PHYSIOLOGICAL 421 



sympathetic nervous system. Blind flat-fishes do not change colour. 

 The message from the outside world affects the eye, then the brain, 

 then the sympathetic nervous system, then the pigment-cells. 

 There is sometimes an enveloping of the pigment-cell with the fine 

 terminal branches of nerve-fibres, an innervation which has never 

 been demonstrated in amphibians. Here, then, are three classes 

 of vertebrates, and the predominant control of the colour change 

 is effected in three different ways! It is possible, however, that 

 there may be auxihary controls. In crustaceans, like the Msop 

 prawn, the chromatophores are usually multicellular, and they 

 often contain more than one kind of pigment, with different sus- 

 ceptibilities and rates of movement. There is a regular rhythm 

 between practical transparency at night and expansion of pigment 

 during the day, and this is due to the direct action of the light on 

 the skin. On the other hand, a response of the crustacean to the 

 nature of its background comes about through the eyes and the 

 central nervous system, the important point being, not the intensity 

 of the light, but the incidence of the rays. In cuttlefishes, which 

 are molluscs, the chromatophores are little multicellular bladders, 

 often visible to the naked eye, and surrounded by radiating muscle- 

 fibres. The play of colour on an octopus is extraordinarily beautiful. 

 There is evidence here that the central nervous system controls 

 the colour change, but there is also strong evidence that the muscle- 

 fibres of the chromatophores may respond directly to the light 

 and differently to different wave-lengths. It is very interesting 

 that there should be such variety of arrangement in effecting the 

 same result, namely, the colour response of the animal to environ- 

 mental change. A fine exposition is given by Hogben in his Pig- 

 mentary Effector System (1924). Where the control is effected by 

 a hormone from the pituitary or the suprarenal, it will be necessary 

 to explain how the environmental stimulus reaches such deeply 

 seated organs from the superficial receptors. 



ORGANIC LUMINESCENCE 



It has been shown that with the contraction of muscle and 

 the secretory activity of glands, with the closing of Venus's Fly- 

 trap (Dionaea) and the collapse of the leaf of the Sensitive Plants, 

 electrical changes are associated. In all vital phenomena there are 

 probably changes of electric potential. But in most cases these 

 electrical changes are not in themselves of importance in the life 

 of the animal or plant; they are merely accompaniments or con- 

 comitants of the movement or the secretion. When, however, as 

 in the Torpedo or the Electric Eel, there is a special and well- 

 defined electric organ which can give a shock, useful in defence and 



