368 
SCANDINAVIAN FISHES. 
that under certain circumstances, e. g. if a species were 
never enabled to exercise it for several generations, it 
may entirely disappear. Furthermore, the same stock 
may eventually be found to have given rise to two 
distinct races, the one deeply and the other faintly 
coloured, according to the different bottom on which 
each of them has taken up its abode; and both of them, 
through want of practice, may have lost their ancestors’ 
power of changing colour. In this manner we may 
explain the origin of several varieties recognised by 
zoology, as due merely to the influence of habit. It 
is even probable that from this point of view we may 
be compelled to revise many species that have been 
selves are generally larger on the blind side. Most 
often too, the muciferous ducts of the head are more 
developed on this side than on the eye side. 
The scales of the body vary considerably in de- 
scription, but are generally well-developed, so much so 
that in this respect, too, the Flatfishes take the same 
place among the Malacopterygian Physoclysts as the 
Scale-finned fishes (fam. Squamipinnes) among flic Acan- 
thopterygians. Sometimes the scales are middle-sized, 
but in that case often deciduous, sometimes small arid 
firmly embedded in the skin. As a rule the scaly co- 
vering of the body advances over the base of the un- 
paired fins, either on the rays or on the membrane 
Fig. 104: Schematic figure of the cephalic system of the lateral line in a Cod: fig. 105: Similar figure of the head of a Turbot. After 
Tkaquair. so, suborbital branch; m, mandibular branch; fr, frontorostral branch; c, connecting branch between the two frontorostral branches; 
spt, supratemporal branch. 
established merely on the strength of characters of this 
description.” 
The sense of sight being now exclusively confined 
to one side of the body, the coloration of the body 
follows it, the other side of the body becoming albinistic. 
At the same time too, the great muscular mass of the 
body is developed more on the eye side than on the 
blind side. In most cases the paired tins are also re- 
duced most and first on the blind side; but sometimes 
not only the beginning of the dorsal fin, but also the 
terminations of this fin and of the anal fin cross over 
to the blind side. The dental equipment of the jaws 
is also stronger, as a rule, on the jaw-bones of this side 
than on those of the eye side, and the jaw-bones them- 
between them; and the head, too, is usually covered 
with scales right out on the snout. In this respect, 
too, the blind side is generally inferior to the eye side; 
and the scales are often wanting on the blind side of 
the head. Sometimes the scales are cycloid (smooth- 
margined), sometimes ctenoid (rough-margined), and 
sometimes a portion of them are changed into spinous 
tubercles. In those of the species with ctenoid scales 
with whose changes of development we are acquainted, 
the scales are cycloid over the whole of the body du- 
ring youth; while in older specimens they are ctenoid 
on the entire eye side or some part thereof, but on 
the blind side may remain cycloid or be changed to 
ctenoid scales there as well. 
