224 A HISTORY OF FISHES 



venomous Sea-snakes, for which it would be to their advantage 

 to be mistaken. The Wcevcr-fish {Trachinus) provides another 

 example of warning coloration, the dorsal fin, which is the 

 only part of the fish visible when it is buried in the sand, being 

 intense black in colour, and in contrast with the pale yellow 

 and brown tints of the rest of the fish, and of the surrounding 

 sand, is clearly visible from a considerable distance (Fig. 58A). 

 Upon provocation this fin is erected and spread out in a 

 conspicuous manner, and it is suggested that this acts as a 

 danger signal to warn predatory fish of the Weever's where- 

 abouts, fish which might otherwise mistake it for a harmless 

 species of similar size and habits. It is significant that the 

 Dragonet {Callionymus) , a fish very like the Weever in many 

 respects, and with the same burying habits, is frequently taken 

 from the stomachs of Gurnards, but rarely, if ever, the Weever 

 itself The Common Sole [Solea) has a deep black patch on the 

 pectoral fin of the upper side, and, when alarmed, is in the 

 habit of burying itself in the sand, and raising this fin vertically 

 like a small fiag. It has been suggested that the pectoral fin of 

 the Sole mimics the dorsal fin of the Weever, and the fish is 

 thus left severely alone. In the Star-gazers {Uranoscopidae) 

 and Flat-heads {Platycephalidae) , likewise armed with poisonous 

 spines, the dorsal fin is generally black and is the only part 

 visible when the fish is buried in the sand. 



The colours of a fish are mainly due to the presence in the 

 dermal layer of the skin, either above or below the scales, of 

 numerous pigment-containing cells known as chromatophores 

 (Fig. 90) . Each of these has the form of a small sac with thin 

 and highly elastic walls, supplied with fine strands of muscle 

 fibres associated with delicate nerve-endings. By the expansion 

 or contraction of these fibres the sac may be drawn up into a 

 minute globe, or flattened out to form a relatively large disc. 

 The granules of pigment deposited in each of the chromato- 

 phores may be either red, orange, yellow, or black, and other 

 shades are produced by the combination or blending of two or 

 more of these primary colours. The exquisite green colour on 

 the back of the Mackerel {Scomber) is due, not to a green 

 pigment, but to a mixing of black and yellow chromatophores 

 in suitable proportions : in the same way, a blending of yellow 

 and black, or of red and black, may give a brownish coloration, 

 and by the appropriate mixing of chromatophores of different 

 colours almost any shade is produced. A black spot or stripe 

 may be due either to the concentration of the black chromato- 



