89 
This is very apparent on many of the Gadidas genus—such as 
the pollack, haddock, whiting, and others ol the same race. 
The Pleuronectidae also show signs of the same periodic 
growth, but not quite so markedly as those of the Gadidae. One 
of the most interesting points to be noticed in the Pleuronectidae, 
or fish swimming on their sides, is the strange metamorphosis 
from a round to a flat fish. They are all hatched as round fish, 
but after six weeks from their birth become swimmers on their 
side, and in the case of soles, plaice, dabs, etc., the left eye moves 
over to the right side, the fish expands in breadth, and 
tilts over on its side, gradually getting deeper in the water till 
about the fortieth day after hatching it matures into a flatfish, 
assumes a brown colour on its top side and white underneath, and 
is then about three inches long. 
The white colour on the under side is accounted for in this 
way. Mr. Cunningham, whose valuable book on the Sole is so 
well known, conducted experiments to show that the white under 
side was entirely owing to the shutting off, so to speak, of the 
rays of light from that portion of the fish. He arranged a series 
of mirrors so as to reflect light underneath a tank at the Laboratory 
at Plymouth, where some plaice were confined, and caused light 
to be reflected to the under side of the fish, and so successful was 
the experiment that he caused the under side to become the same 
colour as the upper side, proving that the absence of rays of light 
to that portion of the fish was the undoubted cause of the white 
appearance. This same experiment is brought home to us by 
tying up a lettuce to bleach the interior, or by putting bulbs in a 
cupboard where light is shut off, and the growth wilPbecome more 
or less bleached. 
Flatfish also have the power of assuming the coloration of 
the sand or bed on which they rest. This is caused by a series of 
pigmentary cells on the brown side having the power of exuding 
the right colour of the sand-bed, and thus the fish becomes prac¬ 
tical!} 7 invisible to its enemies. This is called protective colouring. 
The scales of sea-fish are divided into two classes—cycloid, or 
round-edged, and ctenoid, or comb-like—owing to the spines ,on 
the exposed portions of the scales resembling the teeth of a comb. 
I hese spikes are readily felt in passing the hand over the surface 
of a sole, a dab, and other fish. 
Otoliths. 
Every fish has two little stone-like substances in its head, 
which are called otoliths, or ear-stones, and by means of which fish 
are sensible to the vibrations of the water, much in the same way 
as human beings are sensible to the vibrations of the air, which we 
call hearing. These otoliths are to be found in two little cells at 
the back of the brain, and attached to it by .means of verv fine 
ligaments, so that they are in communication with the brain, and 
are similarly connected with the spinal column. They van - in size 
to a great extent—tor instance, the otolith of a 151b. cod weighs 
