348 EEPOET OF THE COMMISSIONER OF FISHERIES. 



solution, probabl}' in the plasma alone. One bundrcd volumes of 

 arterial blood bold some twenty volumes of oxygen, but onh^ from one 

 to two volumes of nitrogen (Foster, 1895, pp. 5S0, 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 gills are immersed 

 in water instead of air, but this does not greatl}" alter the nature of 

 the breathing process. The blood merel}^ gives up carbon dioxid to 

 and takes up oxygen 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 oxj'gen the osmotic pressure due to ox3^gen is practically nothing, 

 and in this fishes sufi'ocate. The highest osmotic pressure under 

 ordinar}" 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 eyeball 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-e^^e. At the Woods Hole station 

 of the Bureau of Fisheries these symptoms have been observed during 

 the summer for 3'ears 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 dififerent. In aquaria of sea water a few degrees above the 



