172 THE INFLUENCE OF INANIMATE SUEEOUNDINGS. 



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 '^ has shown that the powers of absorption even in nearly 

 related spesies of fresh-water fish are remarkably various. 

 Hence we need not be sui-prised to find that occasionally the 

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

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

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

 astonished to learn that the gejieral 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 resjsiration quite super- 

 fluous. 



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



Fig, to. — Cobitis fossilis. It swallows air-bubbles ^hich pass through the intestine, 

 where the mucous membrane takes up the oxj'gen for respiration. 



Fritz IMiiller observed that crabs of various species [Grapsus, 

 Sesarma, Arc.) often i-aised 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 dro^in a fish than a frog if both are pre- 

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

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

 gilled water-breather." 



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



