CHAPTER XIV. 



COLOURATION OF VERTEBRATA. 



THE vertebrata, as their name implies, are distinguished by the 

 possession of an internal skeleton, of which the backbone is the 

 most essential part, and the general, but not universal, possession of 

 limbs or appendages. 



Consequently we find that the dorsal and ventral surfaces are 

 almost invariably coloured differently, and the dorsal is the darker 

 in the great majority of instances. Generally the spine is marked 

 by a more or less defined central line, and hence this system of 

 colouration may be termed axial, because it is in the direction of the 

 axes, or applied about the axes. 



Fishes. Where fishes have not been modified out of their original 

 form, as are the soles, plaice, and other flat fish, we find the dorsal 

 region darker than the ventral, and even here the under surfaces are 

 the lightest. Even in cases like the Char, Fig. 1, Plate IX., where 

 vivid colour is applied to the abdomen, the dorsum is the darker. 

 The dorsum is often marked by a more or less well-defined dark 

 band, as in the mackerel and perch, Fig. 2, Plate IX. There are 

 sometimes parallel bands at right angles to the above, as in the 

 perch and mackerel ; and this is a common feature, and apparently 

 a very old one, as we find it in the young of fishes whose adults are 

 without these rib-like marks, such as the trout and pike. 



It is only necessary to inspect any drawings of fishes to see that 

 their colouration is on a definite principle, although rather erratic. 

 Important functional parts, like the gills, fins, and tail, are generally 

 marked in colour more or less distinctly, as may be seen, for instance, 

 in our common fresh-water fishes, like the roach and perch. The 

 line of mucus-secreting glands running along the sides is usually 



