102 MARVELS OF FISH LIFE 
through the overflow hole. I show an illustration of the 
ditch from which the food-laden water was gathered, 
and the box below it. Owing to the kindness of my 
friend, Mr. Mower, who visited these fish morning and 
evening, and cleared the box and pipes of scum, this 
simple apparatus never failed the whole summer. 
As the water in the box was only nine inches deep 
the eggs, larve, and young fish could easily be watched, 
and specimens gathered when required for photographic 
purposes. 
Without going into minute details, which would be 
out of place in a book of this kind, I will describe the 
early history of the roach during the first year of its 
life, and this may be taken as an example of the carp 
in our ponds and rivers. Before doing so, however, it 
is necessary to say a few words about the gas bladder of 
fishes, for the habits of the roach before and after this 
appears, are entirely different. Almost all bony fishes 
possess a bladder, which runs along the body just under 
the backbone. This bladder though commonly known 
as an air bladder, does not contain air, but mainly 
oxygen gas, and its presence in the body of the fish 
makes it, bulk for bulk, the same weight as water. Thus 
the fish floats, and having no weight to support, swims 
with the minimum of exertion. A fish is also able to 
diminish or increase the amount of gases in this bladder 
so as to float in different depths of water. The gas 
bladder of a carp has a constriction in the middle as 
shown in the illustration on the opposite page. 
