94 BRITISH FRESH-WATER FISHES 



Second Sub- Order : ANACANTHINI PLEURONECTOIDEI : THE SPINELESS 



SlDE-SwiMMERS 



The second sub-order or division of the Spineless Fishes 

 contains those fish which are distinguished by the asymmetrical 

 form of the head and part of the body. It consists of a 

 single Family. 



PLEURONECTID^:THE FLAT-FISH FAMILY 



Precise expression is not only indispensable to exact science, 

 but inseparable from exact thinking, and the judgment of a 

 loose or obscure talker may safely be set down as not worth 

 having. It is generally right, therefore, to demur to the 

 epithet " extraordinary " being applied to any genus or species. 

 What is usually meant when that word is so applied, is that 

 the creature referred to is of remarkable appearance or behaves 

 in a remarkable way ; but it cannot be correct to speak of 

 an animal as " extraordinary " out of the common which 

 conforms in appearance to the normal character of its genus, 

 and complies with the uniform habits of its kind. Yet when 

 we come to consider the great family of Pleuronectid<e, or flat- 

 fishes, it is almost justifiable to describe them as extraordinary, 

 for they form a group of animals, unique in one respect among 

 all known living creatures, in that they carry both eyes on one 

 side of the head. This is the more remarkable, inasmuch as 

 these fish when hatched from the egg are perfectly symmetrical, 

 swimming in a vertical position like other fishes, and carrying 

 an eye on each side of the head. This much is certain, in 

 regard to all the flat-fishes ; the subsequent changes in posi- 

 tion and structure are still far from being clearly understood. 

 The larval forms of the different genera and species so closely 

 resemble each other as to make it difficult to pronounce upon 

 their respective parentage. All of them lose the power of 

 retaining a vertical position, partly owing to want of an air- 

 bladder, and partly from defective balancing power in the 



