FISHES. 199 



side to all assailants.* Cuvior doubts whctlior 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 dif- 

 fer considerably in their shape, and are yet so uniform in each 

 particular kind that they serve as valuable aids in the discrimi- 

 nation of species. Those along the well-marked line observ- 

 able 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 fivct, the extremity of a tube. Through 

 these orifices a mucus or slime is emitted. This forms a coat- 

 ing to the body, and diminislios 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 l^eautiful provisions 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 back- 

 wards, 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 witli 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 rich- 

 ness of colouring, variety of figure, or grace of movement, to 

 those which are the admired denizens of the air. 



* M. Edwards' "Elemens," p. 305. Roget's Bridgewater Treatise, 

 p. 433. 



t Darwin's Journal, p. 13. " Voj-ages of the Adventure and Beagle." 



J Yan-eir.s History of British Fishe.s, p. 4. 



§ The brilliant metallic colours of the scales of fishes arc thus accounted 

 for by Dr. J. L. Drumniond: — "The scales of fislies are pellucid; and their 

 brilliant ai)pearance is owing to a thin film wiiieh 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 tlieni in water with a stick 

 or other body, so as to detach the films. Tl)c water will then be foinid to 

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



