258 BONY FISHES ix. 9- 



thc trunk-fishes (Ostracioti), one species of which is described as 

 having a green body, yellow belly, and orange tail, while across the 

 body are bands of brilliant blue, edged with chocolate-brown. More- 

 over, the female has another colour scheme and was for long con- 

 sidered as a different species! 



Colour differences between the sexes are frequent in fishes, the male 

 being usually the brighter. Thus in the little millions fish, Lebistes, 

 there are numerous 'races' of males with distinctive colours, but the 

 females are all of a single drab coloration. The genetic factors that 

 produce the various types of male are carried in the Y chromosome. 

 Presumably the colour of the males acts as an aphrodisiac as a part 

 of the mating display, but the significance of the different races is not 

 known. 



Sematic or warning coloration involves the adoption of some strik- 

 ing pattern that does not conceal but reveals the animal. This type 

 of colouring is found in animals that have some special defence or 

 unpleasant taste (such as the sting of the wasp), and its use implies that 

 animals likely to attack are able to remember the pattern and the 

 unpleasant effects previously associated with it. It is not easy to be 

 certain when colours are used in this way, but it is possible that the 

 conspicuous spots on the electric Torpedo ocellata have this function. 

 Among teleosts there is the black fin of the weevers {Trachinus), pos- 

 sibly a warning of their poison spines, and the spiny trigger-fishes 

 and globe-fishes (p. 253) also have conspicuous colours. 



10. Colour change in teleosts 



In spite of the reputation of the chameleon the teleosts are the 

 vertebrates that change their colour most quickly and completely. 

 The melanophores are provided with nerve-fibres (Fig. 154), and 

 these cause contraction of the pigment and hence a paling of the skin 

 colour. The processes of the cells themselves are not withdrawn, the 

 colour change is produced by a movement of pigment within them. 



The nerve-fibres in question are post-ganglionic sympathetic fibres, 

 leaving the ganglia in the grey rami communicantes (Fig. 139) to all 

 the cranial and spinal nerves. The pre-ganglionic fibres that operate 

 them, however, emerge only in a few segments in the middle of the 

 body (Fig. 235), so that severance of a few spinal roots will affect the 

 colour of the whole body. When a nerve to any part of the skin is cut 

 the chromatophores in that region at first expand, making a dark area 

 (Fig. 155). After a few days, however, the skin involved gradually 

 becomes lighter, the process, it is alleged, beginning at the edges and 



