SOME FACTS ABOUT FISHES. 
43 
or pale colour, while the opposite surface, which is turned 
upwards, is of a darker tint, and apparently coloured to secure 
concealment by its resemblance to the surrounding sand, &c. 
In many fishes — perhaps in most — we may remark a similar 
difference between the coloration of the upper and lower sur- 
faces ; hut in these it is the hack and the belly that afford the 
contrast, whereas in the Flat-fishes the two sides of the body 
are thus differently coloured, and the animals swim and lie 
upon the bottom in a position at right angles to that taken by 
all other fishes. The long fins which fringe the two edges of 
the body of a sole or turbot, are really the dorsal and anal fins. 
In order to fit the Pleuronectid fishes for this peculiar con- 
dition of life the two eyes are both placed upon the same side of 
the head, the coloured or upper side, where alone under ordinary 
circumstances they could be of any use to the animal ; and this 
is associated with a curious contortion of the bones of the skull, 
and a twisting of the mouth of the fish, such as often gives it a 
very singular aspect. When very young, however, the Flat- 
fishes do not present any such peculiarities : they are perfectly 
symmetrical, transparent little creatures, having one eye on 
each side of the head like other fishes, and they swim in a ver- 
tical position, like any other deep-bodied member of their class. 
The change from the symmetrical condition of the head in the 
young fish to the distorted form which it presents in the adult, 
takes place at a very early period, but how this wonderful con- 
version is effected does not seem as yet to be quite satisfactorily 
made out. 
4 Whilst some naturalists/ says Dr. Gunther, ‘ believe that 
the eye turning round its axis pushes its way through the 
yielding bones from the blind to the upper side, others hold 
that, as soon as the body of the fish commences to rest on one 
side only, the eye of that side, in its tendency to turn towards 
the light, carries the surrounding parts of the head with it ; in 
fact, the whole of the fore-part of the head is twisted towards 
the coloured side, which is a process of but little difficulty as 
long as the framework of the head is still cartilaginous.’ * 
A j 'priori , the second explanation seems to be by far the more 
probable one, and we must confess that we thought it had been 
finally established by Dr. Traquair in his valuable memoir 
published by the Linnean Society, t 
This change in the Pleuronectidse, from its nature, the 
importance of the organs affected by it, and its singularly 
abnormal results, may perhaps be regarded as an extreme in- 
stance of ‘ hemimetamorphosis but from the very early period 
in the life of the fish at which it occurs, and its uniformity 
* Introduction to the Study of Fishes, p. 553. 
t Linnean Transactions , vol. xxv. p. 263. 
