FISHES, 215 



side to all assailants.* Cuvier doubts whether the Diodon, 

 when in this position, is able to swim; but Mr. Darwin's 

 observations show that it can not only move forward in a 

 straight line, but that it can also turn to either side.f 



COVERING. Most fishes are covered with scales, which 

 differ considerably in their shape, and are yet so uniform in 

 each particular kind that they serve as valuable aids in the 

 discrimination of species. Those along the well-marked line 

 observable on both sides of the body are distinguished from 

 the others in shape, and each of them is found to be pierced 

 with a small hole, which is, in fact, the extremity of a tube. 

 Through these orifices a mucus or slime is emitted. This 

 forms a coating to the body, and diminishes the friction of its 

 passage through the water. These apertures are, in general, 

 larger and more numerous about the head than over the other 

 parts, and may be regarded as one of those beautiful pro- 

 visions of Nature which we are permitted so frequently to 

 observe and to admire, "Whether the fish inhabits the 

 stream or the lake, the current of the water in the one 

 instance, or progression through it on the other, carries this 

 defensive secretion backwards, and spreads it over the whole 

 surface of the body."J The scales are sometimes marked 

 with minute lines, possess a varying metallic lustre, and 

 exhibit a diversity of brilliant colours, which render them 

 highly attractive objects. The poet is perfectly accurate 

 when he describes fishes, which, 



" Sporting with quick glance, 



Show to the sun their waved coats dropt with gold." MILTON. 



Thus the wide-spreading sea has in its waters tribes of beings 

 fitted for that element, and scarcely, if at all, inferior in 

 richness of colouring, variety of figure, or grace of move- 

 ment, to those which are the admired denizens of the air. 



* M. Edwards' "Siemens," p. 305. Koget's Bridgewater Treatise, 

 p. 433. 



f Darwin's Journal, p. 13. "Voyages of the Adventure and Beagle." 



j Yarrell's History of British Fishes, p. 4. 



The brilliant metallic colours of the scales of fishes are thus accounted 

 for by Dr. J. L. Drummond: "The scales of fishes are pellucid; and 

 their brilliant appearance is owing to a thin film which covers the under 

 side of each scale, and is entirely formed of spicula, as is easily proved by 

 scraping off a quantity of scales, and agitating them in water with a stick 

 or other body, so as to detach the films. The water will then be found to 

 contain thousands of moving spicula, which in the sunshine may be dis- 



