TROUT AND GRAYLING, 63 
label to in order to identify specimens and watch their growth 
after placing them in a river. 
The tail is the fish’s organ by which it moves through the 
water, while the fins act as balances, for if you remove one of his 
pectoral fins he rolls over to that side, and if both are removed he 
is unable to keep his head up, while if the dorsal or anal fins 
are removed, he wobbles about from side to side. The line running 
along the side is called the lateral line, and it is composed of a 
row of scales, each with a small hole or tube passing through it, 
from which mucous matter or slime is excreted, which causes the 
well known sliminess of a freshly caught fish. The gills answer 
the same purpose in a fish that the lungs do in the case of the 
warm-blooded animals, that is, they are the apparatus by means 
of which the fish procures a supply of oxygen for the purifi- 
cation of its blood. The red appearance under the gill-covers 
of any fish is really a series of very delicate fringes, and the 
fish taking gulps of water in at its mouth, forces it through the 
gills, which extract the oxygen from the water so that it is taken 
up by the blood as it circulates through these delicate fringes, 
When the fish is deprived of water, these fringes stick together ; 
it is no longer able to procure oxygen for its blood, and it dies of 
asphyxiation in exactly the same manner as one of us would do if 
we were drowned. 
The Salmonide may be roughly divided into two groups, the 
first consisting of those which, like the salmon and the sea trout, 
spend part of their lives in the sea and part in the fresh water, 
and the second group consisting of those which, like the trout 
and the grayling, spend the whole of their lives in fresh water. 
They are distributed more or less all over the arctic and temperate 
portions of the northern hemisphere, but, except in places where 
they have been artificially introduced, they are practically absent 
from any part of the southern hemisphere. 
The fresh water trout is a most beautifully coloured fish, having 
purplish sides plentifully sprinkled with black and scarlet spots, 
with a golden tint on the belly ; but its colour varies very much, 
and depends to a great extent on the locality from which it is 
