MARINE VERTEBRATES 313 



eating with the pharynx as well as with the exterior by a slit-like 

 opening. 



Before leaving the external characters of fishes we must say 

 a word or two about their forms and colours. As regards the 

 former, it is well known that fishes are well adapted for rapid 

 progression through water, but there are many exceptions to this 

 rule. These exceptions, however, apply principally to those species 

 that have no need to swim rapidly, and a study of their habits 

 will show that their form is just as perfectly adapted to their mode 

 of life. They are often species that live on the bottom, or hide in 

 the crevices and holes of rocks, and examples will be given in our 

 future descriptions. 



Variations in colour are even more interesting, especially as 

 they are so commonly connected with the nature of the surround- 

 ings and the protection of the animals. In nearly all cases the 

 colour is darker on the upper surface than on the lower, thus 

 making it appear that the influence of light has something to do 

 with the formation of the pigments of the skin, and experiment 

 proves that this is, at least to a certain extent, the case ; for when 

 fishes have been kept for some time in an aquarium into which 

 light is admitted through the bottom only, pigment spots have 

 formed in the skin on the lower surface. 



Fishes that swim at the surface are generally tinted on the 

 dorsal side with some shade that closely resembles the colour of 

 the water as viewed from above, and are white and silvery below. 

 Such colouring is of course highly protective, for they are not 

 readily distinguished by the sea birds and other enemies that would 

 pounce on them from above, and are almost invisible against the 

 sky to eyes below. This form- of protective resemblance is beauti- 

 fully illustrated in the mackerel, which is barred on the back with 

 black and green, closely imitating the ripples on the surface of the 

 deep green sea, while the under side is of a silvery whiteness that is 

 hardly visible from below with the bright sky as a background. 



The flat fish afford other interesting examples, for these live on 

 the bottom, and are coloured above so as to resemble the bed on 

 which they live ; the tints being those of mud, sand, or gravel. 



But what are we to say of the gaudy colours of the gurnards, 

 rock fishes, &c. ? These are certainly not protective in all cases, 

 for we sometimes find brightly coloured species conspicuous among 

 duller surroundings. Such instances, however, are comparatively 

 rare, the gaudy species living principally among the variously 



