The Book of Household Management by Mrs. Beeton

organs which are placed on each side of the neck, and which are called gills. It is curious to watch the process of breathing as it is performed by the finny tribes. It seems to be so continuous, that it might almost pass for an illustration of the vexed problem which conceals the secret of perpetual motion. In performing it, they fill their mouths with water, which they drive backwards with a force so great as to open the large flap, to allow it to escape behind. In this operation all, or a great portion, of the air contained in the water, is left among the feather-like processes of the gills, and is carried into the body, there to perform its part in the animal economy. In proof of this, it has been ascertained that, if the water in which fishes are put, is, by any means, denuded of its air, they immediately seek the surface, and begin to gasp for it. Hence, distilled water is to them what a vacuum made by an air-pump, is to most other animals. For this reason, when a fishpond, or other aqueous receptacle in which fishes are kept, is entirely frozen over, it is necessary to make holes in the ice, not so especially for the purpose of feeding them, as for that of giving them air to breathe.