62 COMPARATIVE PHYSIOLOGY 



skin glands a very acid slime. Little is known of the 

 mechanism by which such a very high concentration of acid 

 can be attained in the cells which secrete it. We are still 

 in the dark as to the mechanism of acid secretion in the stomach 

 of the mammal. It may be presumed that the source of 

 sulphuric acid must be the sulphur of either proteins or sul- 

 phates of the food. The bionomic significance of acid secre- 

 tion in the lower organisms has been interpreted as a means of 

 softening the calcareous skeletons of animal prey or as a pro- 

 tective device. Neither interpretation is proved ; but the 

 subject would well repay investigation. 



The Secretion of Gas. — Among those fishes (Ganoids 

 and Teleosts) in which a gas bladder is present a peculiar 

 form of secretion is often met with. The bladder (homo- 

 logous with the lungs of air-breathing Vertebrates) is in some 

 cases a true respiratory organ. In those teleosts which dwell 

 in deep water and habitually move over a considerable range 

 of depths, it subserves the function of facilitating movements 

 from one level to another by altering the specific gravity of 

 the fish. It was shown over a century ago that in those teleosts 

 which have a closed duct (physoclistous condition) the bladder 

 contains oxygen only. The oxygen content of the bladder 

 changes during inflation and deflation. Though the tension 

 of oxygen in sea-water is about a fifth of one atmosphere and 

 in the capillaries of the bladder considerably less, the tension 

 in the gas bladder may rise to about a hundred atmospheres. 

 Secretion and absorption of oxygen (in the physoclistous 

 forms) provide an auxiliary mechanism to promote sinking 

 and rising in the water. Deflation in physoclistous fishes is 

 apparently effected by means of the oval, a thin- walled area 

 on the dorsal wall of the bladder overlying the cardinal 

 sinuses and enclosed by a sphincter. That oxygen is re- 

 absorbed into the blood by diffusion through the oval is 

 indicated by the fact that during active gas-secretion the 

 sphincter is completely closed so that the thin-walled area is 

 invisible, while it opens widely when the bladder is com- 

 pletely inflated. 



It is fairly certain that the inflation is brought about by the 



