172 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



essentially the same manner namely, by absorption of oxygen 

 (air) from the water by means of the osmotic action of their 

 membranes. This action varies with the different animals. 

 Bert 72 has shown that the powers of absorption even in nearly 

 related species of fresh-water fish are remarkably various. 

 Hence we need not be surprised to find that occasionally the 

 amount of oxygen which is conveyed to the blood by the typical 

 organs of respiration is not perfectly sufficient, so that the defi- 

 ciency has to be made up in other ways. Nor are we more 

 astonished to learn that the general respiration of the skin can 

 be so increased that under some circumstances it suffices per- 

 fectly for the requirements of the animal, and renders the 

 employment of the special organs of respiration quite super- 

 fluous. 



The former case has been proved in Crustacea and in Fishes. 



FIG. cO. Cobilis fossilis. It swallows air-bubbles which pass through the intestine, 

 where the mucous membrane takes up the oxygen for respiration. 



Fritz Miiller observed that crabs of various species (Grapsus, 

 Sesarma, &c.) often raised the hinder portion of the cephalo- 

 thorax, so as to let the air directly into the branchial cavity, as 

 the amount of oxygen absorbed from the water through the 

 gills was insufficient to supply their requirements. Many of our 

 fresh- water fish, particularly all the species of Bleak (Cyprinoidce), 

 regularly swallow air in order to saturate the water that 

 bathes their gills with oxygen, since the oxygen derived 

 directly from the water is usually insufficient. The amount of 

 oxygen needed by these fishes must be considerable, for it is 

 much more easy to drown a fish than a frog if both are pre- 

 vented from coming to the surface to swallow air ; and yet the 

 frog breathes by lungs, while the fish, on the contrary, is a true, 

 gilled water-breather. 73 



An instance of the second case i.e. that general respiration 



