94 BRITISH FRESH-WATER FISHES 
Second Sub-Order : ANACANTHINI PLEURONECTOIDEI: THE SPINELESS 
Si1pE-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. 
PLEURONECTIDA: 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 Pleuronectide, 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 
