The homology of the air-bladder with the lung is evident. This is often expressed in the phrase that the lung is a developed air-bladder. This is by no means true. To say that the air-bladder is a modified and degenerate lung is much nearer the truth, although we should express the fact more exactly to say that both air-bladder and lung are developed from a primitive cellular breathing-sac, originally a diverticulum from the ventral walls of the oesophagus. The air-bladder varies in size as much as in form. In some fishes it extends from the head to the tail, while in others it is so minute as to be scarcely traceable. It often varies greatly in closely related species. The common mackerel (_Scomber scombrus_) has no air-bladder, while in the closely related colias or chub mackerel (_Scomber japonicus_) the organ is very evident. In other families, as the rockfishes (_Scorpænidæ_), genera with and those without the air-bladder are scarcely distinguishable externally. In general, fishes which lie on the bottom, those which inhabit great depths, and those which swim freely in the open sea, as sharks and mackerel, lack the air-bladder. In the sharks, rays, and chimæras there is no trace of an air-bladder. In the mackerel and other bony fishes without it, it is lost in the process of development. The air-bladder is composed of two layers of membrane, the outer one shining, silvery in color, with muscular fibres, the inner well supplied by blood-vessels. The gas within the air-bladder must be in most cases secreted from the blood-vessels. In river fishes it is said to be nearly pure nitrogen. In marine fishes it is mostly oxygen, with from 6 to 10 per cent of carbonic-acid gas, while in the deep-sea fishes oxygen is greatly in excess. In _Lopholatilus_, a deep-sea fish, Professor R. W. Tower finds 66 to 69 per cent of oxygen. In _Trigla lyra_ Biot records 87 per cent. In _Dentex dentex_, a shore fish of Europe, 40 per cent of oxygen was found in the air-bladder. Fifty per cent is recorded from the European porgy, _Pagrus pagrus_. In a fish dying from suffocation the amount of carbonic-acid gas (CO2) is greatly increased, amounting, according to recent researches of Professor Tower on the weak-fish, _Cynoscion regalis_, to 24 to 29 per cent. This shows conclusively that the air-bladder is to some degree a reservoir of oxygen secreted from the blood, to which channel it may return through a kind of respiration. The other functions of the air-bladder have been subject to much question and are still far from understood. The following summary of the various views in this regard we copy from Professor Tower's paper on "The Gas in the Swim-bladder of Fishes":