatophoral nerves causes the chromato- 

 phores to expand, and severing the nerves causes the denervated area to 

 become pale. It has been maintained that the muscle fibres can show slow 

 tonic contractions, as well as quick single contractions and tetanus. The 

 initial blanching which follows section of a mantle nerve in cephalopods is 

 succeeded by a gradual darkening of the affected area, and it is the opinion 

 of some workers that this darkening phase is due to peripheral autotonus 

 of the radial muscle fibres. Similarly, when a piece of skin is removed, the 

 chromatophores at first contract but afterwards pass into a state of partial 

 expansion. Electrical stimulation of this isolated piece of skin causes con- 

 traction of the partially expanded chromatophores. Moreover, microscopic 

 examination shows that individual chromatophores in the preparation 

 often exhibit rhythmical pulsations. From one viewpoint the dark phase of 

 the animal results from peripheral autonomous tonicity of the radial 



