348 REPORT OF THE COMMISSIONER OF FISHERIES. e 
solution, probably in the plasma alone. One hundred yolumes of 
arterial blood hold some twenty volumes of oxygen, but only from one 
to two volumes of nitrogen (Foster, 1895, pp. 586, 601). 
The physiology of respiration in cold-blooded animals is not so com- 
pletely known, but the broad facts cited above apply equally to fishes. 
There is the interchange of oxygen and carbon dioxid, the corpuscle 
with hemoglobin as the carrier of the oxygen and a set of vascular 
filaments fulfilling the same office as the lungs. The gillsare immersed 
in water instead of air, but this does not greatly alter the nature of 
the breathing process. The blood merely gives up carbon dioxid to 
and takes up exygen from a solution of these gases in water instead of 
directly to and from an atmosphere which they partially constitute. 
The epithelium of the gill filament is the osmotic membrane, and in 
this case the osmotic pressure of the oxygen and of the nitrogen 
depends upon the amount of these gases in solution in the water and 
not directly on the atmospheric pressure, though the latter has an 
influence on the amount of air dissolved in the water. The nitrogen 
is not known to play any part in respiration and the plasma probably 
remains with a fairly constant quota of this gas corresponding to the 
amount of nitrogen dissolved in the water, which is usually air- 
saturated with it. In water recently boiled and containing scarcely 
any oxygen the osmotic pressure due to oxygen is practically nothing, 
and in this fishes suffocate. The highest osmotic pressure under 
ordinary conditions experienced by fishes occurs when water at the 
freezing point—or slightly colder, since salt-water fishes can live in 
water below 0° C.—is so well aerated that it has dissolved all the air 
it will hold at whatever atmospheric pressure exists. Of fishes in 
higher osmotic pressures than this no cases are known to the writers 
save those here described, and experimental observations under such 
conditions seem not to have been made. 
SYMPTOMS AND LESIONS OF THE GAS DISEASE IN FISHES. 
The occurrence in fishes of lesions of a gaseous nature is no recent 
observation. A certain exophthalmia known in fish-cultural parlance 
as ‘‘pop-eye” has long been recognized and is due in many cases to 
the presence of a gas either behind the eyebal] or within it. This 
may be accompanied by inflations of the mucous membrane lining the 
mouth cavity or of the skin elsewhere, and these lesions may exist 
independently of the so-called pop-eye. At the Woods Hole station 
of the Bureau of Fisheries these symptoms have been observed during 
the summer for years among marine fishes held in aquaria for pur- 
poses of exhibition, and have been described by Gorham (1899). In 
very cold water at the same place, other conditions remaining the 
same, the course of the disease is more rapid and the symptoms some- 
what different. In aquaria of sea water a few degrees above the 
